Carbon mineralization of leaves from four Ethiopian agroforestry species under laboratory and field...

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Applied Soil Ecology 35 (2007) 193–202

Carbon mineralization of leaves from four Ethiopian agroforestry

species under laboratory and field conditions

Tesfay Teklay a,*, Anders Nordgren b, Gert Nyberg b, Anders Malmer b

a Wondo Genet College of Forestry, Debub University, P.O. Box 128, Shashemene, Ethiopiab Department of Forest Ecology, Swedish University of Agricultural Sciences, SE-901 83 Umea, Sweden

Received 23 September 2005; received in revised form 23 March 2006; accepted 17 April 2006

Abstract

Green leaves of Albizia gummifera G.F. Gmel, Milletia ferruginea (Hochst.) Baker, Cordia africana Lam., and Croton

macrostachyus Del. were collected from trees growing in fields in southern Ethiopia, and used in laboratory and field experiments.

The aim was to investigate differences in C mineralization parameters related to differences in the leaf qualities of the respective

species, and to examine effects of amending soil (Mollic Andosols) with leaves plus N, P, or N + P on decomposition and microbial

activity. Rates of carbon mineralization were determined by measuring CO2 evolution using an automated respirometer (Respicond

V) in the laboratory and an infrared gas analyser in the field studies. When no nutrients were added about 11–44% and 10–42%, on

average, of the initial C applied as leaves was mineralized within a month in the laboratory and field respiration experiments,

respectively. In both experiments, the rates of C mineralization were highest for C. macrostachyus leaves, followed by M. ferruginea

then A. gummifera and lowest for C. africana leaves. Hence the results of our short-term laboratory study agreed well with those of

the field experiment. Microbial activity (e.g. specific growth rate) was generally stimulated by supplemental nutrients. However, in

most cases cumulative C mineralization was either slightly depressed or not significantly affected by supplemental N or N + P.

Similarly, P addition caused either a reduction in C mineralization or had little effect. Hence the quality of the leaves was more

influential than the nutrient additions. The absence of a pronounced respiratory response to the added N and/or P might be due to

increased microbial C-use efficiency, to adequate amounts of these nutrients being available from the leaves and/or soil, or both.

Further in-depth studies using soils of differing soil fertility are needed to test these hypotheses.

# 2006 Elsevier B.V. All rights reserved.

Keywords: CO2 respiration; Ethiopia; Indigenous trees; Leaf substrates; Microbial activity; Supplemental nutrients

1. Introduction

In agroforestry systems, organic materials enter the

soil system via green manure, litter fall, root detritus

and exudates that are subsequently decomposed by

heterotrophic micro-organisms to obtain C and nutrients

for growth and maintenance. The decomposition of

plant materials in soil is affected by biotic and abiotic

* Corresponding author. Tel.: +251 46 1109900;

fax: +251 46 1109983.

E-mail address: [email protected] (T. Teklay).

0929-1393/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsoil.2006.04.002

factors, one of the most important of which is their

biochemical quality, i.e. litter-quality (Swift et al.,

1979). The initial concentrations of soluble C and N,

lignin and polyphenols are generally recognised as the

main litter-quality variables controlling rates of

decomposition (Palm, 1995; Heal et al., 1997).

However, the importance of any one (or any combina-

tion) of these constituents may vary depending on the

process, timeframe, or type of plant material involved

(Palm, 1995; Palm and Rowland, 1997).

Measurement of weight loss, often using litterbags,

is the most extensively used method to assess rates of

litter decomposition, especially in long-term decom-

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http://dx.doi.org/10.1016/j.apsoil.2006.04.002

T. Teklay et al. / Applied Soil Ecology 35 (2007) 193–202194

position studies in the field (e.g. Berg et al., 1984;

McClaugherty and Berg, 1987; Couteaux et al., 1995).

