The effects of slash burning on ecosystem nutrients during the land preparation phase of shifting...
14
Plant and Soil 220: 247–260, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 247 The effects of slash burning on ecosystem nutrients during the land preparation phase of shifting cultivation Christian P. Giardina 1,3 , Robert L. Sanford Jr. 1 , Ingrid C. Døckersmith 1 & Victor J. Jaramillo 2 1 Department of Biological Sciences, University of Denver, Denver, Colorado 80208, USA; 2 Instituto de Ecologia, Universidad Nacional Autonoma de Mexico, Campus Morelia A.P. 27-3 Xangari, Morelia, Michoacan C.P. 58089 Mexico; 3 Present address: Department of Agronomy and Soil Science, University of Hawaii-Manoa Beaumont Research Center, 461 Lanikaula Street, Hilo, Hawaii 96720 * Key words: cations, fire, nitrogen, nutrients, phosphorus, slash-and-burn, soil, tropical forests Abstract The most commonly observed change in soil following slash-and-burn clearing of tropical forest is a short-term increase in nutrient availability. Studies of shifting cultivation commonly cite the incorporation of nutrient-rich ash from consumed aboveground biomass into soil as the reason for this change. The effects of soil heating on nutrient availability have been examined only rarely in field studies of slash-and-burn, and soil heating as a mechanism of nutrient release is most often assumed to be of minor importance in the field. Few budgets for above and belowground nutrient flux have been developed in the tropics, and a survey of results from field and laboratory studies indicates that soils are sufficiently heated during most slash-and-burn events, particularly in dry and monsoonal climates, to cause significant, even substantial release of nutrients from non-plant-available into plant-available forms in soil. Conversely, large aboveground losses of nutrients during and after burning often result in low quantities of nutrients that are released to soil. Assessing the biophysical sustainability of an agricultural practice requires detailed information about nutrient flux and loss incurred during management. To this end, current conceptual models of shifting cultivation should be revised to more accurately describe these fluxes and losses. Introduction Shifting cultivation is practiced by ∼300 million people annually, and affects ∼400 million ha of the planet’s 1500 million ha of arable land (Brady, 1996; Kleinman et al., 1996; Seubert et al., 1977). Effective management of fire-converted land for sustained pro- duction relies on detailed knowledge of the fluxes and losses of nutrients incurred during and after the burn- ing of slash biomass (Raison, 1979). This information is particularly important for making land management decisions when fertilizers are used minimally. Studies of shifting cultivation have attempted to assess the bio- physical sustainability of shifting cultivation primarily by identifying the magnitude and persistence of in- creased soil fertility following slash-and-burn clearing of forest land (De Rouw, 1994; Juo and Manu, 1996; Nye and Greenland, 1960; Seubert et al., 1977) and * FAX No: 808 9744110. E-mail: [email protected] secondarily by quantifying fluxes and losses of nu- trients from these converted ecosystems (Giardina et al., 2000; Kauffman et al., 1993, 1995). These data have not yet been used to rigorously evaluate currently accepted conceptual models of nutrient flux during slash-and-burn that were developed nearly 40 years ago. Shifting cultivation is a multi-step process that in- cludes site selection, clearing forest by slash-and-burn, cropping, abandonment and fallow. The slash-and- burn phase of shifting cultivation, the focus of this paper, most often includes cutting down forest ve- getation at the beginning of a dry spell or the dry season, drying slashed vegetation in place for weeks to months, and broadcast burning the dried slash be- fore the onset of the rainy season (Ewel et al., 1981; Maass, 1995; Nye and Greenland, 1960; Seubert et al., 1977). The most detrimental changes in site nu- trient status occur during this phase of the shifting cultivation cycle.
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Transcript of The effects of slash burning on ecosystem nutrients during the land preparation phase of shifting...
Plant and Soil220: 247–260, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.
247
The effects of slash burning on ecosystem nutrients during the landpreparation phase of shifting cultivation
Christian P. Giardina1,3, Robert L. Sanford Jr.1, Ingrid C. Døckersmith1 & Victor J. Jaramillo21Department of Biological Sciences, University of Denver, Denver, Colorado 80208, USA;2Instituto de Ecologia,Universidad Nacional Autonoma de Mexico, Campus Morelia A.P. 27-3 Xangari, Morelia, Michoacan C.P. 58089Mexico; 3Present address: Department of Agronomy and Soil Science, University of Hawaii-Manoa BeaumontResearch Center, 461 Lanikaula Street, Hilo, Hawaii 96720∗
Key words:cations, fire, nitrogen, nutrients, phosphorus, slash-and-burn, soil, tropical forests
Abstract
The most commonly observed change in soil following slash-and-burn clearing of tropical forest is a short-termincrease in nutrient availability. Studies of shifting cultivation commonly cite the incorporation of nutrient-richash from consumed aboveground biomass into soil as the reason for this change. The effects of soil heatingon nutrient availability have been examined only rarely in field studies of slash-and-burn, and soil heating as amechanism of nutrient release is most often assumed to be of minor importance in the field. Few budgets forabove and belowground nutrient flux have been developed in the tropics, and a survey of results from field andlaboratory studies indicates that soils are sufficiently heated during most slash-and-burn events, particularly in dryand monsoonal climates, to cause significant, even substantial release of nutrients from non-plant-available intoplant-available forms in soil. Conversely, large aboveground losses of nutrients during and after burning often resultin low quantities of nutrients that are released to soil. Assessing the biophysical sustainability of an agriculturalpractice requires detailed information about nutrient flux and loss incurred during management. To this end, currentconceptual models of shifting cultivation should be revised to more accurately describe these fluxes and losses.
Introduction
Shifting cultivation is practiced by∼300 millionpeople annually, and affects∼400 million ha of theplanet’s 1500 million ha of arable land (Brady, 1996;Kleinman et al., 1996; Seubert et al., 1977). Effectivemanagement of fire-converted land for sustained pro-duction relies on detailed knowledge of the fluxes andlosses of nutrients incurred during and after the burn-ing of slash biomass (Raison, 1979). This informationis particularly important for making land managementdecisions when fertilizers are used minimally. Studiesof shifting cultivation have attempted to assess the bio-physical sustainability of shifting cultivation primarilyby identifying the magnitude and persistence of in-creased soil fertility following slash-and-burn clearingof forest land (De Rouw, 1994; Juo and Manu, 1996;Nye and Greenland, 1960; Seubert et al., 1977) and
∗ FAX No: 808 9744110. E-mail: [email protected]
secondarily by quantifying fluxes and losses of nu-trients from these converted ecosystems (Giardina etal., 2000; Kauffman et al., 1993, 1995). These datahave not yet been used to rigorously evaluate currentlyaccepted conceptual models of nutrient flux duringslash-and-burn that were developed nearly 40 yearsago.
Shifting cultivation is a multi-step process that in-cludes site selection, clearing forest by slash-and-burn,cropping, abandonment and fallow. The slash-and-burn phase of shifting cultivation, the focus of thispaper, most often includes cutting down forest ve-getation at the beginning of a dry spell or the dryseason, drying slashed vegetation in place for weeksto months, and broadcast burning the dried slash be-fore the onset of the rainy season (Ewel et al., 1981;Maass, 1995; Nye and Greenland, 1960; Seubert etal., 1977). The most detrimental changes in site nu-trient status occur during this phase of the shiftingcultivation cycle.
248
Figure 1. The conceptual model proposed by Nye and Greenland(1960) and Sanchez et al. (1991) for nutrient flux during slash burn-ing. Ash from consumed biomass is incorporated into soil resultingin an increase in soil fertility. Carbon and N are largely volatilized; Pand cations are efficiently transferred from biomass to ash and thento soil. During rain some cations may be leached. Soils are relativelyunaffected by burning, but are modified by ash after rainfall.
In 1960, Nye and Greenland reviewed availableinformation on changes in soil fertility followingslash-and-burn clearing of forest, and proposed the‘nutrient-rich ash’ hypothesis to explain post-burn in-creases in soil nutrient availability. Their hypothesiswas straight forward: burning converts slashed veget-ation into nutrient-rich ash that is deposited on the soilsurface and incorporated into soil by rainfall and cul-tivation (Figure 1). In addition to providing an input ofnutrients, the incorporation of ash into soil increasessoil pH, which in acidic soils can increase nutrientavailability. The direct effects of heat on soil nutri-ent availability were hypothesized to be small whencompared with those of ash.
