a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 1 6 4 – 1 7 5

Synchronizing nitrogen availability through application oforganic inputs of varying resource quality in a tropicaldryland agroecosystem

Sonu Singh *, Nandita Ghoshal, K.P. Singh

Center of Advanced Study, Department of Botany, Banaras Hindu University, Varanasi 221005, India

a r t i c l e i n f o

Article history:

Received 20 June 2006

Received in revised form

19 December 2006

Accepted 30 January 2007

Keywords:

Dryland agroecosystem

N-mineralization rate

Available-N

Soil microbial biomass N

Mixed input

a b s t r a c t

A 2-year field experiment was conducted to evaluate the impact of management practices

involving manipulation of quantity and quality of exogenous inputs on soil N-mineraliza-

tion rate, N availability and microbial biomass in a rice–barley rotation in a tropical dryland

agroecosystem. At the beginning of each annual cycle an equivalent amount of N was added

through chemical fertilizer and three organic inputs: Sesbania shoot (high quality resource,

C/N 16, lignin/N 3.2, polyphenol + lignin/N 4.2), wheat straw (low quality resource, C/N 82,

lignin/N 34.8, polyphenol + lignin/N 36.8) and Sesbania + wheat straw (high and low quality

resources mixed). The N-mineralization rate was dominated by ammonification in this

dryland agroecosystem. N-mineralization exhibited a distinct seasonal pattern, decreasing

from the rice period through the summer fallow period, except in Sesbania + wheat straw

and wheat straw treatments which showed a slight increase during the early stages of barley

period. The rate of N-mineralization showed a significant relationship with soil moisture

and microbial biomass N. During the rice period, N-mineralization rate and available-N was

highest in the fertilizer treatment followed by Sesbania > Sesbania + wheat straw > wheat

straw. During the barley period, highest N-mineralization rate and available-N was observed

in Sesbania + wheat straw followed by wheat straw > Sesbania > fertilizer. Adding Sesba-

nia + wheat straw resulted in consistently higher levels of microbial biomass N, N-miner-

alization rate and available-N through the annual cycle compared to single application of

Sesbania and wheat straw, indicating synergy between the two inputs, favoring more

efficient utilization of N. It is suggested that mixed application of high and low quality

resources can modulate N release, resulting in relatively higher synchronization which can

help in minimizing N loss from agroecosystem.

# 2007 Published by Elsevier B.V.

avai lable at www.sc iencedi rec t .com

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1. Introduction

Nitrogen (N) is one of the major nutrient required by crops in an

agroecosystem. Large reserves of N are present in soil organic

matter, but its availability to plants is influenced by several

competing processes which include mineralization, immobili-

zation (by microorganisms and/or plants), nitrification and

* Corresponding author. Tel.: +91 9415811843.E-mail address: sonutg@gmail.com (S. Singh).

0929-1393/$ – see front matter # 2007 Published by Elsevier B.V.doi:10.1016/j.apsoil.2007.01.007

denitrification. Only few studies have examined the kinetics of

gross N-mineralization, immobilization, and nitrification rates

in soil at temperatures above 15 8C (i.e., under tropical

conditions) (Hoyle et al., 2006). Agroecosystems in general

and tropical dryland agroecosystem in particular invariably

require replenishment of N through exogenous N sources.

Chemical fertilizers are most widely used as supplemental N.

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 1 6 4 – 1 7 5 165

However, questions have been raised about the long-term

sustainability of such systems because rate of release of N in

soils often does not match crop demand with fertilizer

applications (Robertson et al., 2000; Campbell et al., 1995;

Groffman et al., 1987). Moreover, highly concentrated inorganic

N inputs can have detrimental environmental impacts (Ven-

terea and Rolston, 2000; Matson et al., 1997). As an alternative N

source, use of organic materials (crop residue, compost or green

manures) has been advocated (Tilman, 1998). Organic amend-

ments are considered to have the potential for conservation of

soil moisture, one of the major constraints to crop productivity

in dryland, besides providing soil nutrients.

Organic resources like crop residues offer sustainable and

ecologically sound alternatives for meeting the N requirement

of crops. Moreover effective management of postharvest crop

residues remains an important issue in many grain-producing

regions of the world (Beare et al., 2002). There is need to

develop and adopt economical and efficient N management

strategies that conserve natural resources while minimizing

adverse environmental impacts. Many of the present manage-

ment practices do not meet these criteria (Giller et al., 2004).