Another approach is based on measuring the CO2

evolved as a product of decomposition to estimate

microbial activity and amounts of litter decomposed

(Marstorp, 1997). From the respiration kinetics

obtained following addition of an organic material,

parameters such as substrate-induced respiration (SIR),

lag-phase lengths and specific microbial growth rates

(m) can be calculated (Nordgren et al., 1988; Stenstrom

et al., 1991; Marstorp, 1996a). Since such parameters

differ considerably between litter components, they can

be used to characterise litter undergoing decomposition

(Marstorp, 1997). Respiration of added C, as green

manure, also correlates well with initial N mineraliza-

tion (Nyberg et al., 2002) and to quality parameters of

the added material, i.e. C/N ratio and lignin/N ratio and

initial N content of the leaves (Mafongoya et al., 2000).

Respiration-based approaches using plant materials

have been applied in a number of studies (e.g. Beare

et al., 1991; Marstorp, 1996a,b; McTiernan et al., 1997;

Schomberg and Steiner, 1997; Bernhard-Reversat,

1998; Sall et al., 2003). There have also been a limited

number of similar studies from Africa that have focused

on plant materials derived from agroforestry species

(e.g. Okeke and Omaliko, 1992; Nyberg et al., 2000,

2002). However, few studies with fine temporal

resolution have concentrated on the short-term

dynamics of microbial activity during the initial phase

of decomposition (e.g. Nyberg et al., 2000, 2002).

The availability of nutrients from the native soil pool

or added inorganic nutrients is presumed to affect

microbial activity and decomposition parameters

(Hobbie and Vitousek, 2000; McGroddy et al., 2004),

especially during the initial phase of decomposition

when nutrient immobilization due to microbial uptake is

considered to be high. Several studies have found

increases in decomposition and microbial activity after

addition of N (Kelly and Henderson, 1978; Goyal et al.,

1999) or P (Cleveland et al., 2002). In contrast, others

have found either decrease in these variables after

addition of N (Soderstrom et al., 1983; Fisk and Fahey,

2001) and P (Kelly and Henderson, 1978) or no effect of

external N/P inputs (Prescott, 1995; Sall et al., 2003;

McGroddy et al., 2004). However, comparison of the

above studies is problematic due to differences in the

sites, soils, climate, plant species, substrate qualities

and nutrients (types and amounts) considered, methods

used and temporal resolution of the measurements.

In this paper, we present the findings of laboratory

and field studies on the short-term decomposition

dynamics of plant materials of varying qualities using

high-resolution CO2 measurement approaches. Green

leaves were used for this purpose from four agroforestry

tree species commonly found in fields in southern

Ethiopia: Albizia gummifera G.F. Gmel, Milletia

ferruginea (Hochst.) Baker, Cordia africana Lam.,

and Croton macrostachyus Del. The potential value of

green manure derived from these species has been

investigated in previous studies using litterbags

(Gindaba et al., 2004; Teklay and Malmer, 2004).

The specific aims of the present study were to

investigate the differences in C mineralization rates

of leaves from the four species, representing different

leaf qualities, under laboratory and field conditions, and

to examine the effect of amending the soil (Mollic

Andosols) with leaves plus N, P, or N + P on

decomposition and microbial activity parameters.

2. Materials and methods

2.1. Description of site and soil

The study site is at Wondo Genet, which is located in

southern Ethiopia (780601400N and 3883701800E, 1800–

2100 m above sea level) and has a sub-humid tropical

climate with mean annual rainfall of 1240 mm and mean

monthly temperature of 20 8C. The major soils in the

study site are Mollic Andosols (FAO, 1988) formed from

late tertiary volcanic parent materials (Anon., 1973).