Temperate field studies of slash burning (reviewedby Raison, 1979 and Walker et al., 1986) and labor-atory heating studies of tropical soil (Kang and Sajja-pongse, 1980; Sertsu and Sanchez, 1978) have shownthat the view of slash-and-burn summarized in Fig-ure 1 is over-simplified. Nonetheless, field studies andreviews of tropical slash-and-burn agriculture have re-lied on this conceptual model to explain increasesin post-burn soil fertility (Beck and Sanchez, 1994;Brady, 1996; Buschbacher et al., 1988; Capistranoand Marten, 1986; Christanty, 1986; De Rouw, 1994;Kleinman et al., 1996; Lal and Cummings, 1979;
Lessa et al., 1996; Maass, 1995; Sanchez, 1976;Sanchez et al., 1991; Seubert et al., 1977).
To adequately assess the biophysical sustainabilityof slash-and-burn agriculture, the quantity of nutri-ents released from above and belowground biomass,and soil organic matter during and after slash burn-ing should be examined (Raison, 1979; Walker etal. 1986), however few studies of shifting cultivationhave examined nutrients at this level of detail (Juoand Manu, 1996). In the following sections, we re-view extant studies of slash-and-burn agriculture toconstruct budgets for total and available nitrogen (N),phosphorus (P) and cation pools in the soil and above-ground biomass of tropical forest. We track changesin these budgets through slash burning, and review theconsequences of burning on biological properties ofsoil. Where tropical field data are not available, weinfer from laboratory or temperate forest studies.
Slash-and-burn effects on aboveground biomass
Nutrients are more concentrated in fine plant material,such as leaves, twigs and small branches, than in thelarger components of a tree. These fine materials makeup a small portion of total aboveground biomass, butoften contain a large portion of the total abovegroundnutrient stock (Buschbacher et al., 1988; Kauffman etal., 1993, 1995). These fine materials are the quick-est to dry following slashing and are the most readilyburned components of a forest (Kauffman et al., 1995;Raison, 1979). Total aboveground nutrient loss dur-ing slash-and-burn clearing of forest land is amongthe highest of any disturbance known. AbovegroundN can undergo large, nearly parallel losses with car-bon (C) during biomass burning (Raison et al., 1985),with release rates ranging from 30% to 90% of totalaboveground stock (Buschbacher et al., 1988; Kauff-man et al., 1993, 1995). In the years following a burn,atmospherically derived N in rainfall and biologicalN2-fixation can replace losses of N (Raison et al.,1985, 1993). Phosphorus in aboveground biomass canalso undergo large volatilization and convective lossesduring slash burning (Giardina et al., 2000; Kauffmanet al., 1993; Raison et al., 1985). The new sourcesof P to replace losses include non-plant available P insoil and in parent material (except in highly-weatheredsoils), the generally low levels of atmospherically de-posited P and fertilizer (Raison et al., 1985). Nutrientcations, such as calcium (Ca), magnesium (Mg) andpotassium (K), have higher volatilization temperaturesthan N or P, but are equally subject to convective losses
249
during a burn and to erosional and leaching losses fol-lowing a burn (Ewel et al., 1981; Raison et al., 1985).As with P, the primary sources of these cations are par-ent material, precipitation and fertilizer. In addition tosubstantial volatilization and convective losses of nu-trients during biomass burning (Kauffman et al., 1993,1995; Mackensen et al., 1996; Raison et al., 1985),nutrients may be lost following a burn by wind andwater erosion of ash (Khanna et al., 1994; Raison etal., 1985).
Across the sites reviewed here, element content ofash was significantly correlated with nutrient contentof the standing crop (Table 1, Figure 2). On average,3%, 49%, 50% and 57% of total aboveground N, P,Ca and K, respectively, were returned to soil as ash.The slopes of the regression lines indicate that elementtransfer, as a proportion of standing stock, increased asfollows:
N� P= Ca< K.
Most N in consumed biomass will be lost from a site,and ash generally contains little N. However, the Ncontent of ash was sometimes substantial when stand-ing stocks of N were large. The large variance for Nin Figure 2 is consistent with the fact that N is muchmore susceptible to volatilization losses than P, Ca orK.
The 50% transfer of aboveground Ca to ash islarger than the 31–34% loss reported by Raison etal. (1985); however, large quantities of Ca are se-questered in tree boles and the low intensity firesexamined by Raison et al. (1985) would have a smallerimpact on these Ca stocks than the intense slash firesreviewed here. The reason why the Mg content of ashcorrelated poorly with Mg content of standing crop(R2 <0.01, P=0.92; Table 1) is not clear. Raison etal. (1985) suggested that organically bound Mg maybe more sensitive to volatilization losses than Ca or K,despite a high volatilization temperature for inorganicMg (1170◦C). Alternatively, the high recovery for Ca,K and P at the two sites with very little recovery of Mg(Mackenson et al., 1996; Zinke et al., 1978) may in-dicate that the Mg numbers from these two sites are inerror. Overall, when the wide range in burn conditionsand the diversity of methodologies are considered, thestrength of the relationships presented in Figure 2 aresurprising.
Fine ash is very susceptible to wind loss and post-burn losses of ash from a site may represent a siz-able export of nutrients, possibly equal to or greaterthan volatilization losses during a burn (Raison et al.,
1985). However, few estimates of ash loss from trop-ical slash-and-burn studies exist. Sampling for ash isdifficult because the distribution of ash is spatiallyvery heterogeneous, and ash is difficult to fully re-cover in the field. Ash is also very heterogeneous withregards to nutrient content. Fine gray or white ashrepresents a small portion of total ash, but is highlyenriched with nutrients and may represent a large por-tion of total ash nutrient content (Raison et al., 1985).Two of the studies included in Table 1 assessed wind-related losses of ash following burning (Giardina etal., 2000; Kauffman et al., 1993). In both, losses ofN and P in ash were substantial. At a wet forest site inCosta Rica, Ewel et al. (1981) also noted substantialash loss due to wind and water in the month followingburning. However, wind related losses were not quan-tified and leaching loss into deeper soils may not haverepresented transport off the site.
Studies of slash-and-burn have viewed the nutri-ents contained in ash as being wholly plant-available(Nye and Greenland, 1960; Seubert et al., 1977; VanReuler and Janssen, 1994). Few tropical studies, how-ever, have assessed the plant-availability of nutrientscontained in ash. Stromgaard (1984) measured plant-available P in ash from a Zambian miombo woodlandsite using a mild acid extract (Bray and Kurtz, 1945)and found that ash contained about 0.8 kg plant-available P per ha. In a temperate study, Ohno andErich (1990) found that 1.9–8.7% of the P, 30–72%of the K, 31–54% of the Mg and 54–87% of the Cain wood ash were extractable with heated ammoniumcitrate. The wide ranges were attributed to species dif-ferences in the source of ash. Khanna et al. (1994)found that 11% of the P, 60% of the sulfur (S), 54–69% of the K and<20% of the Mg and Ca containedin ash from burnt litter, bark and twigs ofEucalyptuspauciflorawere soluble in water. Nutrient availabilityof ash derived from tropical forest would be expectedto follow similar patterns.