The suitability of crop residues as a source of N depends on the

mineralization of N in synchrony with crop demand. Thus, to

use crop residues efficiently as a source of N we need to predict

more accurately their rates of N release. Further, we need to

know more accurately how much N flows through agroeco-

systems and how much N is susceptible for loss (Janzen et al.,

2003).

An important question pertinent to the efficient use of

residues is whether their rates of nutrient release can be

effectively managed to coincide with crop demand (Tilman

et al., 2002). If rates of N release exceed plant demands, then

the N becomes susceptible to various pathways of loss

(Goulding, 2004; Peoples et al., 2004). In some instances,

organic amendments can cause excess accumulation of NO3-

N in soil (Khalil et al., 2005) with potentially detrimental effects

on the environment. Conversely, if rates of N release are too

slow, then crop yields may be constrained.

The rate of N-mineralization, which regulates the avail-

ability of N in soil is governed by microbial biomass (Hadas

et al., 2004). Tillage and residue (straw) management have

been shown to affect microbial biomass, N-mineralization rate

and available-N in rice–barley based dryland agroecosystems

(Kushwaha and Singh, 2005). The environmental hazards

associated with N in agroecosystems stem from various

specific microbial N transformations in soils as well as the

behaviour of mineral N in relation to soil physico-chemical

properties (Peoples et al., 1995). Hence study of relations

between N availability and microbial biomass is of great

importance. This study, therefore, was conducted to examine

the effect of resource quality on N-mineralization rate, N

availability, N synchrony and soil microbial biomass N in a

field trial under dry tropical conditions. The aim was to

increase the temporal resolution of N dynamics. Detailed

information on the temporal resolution of mineralization is

important to achieve synchrony between nutrient release

from added materials and the nutrient requirements of the

crop (Myers et al., 1994). This field study examined three

diverse organic inputs (high quality Sesbania shoot, low quality

wheat straw, and a combination of these (Sesbania + wheat

straw) and fertilizer, applied at equivalent rates of N (80 kg N

ha�1). The objectives of the present study were to evaluate the

effect of resource qualities through the annual cycle on: (i) the

N-mineralization rate (ammonification + nitrification) and

microbial biomass N, and (ii) the seasonal availability of

NH4-N and NO3-N in soil.

2. Materials and methods

2.1. Study site

The experiments were conducted for two annual cycles (2002–

2003, 2003–2004) in the cultivated plots of the Botanical Garden

of the Department of Botany, Banaras Hindu University at

Varanasi (258180N lat. and 83810E long., 76 m above mean sea

level). This region has a dry tropical climate, characterized by

strong seasonal variations in temperature and precipitation,

including a warm rainy season (July–September), a cool winter

(November–February), and a hot summer (April–June). The

average annual rainfall is about 1100 mm, of which about 80%

is received during the rainy season. High temperature

(24–34 8C) and high relative humidity (70–80%) prevail during

the rainy season. In the winter season the temperature range

is 4–25 8C. The summer is dry and hot with a temperature

range of 30–48 8C. Crop production occurs in two seasons: the

rainy season and the winter season, and fields are left fallow

during the summer. The soil of the experimental site (order

Inceptisol, suborder Orchrepts, subgroup Udic Ustocrepts) has

pale brown colour and sandy loam texture.

2.2. Experimental design

The crop sequence studied was rice (Oryza sativa var. NDR 97)-

barley (Hordeum vulgare var. Lakhan)-summer fallow. Rice was

grown from day 0 (day of sowing of first rice crop) to day 115 in

the first annual cycle and from day 367 to 475 in the second

annual cycle; barley was grown from day 130 to 265, and day

490 to 645; summer fallow extended from day 266 to 335 and

from day 646 to 700.