Andosols are important in localised areas in East Africa

(Buresh et al., 1997) and are common and agriculturally

important in the rift valley region of Ethiopia and some

highlands bordering it. They are generally considered to

be P-fixing, but some soils which have received volcanic

ash in recent geological history contain high levels of

available P and thus are relatively fertile (reviews by

Buresh et al., 1997). The soil in the study site had the

following properties in the top 0–20 cm depth: a pH

(H2O) of 6; organic C of 30 g kg�1; total N of

3.1 mg g�1; Bray-II extractable, plant available P of

12 mg kg�1; total P of 515 mg kg�1; oxalate extractable

Al of 1.5 and Fe of 3.3 mg g�1; CEC of 24 cmolc kg�1;

bulk density of 1.01 g cm�3; and a loamy texture with

46% sand, 32% silt, and 22% clay. Details of the soil

chemical analyses can be found in Teklay and Malmer

(2004) and Teklay et al. (2005). For the laboratory

experiment (Experiment 1), field-moist soils were sieved

in the field at Wondo Genet, put in ice-boxes and stored

frozen (�18 8C) until analysis. The soils were thawed

overnight and adjusted to a moisture tension of�25 kPa,

approximating to 50% of the water holding capacity,

which is considered optimal for microbial activity (Ilstedt

et al., 2000).

T. Teklay et al. / Applied Soil Ecology 35 (2007) 193–202 195

Fig. 1. Daily rainfall and temperature (maximum and minimum) at

Wondo Genet during September to October 2004. The horizontal bar

shows the time-span of the short-term field experiment.

About 20 g of soil was weighed into each of 250 ml

plastic jars and incubated in an automatic respirometer

(Respicond V, Nordgren Innovations AB, Sweden)

(Nordgren, 1988). Measurements of CO2 evolution

were taken hourly at a constant temperature of 20 8C for

10–15 days before supplemental nutrient additions (see

below) and for 30 days thereafter. The laboratory study

(Experiment 1) was done at Umea, Sweden, while the

field study (Experiment 2) was done at Wondo Genet,

Ethiopia. Temperature and rainfall were monitored

daily during the short-term field study (Fig. 1).

2.2. Plant materials used

Four agroforestry species (Albizia gummifera, Mill-

etia ferruginea, Cordia africana, and Croton macro-

stachyus) were selected for the present study because of

their abundance in fields and along farm-boundaries in

the area, the diversity in their chemical composition and

their potential as sources of carbon and nutrients. A.

gummifera and M. ferruginea are leguminous while C.

africana and C. macrostachyus are non-legumes. Fresh

Table 1

Initial leaf chemical composition (in mg g�1 DM) of leaves from four agro

Species Sol. C Ct Nt P

A. gummifera 124.5 (4.0) 492.8 (0.7) 34.0 (0.4) 1.9

M. ferruginea 136.3 (29.8) 464.7 (2.7) 35.5 (0.9) 1.6

C. africana 132.0 (9.2) 451.0 (1.2) 21.1 (0.1) 2.9

C. macrostachyus 112.2 (0.9) 425 (2.6) 35.6 (0.3) 5.0

Sol. C: water extractable C; Ct: total C; Nt: total N; P: total phosphorus; P

leaves including petioles, and the rachis of A.

gummifera and M. ferruginea, were collected from

mature trees of each species. Leaves from various stages

of pre-abscission development were included. Fresh

intact leaves and air-dried leaves were used in the field

and laboratory experiments, respectively. To determine

their chemical composition, fresh leaves were first air-

dried, oven-dried at 50 8C, and then ground to<0.1 mm

size. Water-soluble C was determined, after extracting

0.2 g samples in 100 ml deionised water (Thomas,

1977), by analysing the filtrate in a TOC-5000 solution

analyser (Shimadzu Inc., Columbia, MD); total C and N

were determined using a CN analyser (Europa

Scientific, ANCA-NT Systems); and total P was

determined colorimetrically using the ammonium

acetate–molybdate ascorbic acid method (Murphy

and Riley, 1962). Cellulose and lignin were determined

by the acid-detergent-fibre (ADF) method (Van Soest,

1963), and soluble polyphenols were extracted using

70% acetone and determined gravimetrically by the

ytterbium precipitation method (Reed et al., 1985).

Chemical properties of leaves of the four species

considered are presented in Table 1.