With few exceptions (Alegre et al., 1988), mostfield studies of slash-and-burn document increasedsoil pH after burning (Table 2). Burn-related increasesin soil pH are due to the acid neutralizing capacityof ash (Fritze et al., 1994; Khanna et al., 1994; Nyeand Greenland, 1960; Ohno and Erich, 1990; Raison,1979; Romanya et al., 1994; Sanchez, 1976; Singh,1994) and to consumption of hydrogen ions duringthe combustion of organic acids in soil and the forestfloor (D. Binkley, pers. comm.). In acidic soils, in-creases in soil pH may increase the availability ofnutrients such as P (Romanya et al., 1994; Sibanda and
250
Table
1.A
shm
ass,
nutr
ient
cont
enta
ndlo
sses
afte
rtr
opic
alsl
ash
burn
ing
Loc
atio
nM
ean
Sta
ndS
tand
Nut
rient
cont
ento
fN
utrie
ntco
nten
tof
Nut
rient
cont
ento
fash
Re
fere
nce
Ann
ual
Age
Bio
mas
saS
tan
dB
iom
ass
ash
afte
rbu
rnin
gre
ma
inin
ga
fter
loss
es
Ra
infa
ll(y
r)(M
g/ha
)N
PC
aM
gK
NP
Ca
Mg
KN
PC
aM
gK
(mm
)(k
g/ha
)(k
g/ha
)(k
g/ha
)
Ch
am
ela
,Mex
ico
75
0>
100
110
944
2715
35N
D34
628
1169
6N
D90
75
ND
ND
ND
Gia
rdin
ae
tal.,
2000
Dry
deci
duou
sS
teel
ee
tal.,
1998
Per
nam
buco
,Bra
zil
790
nativ
eN
DN
DN
DN
DN
DN
D35
1234
847
61N
DN
DN
DN
DN
DL
ess
ae
tal.,
1996
b
Sem
iarid
deci
duou
s
Per
nam
buco
,Bra
zil
803
1674
551
37N
DN
DN
D16
20N
DN
DN
D7
9N
DN
DN
DK
auffm
ane
tal.,
1993
Sem
iarid
deci
duou
s
Lua
For
est,
Tha
iland
1400
ND
ND
143
1622
811
211
010
ND
567
24N
DN
DN
DN
DN
DZ
inke
etal
.,19
78D
ryse
mi-d
ecid
uous
TaiF
ores
t,Iv
ory
Coa
st18
854
350–
560
351
1725
474
298
2611
125
4212
4N
DN
DN
DN
DN
DV
an
Reu
ler
&Ja
nsse
n,19
93L
owla
ndev
erg
reen
2035
0–56
054
213
401
7820
127
811
847
70N
DN
DN
DN
DN
D
Per
u,Y
urim
agua
s21
0017
ND
ND
ND
ND
ND
ND
676
7516
38N
DN
DN
DN
DN
DS
eube
rtet
al.,
1977
Mo
iste
verg
ree
n
Pa
ra,B
razi
l20
8840
3434
27
311
3575
62
929
20N
DN
DN
DN
DN
DM
acke
nse
ne
tal.,
1996
Hum
idtr
opic
al40
9583
424
783
9527
04
517
716
54N
DN
DN
DN
DN
D(t
ree
s>7
cmdb
hha
d7
3120
79
291
4373
53
112
1522
ND
ND
ND
ND
ND
be
en
ha
rve
ste
d)
Pa
ra,B
razi
l20
88pr
imar
y29
213
9062
90N
D54
547
1847
7N
D21
1N
DN
DN
DN
DN
DK
auffm
anet
al.,
1995
Mo
iste
verg
ree
n
Pa
ra-B
razi
l20
88pr
imar
y43
523
0087
1285
ND
945
5247
640
ND
506
ND
ND
ND
ND
ND
Kau
ffman
etal
.,19
95M
oist
eve
rgre
en+
logg
ing
Ron
doni
a,B
razi
l23
54pr
imar
y29
020
7057
360
ND
420
157
2219
2N
D24
8N
DN
DN
DN
DN
DK
auffm
anet
al.,
1995
Moi
stev
erg
reen
+lo
ggin
g
Ron
doni
a,B
razi
l23
54pr
imar
y36
124
2062
955
ND
500
4035
486
ND
316
ND
ND
ND
ND
ND
Kau
ffman
etal
.,19
95M
ois
teve
rgre
en
Tur
rialb
a,C
osta
Ric
a27
008
5245
914
439
8123
996
1655
510
719
0N
DN
DN
DN
DN
DE
wel
etal
.,19
81P
rem
on
tan
ew
etf
ore
st
aA
bove
grou
ndon
ly.b A
shco
ntam
inat
edw
ithso
ilan
dun
burn
tsla
sh.
251
Table
2.P
re-a
ndpo
st-b
urn
soil
nutr
ient
cont
enta
ndav
aila
bilit
yfo
rst
udie
sof
slas
h-an
d-bu
rn
Loc
atio
nA
nnua
lS
tand
Sta
ndS
ampl
eS
ampl
eTo
tal
CTo
talP
Ava
ilabl
eP
Tota
lNM
iner
alN
Soi
lpH
Re
fere
nce
Ra
infa
llA
geB
iom
ass
dept
hnu
mbe
rP
reP
ost
Pre
Pos
tP
reP
ost
Pre
Pos
tP
reP
ost
Pre
Pos
t(m
m)
(yr)
(Mg/
ha)
(cm
)(M
g/ha
)(k
g/ha
)
Ch
am
ela
,Mex
ico
75
0>
100
110
0–2
12a4.
43.
298
105
731
.975
769
029
117
6.5
8G
iard
ina
etal
.,20
00d
Dry
deci
duou
s2–
512a
ND
ND
137
143
3.7
15.1
720
768c
cN
DN
DS
teel
ee
tal.,
1998
Per
nam
buco
,Bra
zil
803
1674
0–2
125.
15.
550
57N
DN
D44
343
3N
DN
DN
DN
DK
auffm
anet
al.,
1993
d
Sem
iarid
deci
duou
s2–
512
6.0
6.3
7277
ND
ND
522
558
ND
ND
ND
ND
5–10
126.
96.
910
111
7N
DN
D62
566
9N
DN
DN
DN
D
Ibed
an,N
iger
ia12
5015
ND
0–10
ND
2326
.7N
DN
DN
DN
D39
5446
14N
DN
D6.
69.
0L
ala
ndC
umm
ings
,197
9L
owla
nd
rain
fore
st
Lua
For
est,
Tha
iland
1400
9N
D0–
51
17.3
17.7
ND
ND
210
.195
097
0N
DN
D6.
06.
7Z
iuke
eta
l.,19
78D
ryse
mi-d
ecid
uous
Nam
Phr
om,T
haila
nd15
00ol
d33
00–
53
20.5
32.5
ND
ND
3.3
44.4
1500
2100
11.5
33.6
6.3
7.2
Kyu
ma
eta
l.,19
85M
oist
sem
i-dec
iduo
usfo
rest
10–1
53
6.8
12.7
8.8
27.9
555
1045
6.3
19.6
5.6
6.0
Meg
hala
ya,N
.Ind
ia18
5015
ND
0–7
4013
.311
.2N
DN
D2.
52.
518
2017
50N
DN
D5.
17.
8M
ishr
aan
dM
onso
on10
ND
0–7
4012
.611
.9N
DN
D2.
42.
518
2017
50N
DN
D5.
37.
6R
am
akris
hnan
,198
3b
sem
i-dec
iduo
us5
ND
0–7
4011
.211
.2N
DN
D2.
32.
514
7014
00N
DN
D5.
57.
5
Per
u,Y
urim
agua
s21
0017
ND
0–10
4a
6.3
6.9
ND
ND
5.2
16.2
605
690
4164
4.0
4.5
Seu
bert
etal
.,19
77M
ois
teve
rgre
en
Pa
ra,B
razi
l20
88pr
imar
y29
20–
2.5
5N
DN
DN
DN
DN
DN
D36
829
0N
DN
DN
DN
DK
auffm
anet
al.,
1995
Moi
stev
erg
reen
2.5–
105
ND
ND
ND
ND
ND
ND
799
733
ND
ND
ND
ND
Pa
ra,B
razi
l20
88pr
imar
y43
50–
2.5
57.
06.
2N
DN
DN
DN
D62
866
5N
DN
DN
DN
DK
auffm
anet
al.,
1995
Moi
stev
erg
reen
+lo
ggin
g2.
5–10
520
.614
.2N
DN
DN
DN
D16
0316
03N
DN
DN
DN
D
Ron
doni
a,B
razi
l23
54pr
imar
y29
00–
2.5
510
.810
.331
53N
DN
D77
680
3N
DN
DN
DN
DK
auffm
anet
al.,
1995
Moi
stev
erg
reen
2.5–
105
18.8
13.5
8810
0N
DN
D14
7911
29N
DN
DN
DN
D
Ron
doni
a,B
razi
l23
54pr
imar
y36
10–
2.5
512
.3N
D43
ND
ND
ND
938
ND
ND
ND
ND
ND
Kau
ffman
etal
.,19
95M
oist
eve
rgre
en+
logg
ing
2.5–
105
17.1
ND
65N
DN
DN
D14
82N
DN
DN
DN
DN
D
Tur
rialb
a,C
osta
Ric
a27
008
520–
34
20.4
13.4
ND
ND
0.78
0.64
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252
Figure 2. The relation between nutrient content of aboveground biomass and nutrient content of ash deposited on soil surface immediatelyfollowing slash burning at various tropical sites. Points are all taken from Table 1. The study of Ewel et al. (1981) was not included because Pand cation content of ash was higher than that of pre-burn slash biomass.