The study compared residues of contrasting quality:

Sesbania aculeata shoot (N 3.03%, C/N 16.4, lignin/N 3.2, and

polyphenols + lignin/N 4.2) and wheat straw (N 0.61%, C/N

81.8, lignin/N 34.8, and polyphenols + lignin/N 36.8), each

applied to deliver equivalent N. The experimental design

included five treatments: (1) control (no inputs), (2) chemical

fertilizer (80 kg N ha�1 from urea), (3) Wheat straw

(80 kg N ha�1), (4) Sesbania shoot (80 kg N ha�1), and (5) Sesbania

shoot (40 kg N ha�1) + wheat straw (40 kg N ha�1). The experi-

ment was designed to vary the quality of exogenous soil inputs

having an equivalent amount of N application by using high

quality organic input in form of S. aculeata shoot and low

quality input in form of wheat straw. These treatments were

applied to 3 m � 3 m plots, arranged in a random block design

with three replicates, and a 1 m strip between blocks.

The inputs were applied once each year, 1 or 2 days before

sowing of the rice crop, and incorporated into the soil to a

depth of 0–10 cm. Fresh Sesbania shoots were cut into about 2–

3 cm pieces before incorporation. Wheat straw was air-dried

and then incorporated. In the Sesbania shoot + wheat straw

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 1 6 4 – 1 7 5166

treatment both organic inputs were mixed thoroughly and

then incorporated. The N fertilizer was surface-broadcast,

without incorporation. No exogenous inputs were applied to

the barley crop.

2.3. Soil collection

For estimation of available-N, N-mineralization rate and

microbial biomass N soil samples (0–10 cm depth) were

collected eleven times, once during seedling, pre-grain-

forming, grain-forming, post-grain-forming and maturity

stages of rice as well as barley and once during summer

fallow. Three sub-samples of soil were collected from each

replicate plot and composited. After removing visible plant

debris and fauna, the soil was sieved through a 2 mm mesh

screen. The soil was analyzed for available-N (NO3-N and NH4-

N), microbial biomass N, and N-mineralization rate.

2.4. Estimation of available-N and N-mineralization rate

Field moist and sieved soil samples were analyzed for nitrate-

N (NO3-N) by the phenol disulphonic acid method, using CaSO4

as the extractant (Jackson, 1973) and for ammonium-N (NH4-

N) by the phenate method (APHA, 1995), using 2 M KCl as the

extractant. Available-N was calculated as the sum of NO3-N

and NH4-N. N-mineralization rate was measured using the

buried bag technique (Eno, 1960). Two sub-samples of soil

(about 150 g each), with coarse roots and large organic debris

removed to minimize immobilization, were enclosed in sealed

polyethylene bags and buried at 5–10 cm depth in each plot.

Soil NO3-N and NH4-N were determined, as previously

described, before and after field incubation for 30 days.

Ammonification was estimated from the increases in NH4-N

concentration; nitrification was estimated from the increase

in NO3-N concentration; and net N-mineralization was

estimated from the sum of increases in NH4-N and NO3-N

concentrations, during the field incubation. All results were

expressed on an oven dry soil (105 8C) basis.

2.5. Microbial biomass N estimation

Field moist soil samples were pre-conditioned for 7 days at

room temperature in a container with 100% humidity and an

alkali trap to remove CO2. The container was opened for

aeration a few minutes every day. Microbial biomass N (MBN)

was estimated by the chloroform fumigation–extraction

method (Brookes et al., 1985), using a purified CHCl3 treatment,

followed by extraction of both fumigated and non-fumigated

soils with 0.5 M K2SO4, which was then analyzed for total N by

Kjeldahl digestion. The MBN was estimated as: MBN = EN/0.54,

where EN is the difference between the amounts of N extracted

from fumigated and non-fumigated soil (mg N g�1 oven dry

soil); and 0.54 is the fraction of biomass N extracted after

chloroform fumigation.

2.6. Determination of crop N-uptake and apparent N-recovery

In each plot, plants were harvested at ground-level from two

randomly located subplots (each 25 cm � 25 cm) and dried at

80 8C. To estimate root biomass, a soil monolith

(10 cm � 10 cm, 10 cm deep) was excavated from harvested

sub-plot, and washed with a fine jet of water over twin sieves

(2 mm mesh above and 0.5 mm mesh below). Roots recovered

were oven dried at 80 8C and weighed. Above- and below-

ground plant materials were finely ground and analyzed for N

content by a micro-Kjeldahl method (Jackson, 1973). Nitrogen

uptake was calculated by multiplying the biomass of plant

parts for both crops by their N concentration.