2.3. Experiment 1: adding green leaf C to soils

amended with N, P, or N + P

We used 0.5 g of dry leaf powders of each of the four

species (per 20 g of soil) as sources of C in combination

with 7.0 mg (NH4)2SO4-N or 1.4 mg KH2PO4-P or a

mixture of the two to obtain overall ratios of C/N < 20

and C/P < 200. The amount of leaf material, which

corresponded to 15 Mg dw ha�1 of biomass application

in the field, was meant to provide sufficient amount of C

for microbial respiration (see Fig. 2). Soils that had not

been amended with nutrients (only leaves added)

provided the controls. Nutrients were added in the

form of solutions containing enough water to suffi-

ciently moisten the dry leaf powders added. The

experimental design was completely randomised with a

factorial treatment structure and three replicates, i.e.

four species � four nutrient treatments � three repli-

forestry species (numbers in parenthesis show SD; n = 3)

Cellulose Lignin PL C:N ratio

(0.1) 177.6 (8.3) 264.6 (3.8) 124.7 (4.8) 14.5 (0.2)

(0.1) 215.2 (1.1) 164.7 (0.9) 63.6 (0.3) 13.1 (0.3)

(0.2) 241.7 (5.1) 199 (4.2) 86.3 (2.9) 21.3 (0.1)

(1.0) 245.0 (13.2) 61.7 (3.5) 74.0 (5.3) 11.9 (0.1)

L: soluble polyphenols.


Fig. 2. A model of soil respiration kinetics before and after addition of

a C-substrate. The arrow indicates the time when the substrate was

added. BR, SIR, L, and m are as defined in Section 2.5; Rmax:

maximum respiration. The ordinate is assumed to be on a logarithmic

scale (adapted from Nordgren et al., 1988).

cates. After respiration rates had stabilized (about 10

days after the start of incubation) to a basal level, i.e.

CO2 from native soil C, the supplemental nutrients were

added to the soil, thoroughly mixed by gently shaking

the jars and the measurements were restarted.

2.4. Experiment 2—field study using green leaves

The field experiment, on a farm that had been

previously cropped with maize (Zea mays L.), had a

completely randomised design with four replications.

The experimental plots (replicates) were 1.0 m2 in area.

The treatments were additions of fresh leaves of the four

species and a control (no leaf inputs). All plots were

ploughed, cleaned of weeds and prepared a week before

addition of the treatments. Fresh leaves of each of the

four species were incorporated into the top 10 cm of the

1.0 m2 treatment plots at 5.0 Mg dw ha�1, an amount

considered realistic for field application. The control

plots were also ploughed to 10 cm depth in order to

account for the ‘ploughing effect’ which the treated

plots were subjected to while incorporating the leaves.

Then the CO2 evolution rate (g CO2 m�2 h�1) was

measured using an EGM-4 infrared gas analyser

connected to an SRC-1 closed system chamber (PP

Systems, 2002), which had an internal diameter of

10 cm. Five measurements per day were taken from

each plot, starting 1 day before applying the treatments

to measure basal respiration, and continued for the

following six consecutive days. After the sixth day,

measurements were taken at days 26 and 31. The field

study was conducted during September and October of

2004 (see Fig. 1).

2.5. Data analysis

For both experiments, we calculated cumulative

CO2-C mineralization from rates of CO2 evolution

obtained after deducting basal respiration or respiration

from the controls. For the laboratory experiment, basal

respiration was calculated as the average of 40 hourly

measurements before substrate addition when respira-

tion was considered to have stabilized (Ilstedt et al.,

2003). Substrate-induced respiration was the stable

initial respiration shortly after substrate addition

(Nordgren et al., 1988). Specific growth rate constant

(m) was determined from the exponential part of the

respiration curve using the relationship for growth-

associated product formation by exponentially growing

micro-organisms presented by Stenstrom et al. (1991),

i.e.