Young, 1989), reduce the availability of phytotoxicelements such as aluminum (Sanchez, 1976; Walker etal., 1986), and increase microbial activity and nutrientmineralization rates (Fritze et al., 1994; Khanna et al.,1994; Ohno and Erich, 1990).
Additions of ash often increase soil microbialactivity, because labile C in ash is added to soil or min-eralization of native soil C is stimulated by changes inpH and additions of nutrients. Khanna et al. (1994)found that ash additions of 25 Mg per ha increased Crespiration rates by 50–100% in acid soils with widelyranging organic matter contents. Increases were smal-ler in soils with low organic matter content, and atlower ash addition rates. In a Finnish forest soil,in situadditions of up to 5 Mg ash per ha did not significantlyincreasein vitro respiration rates of field moist soils 10d after ash application (Fritze et al., 1994). When soils
were brought to 60% of water holding capacity, respir-ation rates did increase by∼30%, but not at lower ashaddition rates. Raison and McGarity (1980) also foundthat ash additions increased soil respiration rates, butnoted respiration responses were much larger when incombination with soil heating.
Soil heating
Soil heating is controlled by a variety of factors thatinclude fuel quantity, quality, moisture and spatial dis-tribution on the soil surface (Martin, 1990; Raison,1979; Tomkins et al., 1991; Walker et al., 1986).Soil heating also will be influenced by soil texture,soil moisture and initial soil temperature (Dunn et al.,1979; Raison, 1979; Sanchez, 1976; Serrasolsas andKhanna, 1995a; Walker et al., 1986). Topography and
253
Figure 3. Soil temperatures attained at various depths during trop-ical slash burning in Dry (a), Monsoonal (b) and Humid (c) forests.Data are from the following studies: Mexico (Giardina et al., 2000);Brazil (Kauffman et al., 1993); Nam Phrom, Thailand (Kyuma etal., 1985); Ban Pa Pae, Thailand (Zinke et al., 1978); Ivory Coast(Van Reuler and Janssen, 1993); Costa Rica (Ewel et al., 1981); andNigeria (Lal and Cummings, 1979).
meteorological conditions preceding a burn, during aburn and following a burn influence soil heating bymodifying burn times and intensity, as well as the dir-ection and speed of fire spread (Martin, 1990; Raison,1979).
Soil heating during slash-and-burn appears to in-crease with increasing length of dry season (Figure3), a trend that can be at least partly attributed toincreasing soil moisture from dry to humid climates(Ewel et al., 1981; Giardina et al., 2000). Comparis-ons like these are complicated by the diverse methodsused to estimate soil temperatures (temperature sensit-ive paints or crayons, thermocouples). Some methodsmay over-estimate heat flux into soil (e.g. backing ma-terials for paint that conduct heat), and most requiresome disturbance of the soil prior to burning. Whilethese data should be interpreted cautiously, they doprovide a starting point for linking laboratory studies
to the field. This link is important because the surface2 cm of soil can contain from 300 to 1500 kg of Nand 30 to 200 kg of P (Table 2), a large portion ofwhich may be biologically active (organic matter, mi-crobes, roots) and sensitive to heating. Results frommost laboratory heating studies do not accurately pre-dict the effects of soil heating on plant growth becausepotted plants are often grown in uniformly heated soil(Giovannini et al., 1990; Kang and Sajjapongse, 1980)and plant roots are able to grow in the field belowheated soil. Furthermore, few studies have combinedsoil heating with additions of ash. However, laborat-ory studies provide insight into soil changes that arespecific to a given temperature.
The quantity of nutrients in surface soils can equalor exceed the quantity of nutrients in aboveground bio-mass, yet soils have received limited attention in fieldstudies of slash-and-burn. In the following sections,we examine results from studies that have examinedsoil heating in the field and in the laboratory.
Slash-and-burn effects on belowground biomass
Belowground biomass is comprised of soil organisms(bacteria, fungi, macro-fauna), mycorrhizae and plantroots. Microbial biomass can represent a large pool ofnutrients, especially on less fertile sites (Chapin et al.,1986), in disturbed forest stands (Vitousek and Mat-son, 1984, 1985) and in tropical dry forests (Campoet al., 1998; Singh et al., 1989). Soil microbial com-munities also control organic matter decompositionand nutrient mineralization rates and influence nutrientuptake by plants (Paul and Clark, 1996). Burning canhave profound effects on microbial biomass and thecapacity of soil microbes to carry out these functions(Garcia-Oliva et al., 1999; Raison, 1979; Serrasolsasand Khanna, 1995a; Walker et al., 1986).
Heating soil for 10 min at 70◦C kills some fungi,protozoa and bacteria, while temperatures above 127◦C almost completely sterilizes soil (Raison, 1979).Heating of a temperate forest soil (15% water content)to 300◦C to a depth of 1 cm and to 125◦C to a depthof 2 cm killed 98% of soil fungi and 75% of soil het-erotrophic bacteria in the top 2 cm of soil (Dunn etal., 1979). Soil heating to 120◦C in a range of Aus-tralian soils resulted in 34–80% declines in microbialbiomass, while heating to 250◦C resulted in bio-mass reductions of 85–99% (Serrasolsas and Khanna,1995a). For all sites reviewed in Figure 3, slash burn-ing resulted in soil temperatures to 1 cm that were highenough to cause very high rates of microbial mortality.
254
Soil temperatures were sufficiently high at all sites ex-cept the Costa Rican and Nigerian sites, to kill mostmicro-organisms down to 2 cm.
In lieu of data from slash-and-burn plots, estimatesof the quantities of nutrients that would be releasedfrom heat-killed micro-organisms during a burn canbe made using microbial biomass and nutrient contentdata from intact tropical forest. For a range of dryforest sites, dry season soil microbial N averaged from45 to 153µg N per g soil, while microbial P averagedfrom 11 to 34µg P per g soil in the top 10 cm of soil(Campo et al., 1998; Jaramillo et al., in prep.; Singhand Singh, 1995; Srivastava, 1992; Srivastava andSingh, 1988). Jenkinson and Ladd (1981) estimatedmicrobial biomass of an intact Nigerian moist forestsite to contain 90µg N per g soil and 25µg P perg soil. With data from Srivastava (1992) (dry seasonmicrobial nutrient content of 88µg N per g soil and 34µg P per g soil), and the moist forest data of Jenkinsonand Ladd (1981), we illustrate the potential effects ofsoil heating on microbial biomass at these sites. Afterconverting these concentrations to an area basis (withan assumed bulk density of 1 g per cm3), then mul-tiplying by 0.9 to approximate 90% burn mortality,soil heating in the top 2 cm would cause the release of16 kg N per ha and 6 kg P per ha for the dry forest, and16 kg N per ha and 4 kg P per ha for the moist forest.These quantities of N and P are significant relative tothose contained in ash (Table 1).
Microbial biomass of a slashed but not yet burnedforest may be 2–3 times higher than for undisturbedforest because of higher soil moisture from reducedevapo-transpiration and the mulching effect of felledvegetation, reduced competition with plants and in-creased availability of dead roots and litter (Paul andClark, 1996). Additionally, when a site is slashed,microclimate of soil generally is much warmer thanin the previous forest. Therefore, the above estimatesof microbial release are likely conservative for dry,monsoonal and some moist forest sites (Figure 3). Inline with this suggestion, Jaramillo et al. (in prep) ob-served a 90% reduction in microbial biomass in 0–10cm depth soils following slash burning that released∼140 kg N per ha and∼30 kg P per ha.