Apparent N-recovery (ANR) of added N was calculated as

described by Dilz (1988):

ANR ð%Þ

¼ N-uptake in treatment�N-uptake in controlN applied in treatment

� 100

2.7. Soil moisture content estimation

Soil moisture content was estimated as gravimetric soil water

at 0–10 cm depth. Fresh soil (10 g) was sampled in triplicate

from each plot, dried at 105 8C for 24 h and weighed. Soil

moisture content was calculated as:

soil moisture content ð%Þ

¼ weight of fresh soil�weight of dry soilweight of dry soil

� 100

2.8. Statistical analysis

SPSS package was used for the following statistical analyses:

two-way analysis of variance (ANOVA), correlation and least

significant difference (LSD) test. Two-way ANOVA was used to

detect significant differences between the effects of input

(treatments) and sampling date on rate of N-mineralization.

Treatment means were compared using the LSD test at

P < 0.05. The five treatments were regarded as distinct

strategies. N-mineralization rate, available-N (NH4-N and

NO3-N) and MBN measured through the annual cycle were

averaged to assess the variations among three ecologically

distinct phases (two cropping seasons and summer fallow).

3. Results

3.1. Soil N-mineralization rate

Two-way ANOVA indicated that the impact of treatments and

crop stages on the rate of N-mineralization was significant

(P < 0.05). The N-mineralization rate decreased consistently

from rice period to summer fallow through barley period.

However, a slight increase was observed during the early stage

of barley cropping in Sesbania + wheat straw and wheat straw

treatments (Fig. 1). In treatments receiving Sesbania and

fertilizer, a steep decline in N-mineralization rate was

observed from rice period to barley period whereas it was

gradual in Sesbania + wheat straw and wheat straw treat-

ments. The rate of N-mineralization increased throughout the

annual cycle due to application of various soil amendments

Fig. 1 – Seasonal variations in the rate of N-mineralization (mg gS1 monthS1) after applying various soil amendments

through two annual cycles (sowing of first rice crop designated as day 0). Periods are as follows: rice crop period (days 0–115

and days 367–475), barley crop period (days 130–265 and days 490–645) and summer fallow (days 266–335 and days 646–

700). These periods and crop stages sampled are shown in the top left graph. Arrows indicate the date of input application.

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 1 6 4 – 1 7 5 167

(Table 1). A consistently higher rate of N-mineralization was

maintained in the Sesbania + wheat straw treatment through-

out the annual cycle.

During rice the rate of N-mineralization was significantly

higher in the fertilizer treatment (83% increase over control,

mean of 2 years) which decreased sharply and became

comparable to the control during barley and summer fallow.

On the other hand, the wheat straw treatment showed an

initial lag phase during rice period, but became significantly

higher than the control during barley and summer fallow.

Sesbania application also resulted in a higher N-mineralization

rate during rice period but in subsequent crop cycle the rate

became comparable to that of wheat straw. On an annual

basis, maximum N-mineralization rate was observed with

Sesbania + wheat straw (+112%) followed by fertilizer (+50%),

Sesbania (+32%) and wheat straw (+16%). Of the N mineralized,

70-92% originated from ammonification alone (Fig. 2). The rate

of ammonification was highest during the rice period, and

then declined through the barley and summer fallow periods.

In contrast, nitrification was lowest during the rice period and

increased under barley and then decreased again during

summer fallow.

3.2. Variation in available-N (NH4-N and NO3-N)

NH4-N was the predominant form of available-N in this

dryland agroecosystem. The level of NH4-N decreased from

seedling to crop maturity barring a small peak during seedling

stage of rice and later stage of barley crop (Fig. 3). In all

treatments, NH4-N levels were found to be higher during the

rice crop compared to the barley crop (Table 2) except for the

wheat straw treatment where a higher level during barley

Table 1 – Variation in N-mineralization rate (mg gS1 monthS1 W S.E.) following various soil amendments; the values forboth crops are means of samplings done during each crop cycle