d p

dt¼ qN0emt (1)

where dp/dt is the rate of microbial product ( p) forma-

tion (the rate of CO2 evolution in this case), q the

specific activity, N0 the initial concentration of

micro-organisms; and t time. Then, m was calculated

as the slope of the regression of the logarithmically

transformed respiration rate measurements during the

exponential phase against time, i.e. ln(dp/dt) (Marstorp,

1996a; Stenstrom et al., 1991) (see Fig. 2). Estimates of

m from the equation were not affected by whether or not

the values for q and N0 were known. Lag time (L) was

defined as the time between substrate addition and the

start of the resulting exponential increase of the respira-

tion rate (Nordgren et al., 1988) (see Fig. 2). Analysis of

variance was done using SPSS, version 12 (SPSS Inc.,

Chicago, IL) and the significance of differences

between treatments was evaluated with Least Signifi-

cant Difference tests. Spearman’s rank correlation ana-

lysis was performed to examine relationships between

litter-quality variables and microbial respiration vari-

ables. Unless stated otherwise, differences were

regarded as being significant if the probability level

( p) was less than 0.05.

3. Results

3.1. Between-species differences in C

mineralization under laboratory conditions

Temporal patterns of CO2 evolution from decom-

posing leaves of the four tree species after amendment

with different nutrients are shown in Fig. 3. The amount

of leaf C respired (g CO2-C kg�1 of added leaf C)


Fig. 3. Temporal pattern of CO2 evolution from the decomposition of green leaves of (~) C. macrostachyus; (*) M. ferruginea; (*) A. gummifera;

and (~) C. africana under laboratory conditions in soils from Wondo Genet (n = 3; bars represent standard error of difference between means or

SED) (NB: part of the curves beyond 120 h not shown).

during the 1-month period varied considerably and

significantly ( p < 0.05) among the tree species

(Table 2). Without supplemental nutrients, about 11–

44% of the C contained in the applied leaves of the tree

species was mineralized to CO2, the proportions being

highest for C. macrostachyus leaves, followed by M.

Table 2

Mean values* of total C respired as percentage of initial green leaves C adde

week laboratory incubation (n = 3)

Nutrients added Tree species

A. gummifera C. african

None 20.1a 11.0a

N 18.4c 11.0a

P 20.5a 10.1b

N + P 19.4b 11.2a

SED 0.22 0.23

* Values within a column with the same superscripts are not statistically d

from one another, across all the nutrient treatments).

ferruginea then A. gummifera and lowest for C. africana

leaves (Table 2).

Analysis of the effects of the plant substrates on

microbial growth (activity) parameters, when no

nutrients were added, showed that M. ferruginea and

C. macrostachyus induced faster microbial growth rates

d to Mollic Andosol soil amended with/without external nutrients in 4-

a C. macrostachyus M. ferruginea

43.7a 26.8a

37.2c 28.7a

41.5b 28.2a

37.2c 24.3b

0.51 1.00

ifferent ( p > 0.05); (NB: all four species were significantly different


Table 3

Mean values of specific microbial growth rate obtained after adding green leaves from four agroforestry tree species to Mollic Andosol soils, with/

without mineral fertilizers in a laboratory incubation (n = 3)a

Nutrient added Species m (h�1) Species Nutrient added m (h�1)

0 (None) A. gummifera 0.123c A. gummifera 0 0.123c

C. africana 0.071d +N 0.144b

C.macrostachyus 0.155b +P 0.151b

M. ferruginea 0.168a +NP 0.176a

SED 0.005 SED 0.006

+N A. gummifera 0.144b C. africana 0 0.071c

C. africana 0.089c +N 0.089b

C.macrostachyus 0.156b +P 0.085bc


SED 0.006 SED 0.005

+P A. gummifera 0.151c C.macrostachyus 0 0.155a

C. africana 0.085d +N 0.156a

C.macrostachyus 0.161b +P 0.161a


SED 0.004 SED 0.004

+NP A. gummifera 0.176b M. ferruginea 0 0.168b

C. africana 0.114d +N 0.184ab

C.macrostachyus 0.161c +P 0.193a


SED 0.004 SED 0.007

a For each nutrient or species treatment, values within a column with same letter superscripts are not statistically different ( p > 0.05); columns 3

and 6 show statistical comparisons among species and among nutrients, respectively.