Microbial recovery from burning is hypothesizedto be rapid and substantial (Nye and Greenland, 1960),because of elevated soil pH, moisture and temperature,increased nutrient supply and substrate availability,and reduced competition with plants. In a tropical hu-mid forest, the dynamics of microbial activity appearto fit this model (Matson et al., 1987; Nye and Green-
land, 1960). In contrast, slash burning in Mexicandry forest resulted in a dramatic reduction in micro-bial biomass that persisted in the top 10 cm of soilfor up to 2 years (Jaramillo et al., in prep.). Nitro-gen mineralization rates following burning at this siteshowed a similarly long recovery time (Døckersmithet al., 1999). In a temperate study of dry woodland,microbial biomass N was reduced for 90 days fol-lowing heating (the length of the incubation), withlarger reductions for initially drier soils (Klopatek etal., 1990). In another temperate study, reductions insoil microbial biomass were inversely related to soilheating, itself related to soil moisture at the time ofburning (Dunn et al., 1979), which suggests that mi-crobial recovery is increasingly delayed with increas-ing severity of soil heating. Serrasolsas and Khanna(1995a) found a mixture of responses and persistenceof these responses depending on degree of heating,soils type, clay content and the population of organ-isms in question. In the field, microbial recovery fromslash-and-burn will also be influenced by the dramaticchanges in soil micro-climate that accompany land-use. Without the moderating influence of the forestcanopy, soils will be warmer during the day and coolerat night. Additionally, soils will be wetter in clearedsites (Paul and Clark 1996), particularly at the start ofthe rainy season because canopy interception is elim-inated. Overall, assessing the long-term response ofsoil micro-organisms to slash-and-burn represents aresearch priority.
Coarse and fine roots in surface soil are killed andsometimes consumed during slash burning. For ex-ample, in a Mexican dry forest, slash burning reducedlive and dead fine roots (<1 mm in diameter) in theupper 2 cm of the soil by 55% and 32%, respectively(Castellanos, 1998). Notably, on an area basis fineroots contain low amounts of nutrients compared tothe other nutrient fluxes reviewed here (Kummerow etal., 1990; Lessa et al., 1996), especially at the endof a dry season when burning typically occurs andwhen fine root biomass is lowest (Castellanos, 1998;Kavanagh and Kellman, 1992; Roy and Singh, 1995).
Slash-and-burn effects on soil organic matter
Soil organic matter is comprised of the decay resist-ant remains and the precipitated by-products of plantresidue decomposition, and is affected by slash burn-ing on various levels. Thermally induced losses oftotal soil C, the focus of most slash-and-burn stud-ies, are typically small (Table 1). Larger losses may
255
be incurred in surface soil when large quantities offuel are consumed. Soil heating can also alter thechemical composition of organic matter (Fernandezet al., 1997), either through soil desiccation, thermalmodification or release of microbial biomass. Finally,soil heating can thermally mineralize nutrients with orwithout loss of total soil C (Giovannini et al., 1990;Giardina et al., 2000). Because the effects of soil heat-ing are often element specific, we address individualnutrients separately below.
Phosphorus
The processes regulating the geochemical release ofinorganic P and the biochemical mineralization oforganic P are affected by soil pH, vegetation and mi-crobiology (Cross and Schlesinger, 1995; Giardina etal., 1995; Hedley et al., 1982; Stewart and Tiessen,1987), all of which are significantly, but not uniformly,affected by fire. Studies of slash-and-burn agriculturehave assessed changes in plant-available P by usingextracts that remove pools of soil P that correlate withplant uptake of P in controlled agronomic studies. Theutility of these methods in studies of slash-and-burnis unclear because the longer-term supply of P in soilis controlled by the replenishment of depleted solu-tion P pools and the turnover of organic P, not thesize of a specific extractable pool (Tiessen and Moir,1993). Because the capacity of an extract to removeP varies with the chemical properties of a soil, thiscapacity will also be influenced by fire. For example,the slightly basic, sodium bicarbonate solution oftenused to estimate plant-available P in soil (Olsen et al.,1954) functions by increasing soil pH and providingan overwhelming supply of anions that compete with Pfor adsorption sites (Tiessen and Moir, 1993). Burningalso causes an increase in soil pH and an increase inthe concentration of anions that compete with P foradsorption sites.
Plant-available inorganic P, as determined by pHdependent extracts, generally increases when soil isheated (Tables 3 and 4). Between 170 and 300◦C,these increases are matched by declines in organic Pas P is thermally mineralized from soil organic matter,often with little loss of total soil C (Giardina et al.,2000; Giovannini et al., 1990; Kang and Sajjapongse,1980). The heat-induced death of soil microbial pop-ulations is a likely source for part of the increase inextractable soil P (DeBano and Klopatek, 1988; Serra-solsas and Khanna, 1995b). Between 220◦C and 500◦C, P is liberated during the thermal oxidation of soil C
Figure 4. An alternative conceptual model for the fluxes of nutri-ents during slash-and-burn clearing of tropical forest (Modified fromWalker et al., 1986). Variable quantities of biomass are consumedand converted to ash, which may be lost from a site by wind orwater erosion, or incorporated into soil. Substantial quantities ofC, N and P are volatilized, and C, N, P and cations may be lostvia convective transport during the burn. Soils are influenced byburning; heating is variable and dependent upon the many factorsthat control heat penetration into soil. Soil heating causes the re-lease of nutrients from non-plant-available sources (roots, microbialbiomass, organic matter) to plant-available forms. Heating and ashalso cause an increase in soil pH, which can solubilize previouslynon-plant-available nutrients.
(Table 4; Andriesse and Koopmans, 1984; Giovanniniet al., 1990; Kang and Sajjapongse, 1980).
Alternatively, heating in heavily weathered acidsoils can reduce P availability by increasing the P ad-sorption capacity of soil (Sibanda and Young, 1989);heating soil to 200◦C destroyed carboxyl groups onorganic matter that compete with P for adsorption siteson Fe and Al minerals. Organic matter reductionsof <15% can correspond to large (>50%) losses ofcarboxyl functional groups, and increased P adsorp-tion capacity may largely offset any increased supplyof P from mineralized organic matter. In contrast,Romanya et al. (1994) found that despite higher P ad-sorption capacity in intensively heated soils, seedlinggrowth and P content in these soils was higher thanin either unburnt or lightly burnt soils. Seven monthsafter burning, Bray-1 extractable P was significantlyhigher in burnt soils, with the largest increases foundfor the most intensely burnt soils. These increasesin Bray-1 extractable P corresponded to significantreductions in organic P.
Burning can profoundly affect phosphatase activ-ity, the primary exo-enzymes responsible for miner-alizing P from soil organic matter. Serrasolsas andKhanna (1995b) reported a steady decline in phos-
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Table 3. Effects of laboratory heating on soil C and N. Values for sand, clay, C and N are % of soil mass
Soil Sand Clay Total C Total N NH4+NO3 Referencetype (%) (%) (%) (%) (mg/kg soil)
Australia a20 60 120 250 20 60 120 250 20 60 120 250 Serrasolsasand Khanna,1995a,b
Yellow podzolic 73 8 1.7 1.7 1.8 1.7 0.06 0.05 0.05 0.05 10 11 13 39Leached sand 98 0 6.9 6.6 6.8 ND 0.22 0.12 0.12 ND 19 19 39 35Yellow podzolic 72 11 3.2 3 2.9 3.4 0.08 0.08 0.07 0.07 8 9 23 38Yellow earth 65 12 6.2 6.3 6.4 6.3 0.12 0.12 0.14 0.17 20 21 59 51Red earth 34 26 10.6 10.4 10.5 10.9 0.36 0.37 0.37 0.39 39 40 122 60
Italy a25 170 220 460 700 25 170 220 460 700 25 170 220 460 700Giovannini etal., 1990
Arnino Silty Clay 2.4 2.3 2.1 0.3 0.0 0.13 0.122 0.11 0.008 0.005 13 22 54 8 7Torretta Sandy Loam 1.1 1.0 0.9 0.1 0.0 0.099 0.078 0.064 0.005 0.004 12 20 42 8 7
Nigeria a25 100 200 500 600 25 100 200 500 600 25 100 200 500 600Kang andSajjapongse, 1980
Paleustalf 63 21 1.75 1.7 0.8 0.1 0.05 0.24 0.24 0.21 0.03 0.01 ND ND ND ND ND
USA, India& Ethiopia a25 100 200 400 600 25 100 200 400 600 25 100 200 400 600Sertsu and
Sanchez, 1978Ethiopian 12 50 3.4 3.9 2.5 0 0 0.85 1.19 1.2 0.6 0.21 1 2 75 1 0Paleudult 15 32 2.9 2.4 1.8 0 0 1.1 1.1 1.1 0.65 0.6 1 1 12 1 0Vertisol 25 55 2 2 1.9 0 0 0.95 0.8 0.94 0.59 0.61 2 2 14 1 0
a Incubation temperature (◦C).
phatase activity with increasing temperature, and anear complete loss of phosphatase activity in soilsheated to 250◦C. Burning on dry pine soils re-duced phosphatase activity to 30% of unburnt controls(DeBano and Klopatek, 1988). This loss of activitypersisted for the 90 day duration of the experiment.Losses of phosphatase activity were also observed inburnt juniper soils (dry or wet), but declines in controlsoil phosphatase activity complicated interpretations.Phosphatase activity in burnt, wet juniper soil re-covered after 45 d and surpassed control soil activityby day 90.