Crop Treatments

Control Fertilizer Wheat straw Sesbania Sesbania + wheat straw

Period: 2002–2003 annual cycle

Rice 9.9 � 0.68 a 18.7 � 2.49 b 10.3 � 0.85 a 16.7 � 1.71 b 18.1 � 0.93 b

Barley 7.3 � 0.32 a 7.5 � 0.22 a 9.3 � 0.23 b 9.6 � 0.23 b 17.6 � 0.39 c

Summer fallow 4.9 � 0.46 a 5.1 � 0.08 a 6.9 � 0.26 b 6.1 � 0.10 ab 14.0 � 0.60 c

Annual 7.9 � 0.42 a 11.0 � 1.34 b 9.4 � 0.35 ab 11.6 � 0.94 b 17.3 � 0.44 c

Period: 2003–2004 annual cycle

Rice 10.8 � 0.70 a 19.2 � 0.80 b 10.6 � 0.64 a 17.5 � 0.77 b 18.4 � 0.72 b

Barley 7.1 � 0.27 a 8.0 � 0.42 b 9.1 � 0.26 c 9.1 � 0.35 c 17.0 � 0.18 d

Summer fallow 4.7 � 0.54 a 5.4 � 0.17 a 7.5 � 0.08 b 6.3 � 0.20 c 15.6 � 0.17 d

Annual 8.5 � 0.51 a 12.8 � 1.10 b 9.6 � 0.35 a 12.7 � 0.88 b 17.5 � 0.37 c

In each row values having different letters are significantly different from each other (P < 0.05).

Fig. 2 – Seasonal variations in the rate of ammonification (mg gS1 monthS1) and nitrification (mg gS1 monthS1) due to

application of various soil amendments; the periods and crop stages sampled are shown in the top left graph. Arrows

indicate the date of input application.

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 1 6 4 – 1 7 5168

Fig. 3 – Seasonal variations in the levels of available ammonium-N and nitrate-N (mg gS1 soil) due to application of various

soil amendments; the periods and crop stages sampled are shown in the top left graph. Arrows indicate the date of input

application.

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 1 6 4 – 1 7 5 169

period compared to rice period was found. During summer

fallow, the lowest level of NH4-N was recorded in all

treatments. Seasonal variations were very distinct in case of

Sesbania and fertilizer treatments whereas in case of control,

Table 2 – Variation in NH4-N levels (mg gS1 soil W S.E.) followinmeans of samplings done during each crop cycle

Crop

Control Fertilizer W

Period: 2002–2003 annual cycle

Rice 11.9 � 0.62 a 19.6 � 0.85 b

Barley 9.2 � 0.23 a 9.7 � 0.32 a 1

Summer fallow 6.0 � 0.35 a 7.4 � 0.17 b 7

Annual 10.1 � 0.43 a 14.0 � 0.99 b 1

Period: 2003–2004 annual cycle

Rice 12.0 � 0.56 a 19.7 � 0.86 b 1

Barley 9.6 � 0.27 a 10.0 � 0.66 a 1

Summer fallow 6.4 � 0.28 a 7.7 � 0.25 bc

Annual 10.5 � 0.41 a 14.6 � 1.00 b 1

In each row values having different letters are significantly different fro

wheat straw and Sesbania + wheat straw treatments the

variations were moderate.

During the rice period higher levels of NH4-N were observed

in the treatments receiving fertilizer (+65%), Sesbania (+36%)

g various soil amendments; the values for both crops are

Treatments

heat straw Sesbania Sesbania + wheat straw

9.7 � 0.33 c 16.0 � 0.75 d 15.7 � 0.5 d

1.3 � 0.49 b 10.6 � 0.65 ab 14.0 � 0.59 c

.76 � 0.27 b 6.9 � 0.32 ab 10.5 � 0.38 c

0.2 � 0.33 a 12.7 � 0.71 b 14.5 � 0.43 b

0.5 � 0.47 a 16.5 � 0.72 c 16.0 � 0.41 c

1.4 � 0.42 b 10.6 � 0.50 ab 14.6 � 0.38 c

8.7 � 0.36 c 7.2 � 0.35 ab 11.0 � 0.42 d

0.7 � 0.31 a 13.3 � 0.70 b 15.0 � 0.34 b

m each other (P < 0.05).