Table 4

Spearman’s rank correlation coefficients (r) between initial leaf

chemistry variables and microbial respiration parameters after adding

green leaves to Mollic Andosols in a 4-week laboratory incubation

based on hourly measurements of CO2

Leaf quality variables Microbial respiration variables

Ct respireda m

Sol. C �0.581 (0.047) �0.161 (0.617)

Total C �0.389 (0.212) 0.014 (0.966)

N 0.858 (<0.001) 0.774 (0.003)

P 0.193 (0.549) �0.382 (0.221)

Cellulose 0.130 (0.688) �0.154 (0.632)

Lignin �0.753 (0.005) �0.571 (0.053)

Polyphenols �0.592 (0.043) �0.760 (0.004)

C/N ratio �0.949 (<0.001) �0.732 (0.007)

Lignin/N ratio �0.914 (<0.001) �0.732 (0.007)

PL/N ratio �0.760 (0.004) �0.928 (<0.001)

(LG + PL)/N ratio �0.942 (<0.001) �0.739 (0.006)

Values within parentheses show probability levels (n = 12).a Total respired C as percentage of initially applied green leaf C.

(m) than A. gummifera and C. africana leaves

(Table 3). Similar to the cumulative C respiration,

the maximum respiration rate (Rmax) was highest for

C. macrostachyus and lowest for C. africana (Fig. 3).

In addition, the cumulative C respired and specific

growth rate (m) were positively correlated with total N

and negatively correlated with the lignin and poly-

phenol contents, and the C/N, lignin/N, polyphenol/N

and [lignin + polyphenol]/N ratios of the leaf sub-

strates (Table 4).

3.2. Effects of N and P amendments on C

mineralization under laboratory conditions

Nutrient input and the type of nutrient had significant

effects on C mineralization (Table 2), but there were

also significant interactions between leaves and added

nutrients. In comparison to the control (i.e. no nutrients

added), supplemental N or N + P mostly depressed the

cumulative C respired. Similarly, supplemental P either

depressed the cumulative percent C respired or had little

effect. Overall, nutrient effects were not as pronounced

(Table 2, Fig. 3). However, the specific growth rate (m)

was enhanced by the addition of any of the nutrients (or

nutrient combinations) for leaves of all the species

except C. macrostachyus (Table 3).

3.3. Field measurements of C mineralization from

applied fresh leaves

The total C respired (g m�2), due to leaves added,

during the 1-month period in the field study differed

significantly among the plant species (Fig. 4); an


Fig. 4. Time course of CO2 evolution from the decomposition of fresh

green leaves from four agroforestry trees under field conditions at

Wondo Genet (n = 4; bars represent SED).

amount of C equivalent to 42, 17, 15 and 10% of the

total C initially applied as leaves from C. macro-

stachyus, M. ferruginea, A. gummifera and C. africana,

respectively, was mineralized to CO2 (Fig. 5b) (NB:

respiration between days 7 and 26 was interpolated

using first order decomposition kinetics). Addition of C.

macrostachyus leaves differed significantly from all

other treatments, and adding leaves of all four species

differed from the control with respect to the cumulative

amounts of CO2-C (g m�2) respired.

The maximum respiration rates (Rmax), which

followed a similar pattern, occurred 3 days after

incorporating fresh leaves in the soil, with mean

values ranging from 0.71 g CO2-C m2 h�1 (control) to

2.0 g CO2-C m2 h�1 (C. macrostachyus) (Fig. 4). On

the 26th day of measurement and afterwards, the

respiration subsided to values close to the basal level

Fig. 5. Cumulative C respired during 1 month as a percentage of total C i

ferruginea; (*) A. gummifera; and (~) C. africana under: (a) laboratory an

(i.e. 0.12 g CO2-C m2 h�1). A downward shift in

respiration was also observed during the second day,

which we attributed to heavy rainfall during the

preceding evening/night.