Few field studies of tropical slash-and-burnprovide enough information to assess changes in soil Pavailability (Table 2). One month after slash burning ata site in the Peruvian Amazon and several weeks intothe growing season, Seubert et al. (1977) measuredincreases in bicarbonate extractable P (Olsen P) of 11kg per ha. These authors found that ash contained 6kg P per ha immediately after burning and suggestedthat ash contributed 6 of the 11 kg P per ha increase.The size of the increase in Olsen P was likely under-estimated for several reasons. Soil pH increased afterburning, potentially reducing the capacity of the ex-tract to remove P. Only a small portion of P in ash
appears to be plant-available (Khanna et al., 1994;Ohno and Erich, 1990). Furthermore, in the monthbefore sampling, post-burn soils were moist, and bio-logical and geochemical fixation (especially importantin the heavily weathered, acidic soils of this Peruvianstudy) would have reduced the elevated quantities oflabile soil P.
At a Mexican dry forest site, soil heating dur-ing slash burning transformed 35 kg of non-plant-available organic and occluded soil P per ha intoplant-available mineral forms (Giardina et al., 2000).About 7 kg per ha of volatilized P were transferredduring burning from biomass to soil (Table 2). Ashfrom consumed slash contained 11 kg P per ha im-mediately after burning (Table 1), 6 kg of which weretransported off the site by wind. Because soils at thisdry forest site were near neutral before burning, ash in-duced increases in soil pH probably played little part inincreasing soil P availability (Giardina et al., 2000). Incomparing inputs to soil, P from ash was of secondaryimportance compared with immediate changes withinthe soil. The longer-term supply of mineralized P wasnot examined.
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Table 4. Effects of laboratory heating on soil phosphorus and pH. Available P in the Australian and Nigerian soils were determined usingBray-1 extract (Bray and Kurtz, 1945), and by Olsen extract (Olsen et al., 1954) in the others
Soil Total P Available P Soil pH Referencetype (mg/kg soil) (mg/kg soil)
Australia a20 60 120 250 20 60 120 250 20 60 120 250 Serrasolsasand Khanna,1995a,b
Yellow podzolic 252 ND ND ND 23 21 22 42 ND ND ND NDLeached sand 45 ND ND ND 1 0.5 1 4 3.6 3.5 3.4 3.1Yellow podzolic 68 ND ND ND 0.5 0.5 1.5 4 4.2 4.7 4.6 4.3Yellow earth 94 ND ND ND 0.5 0.5 0.7 3 4.0 4.7 4.6 4.1Red earth 458 ND ND ND 3.5 3.5 7 8.5 ND ND ND ND
Italy a25 170 220 460 700 25 170 220 460 700 25 170 220 460 700Giovannini etal., 1990
Arnino 650 650 650 650 650 5 23 42 92 35 7.6 7.3 7.0 7.6 10.7Torretta 700 700 700 700 700 8 37 55 100 40 7.8 7.3 7.2 7.9 11
Nigeria a25 100 200 500 600 25 100 200 500 600 25 100 200 500 600Kang andSajjapongse, 1980
Paleustalf 210 ND 206 ND 259 7.2 9.6 40 20 18 6.1 6.1 6.2 6.9 6.8
USA, India& Ethiopia a25 100 200 400 600 25 100 200 400 600 25 100 200 400 600Sertsu and
Sanchez, 1978Ethiopian ND ND ND ND ND 2 4 33 59 26 5.9 5.2 5 6.5 5.4Paleudult ND ND ND ND ND 10 13 49 48 56 5.1 4.9 5.1 6 5.3Vertisol ND ND ND ND ND 3 2 10 20 19 8.2 8 7 6.9 7.7
a Incubation temperature (◦C).
Nitrogen
Soil N is very sensitive to biological transformations,and to losses due to leaching, volatilization, oxida-tion and denitrification (Matson et al., 1987; Raison,1979). Volatilization of ammonia (NH3) and nitricacid (HNO3) increase with temperature. At temperat-ures above 300◦C, soil organic N is lost during thethermal oxidation of organic matter in the form of ox-idized N gases and N2 (Raison, 1979). When soil isheated above 100◦C, ammonium levels also generallyincrease (Table 3) as NH3 is released from microbialbiomass (DeBano and Klopatek, 1988; Serrasolsasand Khanna, 1995a), thermal decomposition organicmatter and protein hydrolysis (Russell et al., 1974)and the desiccation of soil minerals (Raison, 1979).Most slash-and-burn studies examining soil N have fo-cused on changes in total N (Table 2), which may notcorrespond to short-term changes in N availability. Intropical and temperate studies where N mineralizationhas been examined, the influence of slash burning onN mineralization rates appears variable, with studiesreporting increases (Matson et al., 1987), decreases(Døckersmith et al., 1999) or no significant change(Bauhus et al., 1993).
At a wet forest site in Costa Rica, slash-and-burnclearing of a 75-yr-old stand resulted in a 7–12 foldincrease in extractable NH+4 and NO−3 , and a 3 fold in-crease net N mineralization rates (Matson et al., 1987).Large differences between burnt and unburned controlplots persisted for up to 6 months. At a dry forestsite in Mexico, slash-and-burn clearing of a 100 yrold stand resulted in a decrease of 150 kg non-plantavailable N; 68 kg per ha was lost from soil and theremaining 82 kg per ha was transformed into mineralform (Døckersmith et al., 1999). Net N mineralizationrates in 0–10 cm depth soils decreased dramaticallyfollowing burning and continued to decline throughthe first growing season.
CationsCation availability and soil cation exchange capacitycan be affected by soil heating, but the influences arevariable. At low temperatures, cation availability hasbeen shown to increase, decrease or remain unchangedin response to heating. Sertsu and Sanchez (1978)found that both exchangeable Mg and Ca declinedsteadily while exchangeable K increased steadily withheating to 600◦C; effective cation exchange capa-
258
city declined steadily with heating. In contrast, Kangand Sajjapongse (1980) found a steady increase in ex-changeable cations and cation exchange capacity withincreased temperature to 200◦C, after which the avail-ability of these cations and cation exchange capacitydropped dramatically. In the field, slash burning usu-ally results in an increase in soil cation availability. Itis generally assumed that increases in an exchangeablecation represents the input of that cation from ash. In-terpreting these results is complicated, however, by theinfluence that ash, heating and changing pH can haveon cation availability in soil.
Soil pHThe influence of heating on soil pH are not consist-ent across studies, temperatures or soil type (Table 4).Several laboratory heating studies have documentedincreased soil pH after heating alone (Andriesse andKoopmans, 1984; Kang and Sajjapongse, 1980; Khareet al., 1982; Kutiel and Shaviv, 1989), with changesoccurring at temperatures as low as 200◦C. Con-versely, other studies have documented that soil heat-ing to 200◦C (Sertsu and Sanchez, 1978) or to 220◦C (Giovannini et al., 1990) lowers soil pH, and it isnot until soils are heated above 400◦C that soil pHincreases over that of unheated soils. Lowered soil pHhas been attributed to reduced buffering capacity ofsoils, newly exposed soil surfaces, and released Alduring heating. Elevated soil pH has been attributedto consumption of organic acids.
Conclusion
The biophysical sustainability of shifting cultivationhas been the focus of several recent reviews (Brady,1996; Harwood, 1996; Kleinman et al., 1996; Tinkeret al., 1996); however, assessing the biophysical sus-tainability of shifting cultivation is difficult because ofthe long time periods involved and the wide array ofsoils, climates and population pressures under whichit is practiced, and because adequate data are lacking(Juo and Manu, 1996). Biomass burning is known tohave significant long-term consequences for the nutri-ent balance of forest ecosystems (Raison et al., 1993),and Walker et al. (1986) have identified three mainsources of nutrients that supply post-burn increases insoil fertility. These include soil organic matter, soilmicrobial biomass and aboveground biomass (Figure4). The extent to which slash-and-burn alters these nu-trient sources, and the persistence of these alterations
is poorly characterized for most shifting cultivationpractices (Table 2). In perspective, the following com-ponents would be useful to address in a single study:(i) the quantity of nutrients contained in slash biomass;(ii) the nutrient content of ash and unburnt debris; (iii)the quantity of nutrients in ash that are lost from a site;(iv) the net quantity of non-plant-available soil nutri-ents that are thermally transformed into mineral formsavailable to plants (i.e. changes in both exchangeableand total soil nutrient pools); (v) the extent to whichpost-burn increases in soil pH modify nutrient avail-ability (e.g. P sorption capacity); and (vi) the impactof burning on microbial biomass and activity (e.g. Cand N mineralization rates). Ecosystem level studiesaddressing these questions are needed across a rangeof forest types and field conditions before the biophys-ical sustainability of slash-and-burn can be accuratelyassessed.