Table 3 – Variation in NO3-N levels (mg gS1 soil W S.E.) following various soil amendments; the values for both crops aremeans of samplings done during each crop cycle

Crop Treatments

Control Fertilizer Wheat straw Sesbania Sesbania + wheat straw

Period: 2002–2003 annual cycle

Rice 1.04 � 0.04 a 1.71 � 0.12 b 1.32 � 0.08 c 1.62 � 0.09 b 1.56 � 0.10 bc

Barley 1.37 � 0.08 a 1.80 � 0.10 b 1.93 � 0.06 b 1.91 � 0.08 b 2.03 � 0.06 b

Summer fallow 0.79 � 0.04 a 1.23 � 0.11 b 0.98 � 0.04 a 1.32 � 0.06 b 1.71 � 0.01 c

Annual 1.17 � 0.05 a 1.70 � 0.08 bc 1.56 � 0.07 b 1.72 � 0.06 bc 1.78 � 0.06 c

Period: 2003–2004 annual cycle

Rice 0.91 � 0.04 a 1.64 � 0.09 b 1.23 � 0.11 c 1.66 � 0.09 b 1.39 � 0.11 bc

Barley 1.33 � 0.05 a 1.85 � 0.53 b 1.92 � 0.06 bc 1.76 � 0.04 b 2.03 � 0.05 c

Summer fallow 0.84 � 0.02 a 1.28 � 0.01 b 1.06 � 0.04 c 1.27 � 0.04 b 1.79 � 0.02 d

Annual 1.07 � 0.04 a 1.70 � 0.06 b 1.50 � 0.09 b 1.67 � 0.05 b 1.69 � 0.07 b

In each row values having different letters are significantly different from each other (P < 0.05).

Fig. 4 – Impact of various soil inputs on seasonal mean

values of microbial biomass N (mg gS1). The values are

means of samplings during each cropping season. In each

cropping season, bars having different letters are

significantly different from each other (P < 0.05). Code: C,

control; F, fertilizer; W, wheat straw; S, Sesbania; M,

Sesbania + wheat straw.

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 1 6 4 – 1 7 5170

and Sesbania + wheat straw (+33%) relative to the control,

while wheat straw recorded lower levels (�15%). However,

trends were reversed during barley period with higher NH4-N

levels in treatments receiving Sesbania + wheat straw (+52%)

and wheat straw (+21%). On an annual mean basis, NH4-N

levels were significantly higher in Sesbania + wheat straw

(+43%), fertilizer (+39%), and Sesbania (+26%) treatments than

in other treatments.

The levels of NO3-N increased consistently from rice period

to barley period and thereafter it decreased during summer

fallow (Fig. 3). During the rice period, higher levels of NO3-N

were recorded in the fertilizer treatment (+72%) followed by

Sesbania (+69%), Sesbania + wheat straw (+51%) and wheat

straw (+31%). During the barley period maximum NO3-N was

recorded in Sesbania + wheat straw (+50%), followed by wheat

straw (+43%), Sesbania (+36%) and fertilizer (+35%). On an

annual basis, relative to the control, treatments receiving

Sesbania + wheat straw (+55%), fertilizer and Sesbania (+52%)

showed higher NO3-N levels compared to low quality input

wheat straw (+37%) (Table 3). The mean values of available N

(NH4-N + NO3-N) during rice, barley and summer fallow

periods were strongly correlated with the corresponding rates

of N-mineralization (r = 0.93, d.f. = 28, P < 0.01).

3.3. Microbial biomass N

Across all treatments accumulation of MBN was found to

increase from rice period to the maximum at summer fallow

through barley period (Fig. 4). In case of theSesbania treatment, a

slight decrease was noticed during barley period however, this

trend was observed only during the second annual cycle.

Consistently higher levels of MBN were observed in the

Sesbania + wheat straw treatment through rice (+85%), barley

(+74%) and summer fallow (+75%). The Sesbania treatment

showed the highest biomass N during rice period (+95%) which

decreased during barley period (+43%). On an annual mean

basis, the MBN was consistently and significantly higher in

Sesbania + wheat straw (+79%) and Sesbania (+64%) treatments

compared to other treatments. The MBN remained lower in case

of wheat straw and fertilizer treatments throughout the annual

cycle. The seasonal variations in MBN levels across treatments

were positively correlated with changes in the corresponding

rate of N-mineralization (r = 0.61, d.f. = 28, P < 0.01).

3.4. Crop N-uptake and apparent N-recovery

Total N-uptake ranged from 50 to 81 kg ha�1 in rice, and from

31 to 50 kg ha�1 in barley (Table 4). The maximum N-uptake in

rice occurred in the fertilizer treatment (+56%), followed by

Sesbania (+38%) and Sesbania + wheat straw (+33%). During the

barley period N-uptake was maximized with Sesbania + wheat

Table 4 – Crop N-uptake (kg N haS1) of rice and barley under different soil inputs; the values are mean W S.E.