4. Discussion

4.1. Leaf quality and supplemental nutrient effects

The results from our short-term (1-month) laboratory

and field studies clearly indicated that the C losses from

the leaves of the species examined differed markedly,

ranging between 10 and 44%. The chemical quality of

the leaves was the major factor controlling C losses,

while supplemental nutrients were less influential.

Strong correlations were also found in our study

between litter-quality and respiration variables, which is

in accordance with findings of various other investiga-

tions in which litter-quality has been related to

decomposition and nutrient release in the tropics (Palm,

1995; Mafongoya et al., 2000; Seneviratne, 2000;

Nyberg et al., 2002; Teklay and Malmer, 2004). In the

present study, the C/N ratio, total N, and measures of

carbon quality, particularly polyphenol and lignin

contents, and their ratios with N, seem to be the quality

factors with the strongest influence on total C respired

and microbial growth rates (m) (Table 2). This could

explain why adding C. macrostachyus leaves, which

also had the lowest C/N ratio of the four species,

resulted in the highest %C respired values. On the other

hand, relatively high contents of polyphenols and lignin

in the two legumes (despite their high N contents), and a

combination of low nitrogen content and high lignin and

polyphenol levels in the leaves of C. africana might

explain the observed results. However, we should note

nitially applied in green leaves from (~) C. macrostachyus; (*) M.

d (b) field conditions (n = 3 and 4, respectively; bars represent SED).


here that although the polyphenol measurements we

obtained using the method described by Reed et al.

(1985) were good enough for comparison within the

present study, they may not be directly comparable with

measurements from studies employing other commonly

used methods for analysing polyphenols (e.g. Anderson

and Ingram, 1993).

Overall, in all the species except C. africana, an

amount of C close to or even more than the total soluble

C was respired during the 1-month period, suggesting at

least in the case of C. macrostachyus, that leaf

constituents other than water-soluble compounds, e.g.

cellulose, started to decompose. The detection of more

than one respiration peak following the addition of

leaves of all species except A. gummifera (Fig. 3),

possibly due to decomposition of different C compo-

nents (substrates) of the leaves, provides a further

indication of this possibility, since the m and lag-phases

associated with each component would differ (Mar-

storp, 1996b), resulting in a respiration pattern with

multiple peaks. The different peaks observed in our

study might also represent what Marstorp (1996b)

described as ‘kinetically defined fractions’, but we

could not test this hypothesis since our study was not

designed to include water-extracted leaves.

There could be several reasons for the apparent

absence of nutrient effects on cumulative C respired,

despite the enhanced microbial activity (as indicated by

the increased m) observed in our study. Firstly, a strong

possibility supported by a number of studies, is that the

reduction in respiration could be attributed to C

utilisation tending to be more efficient, due to C being

redirected from waste respiration to microbial growth,

when supplemental nutrients (N and/or P) are added

(Dilly, 1999; Thirukkumaran and Parkinson, 2002;

Schimel and Weintraub, 2003). Using a modelling

approach, Schimel and Weintraub (2003), for instance,

showed that adding a pulse of C to an N-limited system

could increase respiration, while adding N actually

decreased it. They argued that the lack of a respiratory

response by soil microbes to added N does not

necessarily imply that they are not N-limited. We have

also seen a similar phenomenon following the addition

of glucose-C, where total respiration was lower when

both N and P were added than when either one was

added separately (Teklay et al., 2005).

Secondly, sufficient nutrients to meet microbial

needs could already have been present in the leaves and/

or soil, especially since the amount of available P in the

soil we used was relatively high (i.e. 12 mg kg�1).

Although inorganic N contents were not analyzed here,

a recent study on nutrient limitation (Teklay et al., 2005)

found that N was more limiting to micro-biota than P in

the study soil.