Acknowledgements
We thank D. Binkley, J. Fownes, R. Menezes and twoanonymous reviewers for ideas, comments and sug-gestions on this manuscript. We thank B. Kauffman,F. Garcia-Oliva and C. Rhoades for helpful discussionson fire and nutrient cycling.
References
Alegre J, Cassel D and Brandy D 1988 Effect of land clearingmethod on chemical properties of an Ultisol in the Amazon. SoilSci. Soc. Amer. J. 52, 1283–1288.
Andriesse J and Koopmans T 1984 A monitoring study on nutrientcycles in soils used for shifting cultivation under various climaticconditions in tropical Asia. I. The influence of simulated burn-ing on form and availability of plant nutrients. Agric. Ecosyst.Environ. 12, 1–16.
Bauhus J, Khanna, P K and Raison R J 1993 The effect of fire on car-bon and nitrogen mineralization and nitrification in an Australianforest soil. Aust. J. Soil Res. 31, 621–639.
Beck M and Sanchez P 1994 Soil phosphorus fraction dynamicsduring 18 years of cultivation on a typic Paleudult. Soil Sci. 34,1424–1431.
Brady N 1996 Alternatives to slash-and-burn: a global imperative.Agric. Ecosyst. Environ. 58, 3–11.
Bray R and Kurtz L 1945 Determination of total, organic andavailable forms of phosphate in soils. Soil Sci. 59, 39–45.
Buschbacher R, Uhl C and Serrao E 1988 Abandoned pastures ineastern Amazonia. II Nutrient stocks in soil and vegetation. J.Ecol. 76, 682–699.
Campo J, Jaramillo V J and Maass J M 1998 Pulses of soil phos-phorus availability in a Mexican tropical dry forest: effects ofseasonality and level of wetting. Oecologia 115, 167–172.
259
Capistrano A and Marten G 1986 Agriculture in Southeast Asia.In Traditional Agriculture in Southeast Asia. Ed. G Marten. pp6–19. Westview Press, Boulder, Colorado.
Castellanos J 1998 Efecto de a roza, tumba y quema sobre a dinám-ica de las raices finas de una selva baja caducifolia. MSc Thesis.Universidad Nacional Autónoma de México, México, DF.
Chapin III F, Vitousek P and Van Cleve K 1986 The nature ofnutrient limitation in plant communities. Am. Nat. 127, 48–57.
Christanty L 1986 Shifting cultivation and tropical soils: patterns,problems and possible improvements.In Traditional Agriculturein Southeast Asia. Ed. GG Marten. pp 226–240. Westview Press,Boulder, Colorado.
Cross A and W Schlesinger 1995 A literature review and evaluationof the Hedley fractionation: applications to the biogeochemicalcycle of soil phosphorus in natural ecosystems. Geoderma 64,197–214.
DeBano L and Klopatek J 1988 Phosphorus dynamics of pinyon-juniper soils following simulated burning. Soil Sci. Soc. Am. J.52, 271–277.
De Rouw A 1994 Effect of fire on soil, rice, weeds and forest re-growth in a rain forest zone (Cote d’Ivoire). Catena 22, 133–152.
Døckersmith I C, Giardina C P and Sanford Jr R L 1999 Persistenceof tree related patterns in soil nutrients following slash-and-burndisturbance in the tropics. Plant Soil 209, 137–156.
Dunn P, DeBano L and Eberlein G 1979 Effects of burning on cha-parral soils: II. Soil microbes and nitrogen mineralization. SoilSci. Soc. Am. J. 43, 509–514.
Ewel J, Berish C, Brown B, Price N and Raich J 1981 Slash and burnimpacts on a Costa Rican wet forest site. Ecology 62, 816–829.
Fernandez I, Cabaneiro A and Carballas T 1997 Organic matterchanges immediately after a wildfire in an Atlantic forest soil andcomparison with laboratory soil heating. Soil Biol. Biochem. 29,1–11.
Fritze H, Smolander A, Levula T, Kitunen V and Malkonen E1994 Wood-ash fertilization and fire treatments in a Scots pineforest stand: effects on the organic layer, microbial biomass andmicrobial activity. Biol. Fertil. Soils 17, 57–63.
Garcia-Oliva F, Sanford Jr R and Kelly E 1999 Effect of burningof tropical deciduous forest soil in Mexico on the microbialdegradation of organic matter. Plant Soil 206, 29–36.
Giardina C P, Huffman S, Binkley D and Caldwell B A 1995 Aldersincrease soil phosphorus availability in a Douglas-fir plantation.Can. J. For. Res. 25, 1652–1657.
Giardina C, Sanford Jr R and Døckersmith I 2000 Changes in soilphosphorus and nitrogen during slash burning of a dry tropicalforest. Soil Sci. Soc. Am. J. 64, 399–405.
Giovannini G, Lucchesi S and Giachetti M 1990 Effects of heatingon some chemical parameters related to soil fertility and plantgrowth. Soil Sci. 149, 344–350.
Harwood R 1996 Development pathways toward sustainable sys-tems following slash-and-burn. Agric. Ecosyst. Environ. 58,75–86.
Hedley M J, Stewart J W B and Chauhan B 1982 Changes ininorganic soil phosphorus fractions induced by cultivation prac-tices and by laboratory incubations. Soil Sci. Soc. Amer. J. 46,970–976.
Hossain A K M A, Raison R J and Khanna P K 1995 Effects offertilizer application and fire regime on soil microbial biomasscarbon and nitrogen, and nitrogen mineralization in an Australiansubalpine eucalypt forest. Biol. Fertil. Soils 19, 246–252.
Jenkinson D and Ladd J 1981 Microbial biomass in soil: measure-ment and turnover.In Soil Biochemistry Vol 5. Eds EA Paul andJN Ladd. pp 415–471. Dekker, New York.
Juo A and Manu A 1996 Chemical dynamics in slash-and-burnagriculture. Agric. Ecosyst. Environ. 58, 49–60.
Kang B and Sajjapongse A 1980 Effect of heating on properties ofsome soils from southern Nigeria and growth of rice. Plant Soil55, 85–95.
Kauffman J B, Sanford Jr R, Cummings D, Salcedo I and Sampaio E1993 Biomass and nutrient dynamics associated with slash firesin neotropical dry forests. Ecology 74, 140–151.
Kauffman J B, Cummings D, Ward D and Babbitt R 1995 Fire inthe Brazilian Amazon: 1. Biomass, nutrient pools and losses inslashed primary forests. Oecologia (Berlin) 104, 397–408.
Kavanagh T and Kellman M 1992 Seasonal pattern of fine rootproliferation in a tropical dry forest. Biotropica 24, 157-165.
Khanna P K, Raison R J and Falkiner R A 1994 Chemical propertiesof ash derived fromEucalyptuslitter and its effects on forestsoils. For. Ecol. Manage. 66, 107–125.
Khare D, Khare P and Mishra G 1982 Effect of heat on nutrientrelease from soil under tropical dry deciduous forest. Jap. J. Ecol.32, 107–110.
Kleinman P, Pimentel D and Bryant R 1996 The ecological sustain-ability of slash-and-burn agriculture. Agric. Ecosyst. Environ.52, 235–249.
Klopatek J M, Klopatek C C and DeBano L F 1990 Potential vari-ation of nitrogen transformations in pinyon-juniper ecosystemsresulting from burning. Biol. Fertil. Soils 10, 35–44.
Kumada K, Ohta S, Arai S, Kitamura M and Imai K 1985 Changesin soil organic matter and nitrogen fertility during the slash-and-burn and cropping phases in experimental shifting cultivation.Soil Sci. Pl. Nutr. 31, 611–623.
Kummerow J, Castellanos J, Maass M and Larigauderie A 1990Production of fine roots and the seasonality of their growth ina Mexican deciduous dry forest. Vegetatio 90, 73–80.