Treatments 2002–2003 annual cycle 2003–2004 annual cycle

Rice Barley Rice Barley

Control 50.4 � 0.70 a 30.5 � 1.35 a 51.5 � 1.59 a 30.8 � 1.51 a

Fertilizer 76.4 � 1.90 b 35.2 � 1.30 a 81.0 � 1.20 b 37.3 � 1.02 b

Wheat straw 55.0 � 2.70 a 45.0 � 1.75 b 57.6 � 2.76 a 45.7 � 2.13 c

Sesbania 70.1 � 3.38 c 36.1 � 1.89 a 71.1 � 0.58 c 36.9 � 0.74 b

Sesbania + wheat straw 66.9 � 0.93 c 48.4 � 2.79 b 69.0 � 4.78 c 49.9 � 1.18 c

In each column values having different letters are significantly different from each other (P < 0.05).

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 1 6 4 – 1 7 5 171

straw (+60%) followed by wheat straw (+48%). Apparent N-

recovery of added N by the crops from the soil was found to be

higher in Sesbania + wheat straw and fertilizer treatments

than in the wheat straw treatment (Table 5).

3.5. Soil moisture content

Soil moisture content followed a seasonal pattern influenced

by precipitation with higher soil water content (13.4–14.9%)

during rice period (rainy season), thereafter decreasing to the

minimum (6.5–7.9%) during summer fallow (Fig. 5). Soil

moisture was not affected (P < 0.05) by treatments throughout

the annual cycle. The seasonal variations in moisture content

across treatments were positively correlated with the corre-

sponding rate of N-mineralization (r = 0.76, d.f. = 28, P < 0.01).

4. Discussion

4.1. Soil N-mineralization rate and microbial biomass N

A marked seasonal variation was observed in soil N-miner-

alization rate, which was higher in the wet season crop (rice)

relative to the dry season crop (barley). During the wet season

the higher soil moisture level, among several factors, seems to

be the chief determinant responsible for greater N-miner-

alization rates. Throughout the annual cycle, across all the

treatments, N-mineralization was found to be influenced

strongly by soil moisture (r = 0.76). After the rainy season, due

to drop in soil moisture during the rest of the annual crop

cycle, there was a decrease in N-mineralization in winter and

summer. Ghoshal (2002) also found strong correlation

between N-mineralization rate and soil moisture content

(r = 0.86, P < 0.01). Available-N concentrations in tropical soils

Table 5 – Annual apparent N-recovery (%) in rice andbarley (combined) under different soil inputs; the valuesare mean W S.E.

Treatments 2002–2003annual cycle

2003–2004annual cycle

Fertilizer 40.8 � 2.4 a 45.1 � 0.1 a

Wheat straw 23.9 � 5.6 b 26.4 � 6.1 b

Sesbania 31.6 � 2.3 ab 32.2 � 0.9 ab

Sesbania + wheat straw 43.2 � 3.3 a 45.4 � 7.3 a

In each column values having different letters are significantly

different from each other (P < 0.05).

fluctuate considerably with seasonal changes in soil water

potential (Wong and Nortcliff, 1995). Rewetting of a dry soil has

been noticed as a major factor for the acceleration of N-

mineralization especially under semi-arid and subtropical

conditions (Dalal and Mayer, 1986). Inaccessible soil organic

matter and dead N-rich microbial cells become accessible to

the microorganisms as a new pool of readily mineralizable N

due to drying and rewetting (Campbell and Biederbeck, 1982).

The N-mineralization rate during the rice period may also

have been enhanced by tillage, which disrupts soil aggregates,

exposing the native soil organic matter to microbes.

The rate of N mineralization varied among treatments,

especially in the early phase of the annual cycle, even though

Fig. 5 – Variation in soil moisture content (%) during various

crop and fallow periods through two annual cycles. Values

are means of samplings during each cropping season.

Seasonal means were not significantly different from each

other (P < 0.05). Code: C, control; F, fertilizer; W, wheat

straw; S, Sesbania; M, Sesbania + wheat straw.