Thirdly, increases in pH (e.g. 0.6–1.2 observed in the

soils amended with only leaves, i.e. controls) possibly

due to the consumption of H+ ions during the

decomposition of amino acids, or the release of base

cations from the leaves, might have enhanced nutrient

availability. Increase in pH can also occur as decom-

posing residue or the organic anions released form

complexes with inorganic Al, thereby decreasing Al

solubility in soil solution (Wong et al., 1995).

In some studies (e.g. Fenn and Kirk, 1981; Berg and

McClaugherty, 2003), reductions in decomposition

rates (respiration) have also been attributed to the

repression of lignin degradation in the presence of high

initial amounts of N, but the duration of our study might

have been too short for this phenomenon to be of any

significance. Other studies (Prescott, 1995; Hobbie and

Vitousek, 2000) have put forth the possibility that more

N may be immobilised per unit of CO2 evolved due to

‘luxury uptake’ by micro-organisms when N is

abundant. Generally, while most of the aforementioned

studies and a number of investigations reviewed by Fog

(1988) have observed reductions in rates of decom-

position or respiration due to nutrient (especially N)

addition, several others have observed increases (e.g.

Adedeji, 1986; Priess and Folster, 2001; Allen and

Schlesinger, 2004; Conde et al., 2005).

4.2. Field respiration study and conformity with the

laboratory study

The C mineralization values from our field and

laboratory experiments generally showed similar trends

and were in good agreement with each other, although

the laboratory conditions were more tightly controlled

and the soil was frozen prior to its use. The only marked

differences were that decomposition of A. gummifera

and M. ferruginea leaves seemed to be slower in the

field study and that M. ferruginea leaves decomposed

significantly more than A. gummifera leaves in the

laboratory study (Fig. 5a and b). These differences

might have been due to our use of ground leaves (with

more surface area) and intact leaves in the laboratory

and field experiments, respectively. While drying green

leaves might reduce decomposition rates, grinding the

leaves might increase their surface area and promote

microbial activity (Oglesby and Fownes, 1992). In

general, the 10–42% average C losses we found in the

field study are lower than those reported by Nyberg et al.

(2002) who used a d13C natural abundance technique

and estimated C losses of 70–90% for Sesbania sesban.


However, our results are comparable to the 30–40%

they obtained for Grevillea robusta green manures in a

40-day field incubation. Nevertheless, the figures in our

study (obtained by deducting control values) should be

treated with caution as they might underestimate the

contribution from the decomposing leaves to the hourly

CO2 respiration measurements. The contribution from

‘endogenous C’, after adding green leaves, could be

lower than the values recorded in the control plots

because the decomposers could have shifted from using

the relatively poor quality endogenous C to the higher

quality added green leaves as a C source. A similar

explanation was forwarded by Fontaine et al. (2003),

who postulated that a ‘priming effect’ occurs due to

competition for energy and nutrients between micro-

organisms specialized in the decomposition of fresh

organic matter and those feeding on polymerised soil

organic matter.

5. Conclusions

The C mineralization results obtained from our

short-term laboratory study agreed well with those from

the field experiment. Our study indicated that, for the

soil and site we examined, the quality of the leaves had a

much stronger influence on the decomposition para-

meters than supplemental nutrients, although the

latter stimulated microbial activity. The absence of a

pronounced respiratory response to added N and/or P

might be due to adequate amounts of these nutrients

being available from the leaves and/or soil, or to

increased C-use efficiency, or both. Further in-depth

study using soils of contrasting soil fertility are needed

to test these hypotheses.

Acknowledgements

This study was funded by Swedish International

Development Agency (Sida) through a long-term

institutional cooperation between Wondo Genet Col-

lege of Forestry (WGCF) and the Swedish University of

Agricultural Sciences (SLU). We thank WGCF for

providing the first author access to the required

facilities, Dr. J. Blackwell for editing the language

and the anonymous reviewers for their helpful com-

ments.

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