Kutiel P and Shaviv A 1989 Effect of simulated forest fire on theavailability of N and P in Mediterranean soils. Plant Soil 120,57–63.
Kyuma K, Tulaphitak T and Pairintra C 1985 Changes in soil fer-tility and tilth under shifting cultivation. Soil Sci. Pl. Nutr. 31,227–238.
Lal R and Cummings D 1979 Clearing a tropical forest I. Effects onsoil and micro-climate. Field Crops Res. 2, 91–107.
Lessa A, Anderson D and Moir J 1996 Fine root mineralization,soil organic matter arid exchangeable cation dynamics in slashand burn agriculture in the semi-arid northeast of Brazil. Agric.Ecosyst. Environ. 59, 191–202.
Maass J 1995 Conversion of tropical dry forest to pasture and ag-riculture. In Seasonally Dry Tropical Forests. Eds S Bullock,H Mooney and E Medina. pp 399–422. Cambridge UniversityPress, Cambridge.
Mackensen J, Holscher D, Klinge R and Folster H 1996 Nutri-ent transfer to the atmosphere by burning of debris in easternAmazonia. For. Ecol. Manage. 86, 121–128.
Martin R E 1990 Goals, methods and elements of prescribed burn-ing. In Natural and Prescribed Fire in Pacific Northwest Forests.Eds JD Walstad, SR Radosevich and DV Sandberg. pp 55–66.Oregon State University Press, Corvallis, OR.
Matson P, Vitousek P, Ewel J, Mazzarino M and Robertson G 1987Nitrogen transformations following tropical forest felling andburning on a volcanic soil. Ecology 68, 491–502.
Mishra B and Ramakrishnan P 1983 Slash and burn agriculture athigher elevations in north-eastern India. II. Soil fertility changes.Agric. Ecosyst. Environ. 9, 83–96.
Nye P and Greenland D 1960 The soil under shifting cultivation.Technical Communication No. 51. Commonwealth AgriculturalBureau, England. 156 p.
260
Ohno T and Erich M 1990 Effect of wood ash application on soil pHand soil test nutrient levels. Agric. Ecosyst. Environ. 32, 223–239.
Olsen S, Cole V and Watanabe F 1954 Estimation of available phos-phorus in soils by extraction with sodium bicarbonate. USDACirc. 939.
Paul E A and Clark F E 1996 Soil Microbiology and Biochemistry.Academic Press, Inc., San Diego, California. 126 p.
Raison R J 1979 Modification of the soil environment by vegeta-tion fires, with particular reference to nitrogen transformations:a review. Plant Soil 51, 73–108.
Raison R J and McGarity J W 1980 Effects of ash, heat, and the ash-heat interaction on biological activities in two contrasting soils –I. Respiration rate. Plant Soil 55, 363–376.
Raison R J, Khanna P and Woods P 1985 Mechanisms of elementtransfer to the atmosphere during vegetation burning. Can. J. For.Res. 15, 132–140.
Raison R J, O’Connell A M, Khanna P K and Keith H 1993 Ef-fects of repeated fires on nitrogen and phosphorus budgets andcycling processes in forest ecosystems.In Fire in MediterraneanEcosystems. Eds L Trabaud and R Prodon. pp 347–363. EECPublication, Belgium.
Romanya J, Khanna P K and Raison R J 1994 Effects of slash burn-ing on soil phosphorus fractions and sorption and desorption ofphosphorus. For. Ecol. Manage. 65, 89-103.
Roy S and Singh J 1995 Seasonal and spatial dynamics of plant-available N and P pools and N-mineralization in relation to fineroots in a dry tropical forest habitat. Soil Biol. Biochem. 27, 33–40.
Russell J, Fraser A, Watson J and Parsons J 1974 Thermal de-composition of protein in soil organic matter. Geoderma 11,63–66.
Sanchez P 1976 Properties and Management of Soils in the Tropics.John Wiley and Sons, New York, USA.
Sanchez P, Palm C and Smyth T 1991 Phosphorus cycles in ter-restrial and aquatic ecosystems.In SCOPE-UNEP RegionalWorkshop 3: South Central America. Eds H Tiessen et al. pp146–160. Inst. Pedology, Saskatoon, Canada.
Serrasolsas I and Khanna P 1995a Changes in heated and auto-claved forest soils of S.E. Australia. I. Carbon and nitrogen.Biogeochemistry 29, 3–24.
Serrasolsas I and Khanna P 1995b Changes in heated and autoclavedforest soils of S.E. Australia. II. Phosphorus and phosphataseactivity. Biogeochemistry 29, 25–41.
Sertsu S and Sanchez P 1978 Effects of heating on some changesin soil properties in relation to an Ethiopian land managementpractice. Soil Sci. Soc. Am. J. 42, 940–944.
Seubert C, Sanchez P and Valverde C 1977 Effects of land clearingmethods on soil properties of an ultisol and crop performance inthe Amazon jungle of Peru. Tropical Agriculture (Trinidad) 54,307–321.
Sibanda H and Young S 1989 The effect of humus acids and soilheating on the availability of phosphate in oxide-rich tropicalsoils. In Mineral Nutrients in Tropical Forest and Savanna Eco-systems, Special Publication no. 9 of the British EcologicalSociety. Ed. J Proctor. pp 71–83. Blackwell Science Publications,Oxford.
Singh R 1994 Changes in soil nutrients following burning of drytropical savanna. International Journal of Wildland Fire 4, 187–194.
Singh J S, Raghubanshi A S, Singh R S and Srivastava S C 1989 Mi-crobial biomass acts as a source of plant nutrients in dry tropicalforest and savanna. Nature 338, 499–500.
Singh S and Singh J 1995 Microbial biomass associated with water-stable aggregates in forest, savanna and cropland soils of aseasonally dry tropical region, India. Soil Biol. Biochem. 27,1027–1033.
Srivastava S C 1992 Microbial C, N and P in dry tropical soils: sea-sonal changes and influence of soil moisture. Soil Biol. Biochem.24, 711–714.
Srivastava S C and Singh J S 1988 Carbon and phosphorus in thesoil biomass of some tropical soils of India. Soil Biol. Biochem.20, 743–747.
Srivastava S C and Singh J S 1989 Effect of cultivation on microbialcarbon and nitrogen in dry tropical forest soil. Biol. Fertil. Soils8, 343–348.
Steele M 1999 Biomass dynamics associated with deforestation,biomass burning and conversion to pasture in a tropical dry forestin Mexico. MS Thesis. Oregon State Univ., Corvallis, OR.
Stewart J W B and Tiessen H 1987 Dynamics of soil organicphosphorus. Biogeochemistry 4, 41–60.
Stromgaard P 1984 The immediate effect of burning and ash-fertilization. Plant Soil 80, 307–320.
Tiessen H and Moir J 1993 Characterization of available P by se-quential extraction.In Soil Sampling and Methods of Analysis.Ed. MR Carter. pp 75–86. Canadian Society of Soil Science,Lewis Publishers.
Tinker P, Ingram J and Struwe S 1996 Effects of slash-and-burnagriculture and deforestation on climate change. Agric. Ecosyst.Environ. 58, 13–22.
Tomkins I B, Kellas J D, Tolhurst K G and Oswin D A 1991 Effectsof fire intensity on soil chemistry in a eucalypt forest. Aust. J.Soil Res. 29, 25–47.
Van Reuler H and Janssen B 1993 Nutrient fluxes in the shiftingcultivation system of south-west Cote d’lvoire. Plant Soil 154,169–177.
Vitousek P and Matson P 1984 Mechanisms of nitrogen retention inforest ecosystems: a field experiment. Science 225, 51–52.
Vitousek P and Matson P 1985 Disturbance, nitrogen availabil-ity and nitrogen losses in an intensively managed loblolly pineplantation. Ecology 66, 1360–1376.
Walker J, Raison R J and Khanna P K 1986 Fire.In AustralianSoils: the Human Impact. Eds J Russell and R Isbell. pp 185–216. University of Queensland Press, Brisbane, Queensland,Australia.
Zinke P, Sabhasri S and Kunstadter P 1978 Soil fertility aspects ofthe Lua’ forest fallow system of shifting cultivation.In Farmersin the Forest. Eds P Kunstadter, EC Chapman and S Sabhasri. pp134–159. The University Press of Hawaii, Honolulu, USA.
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