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 1 6 4 – 1 7 5172

an equivalent amount of N was applied through all the

exogenous inputs. In the plots receiving Sesbania and fertilizer

the pattern of N-mineralization followed the classical pattern

of initial rapid release followed by a slower linear release

(Fig. 1). Such progress of N-mineralization with time was also

recorded by El-Harris et al. (1983) and Bonde and Rosswall

(1987). Fertilizer application significantly increased the N-

mineralization rate in this study, in agreement with early

studies (Singh and Singh, 1994) in a dryland agroecosystem

(24–66% increase relative to the control); probably due to ready

availability of the added nutrients. According to Woods et al.

(1987), this increased rate of N-mineralization may reflect the

priming effect of fertilizer N on indigenous soil N. Sesbania

having a low C:N ratio decomposed rapidly (instantaneous

decay constant, k = 0.028 day�1, t95 = 109 days; Singh et al.,

2007) after application resulting in a high rate of N-miner-

alization during rice period. Sesbania decomposed completely

in around 120 days which resulted in a low rate of N-

mineralization during barley period. Aulakh et al. (2000) also

observed an increase in N-mineralization rate when Sesbania

was applied in a rice–wheat crop rotation.

The wheat straw treatment showed an initial lag phase in N

release, reflecting early immobilization of N by the microbial

biomass. During the later phase of the annual cycle the

immobilized N was re-mineralized and resulted in a shift of

dominance from immobilization to mineralization. Mubarak

et al. (2001) similarly reported N-immobilization shortly after

incorporating crop residue with high C:N ratio; when soil was

incubated with straw alone, about 30% of the straw N was held

in the microbial biomass within 5 days. When inorganic N was

added with the straw, the consumption of straw N by the

biomass decreased slightly. Azmal et al. (1996) observed net

immobilization throughout the incubation period both at 25

and 35 8C, after adding rice straw (C/N Ratio 60) to upland soils.

Residue with less than 1–1.2% N usually immobilizes mineral

N (Vigil and Kissel, 1991). De Neve et al. (2004) observed initial

immobilization followed by re-mineralization of wheat straw

after 50 days of incubation. Khalil et al. (2005) found that,

irrespective of soil type, chicken manure (C:N 10.6) resulted in

net N-mineralization whereas wheat residue (C:N 75.4)

resulted in net N-immobilization.

Palm et al. (2001) suggested that mixing low quality and

high quality organic inputs generally results in a mineraliza-

tion pattern reflecting the weighted average of the two

separate materials. However, in some cases there have been

non-additive nutrient availability patterns from mixes of low

and high quality materials but the results are difficult to

predict and again do not necessarily result in a synchronous

curve of nutrient availability (Mafongoya et al., 1998; Palm and

Rowland, 1997). If a poor quality residue has high amounts of

available C, the capacity for N immobilization may exceed the

amounts of N available from the soil, thereby immobilizing the

N from the accompanying N-rich residue. Thus, for example,

Sakala et al. (2000) observed that N-mineralization from poor

quality maize residues mixed with leaves of pigeonpea was

much less than that which would have been predicted from

the individual amendments.

The effect of mixed application of high and low quality

resources on N-mineralization rate under dryland condi-

tions has rarely been investigated. Mixed application of

Sesbania + wheat straw resulted in a consistently higher rate

of N-mineralization throughout the annual cycle, probably

due to an interactive effect of Sesbania and wheat straw,

which modulated the N-mineralization rate and showed

synergistic behaviour with reference to N-mineralization.

Our data suggest that mixed application resulted in a

gradual release of inorganic N. Moreover, the greater C

availability in the organic system apparently supports a

more active microbial biomass with greater N demand,

thus promoting immobilization and mineralization of NO3

(Burger and Jackson, 2003) in accordance with crop require-

ment. The challenge in managing high soil organic

matter input systems is to balance C and N inputs to

minimize accumulations of NO3, yet avoid high rates of

microbial N immobilization during peak periods of crop N

demand.

The size and activity of the microbial biomass are key

factors controlling the rates of N-mineralization (Paul and

Voroney, 1984; Azmal et al., 1996). In the present study across

all the treatments through the two annual cycles, positive

correlation was observed between the MBN and the N-

mineralization rate (r = 0.61). Similar relations between N in

soil microbial biomass and N-mineralization rate were

recorded by Kushwaha et al. (2000). Bremer and Kessel

(1992) reported that the relationship of N-mineralization to

microbial biomass dynamics depended on the phase of

decomposition and concluded that the microbial N was the

main source of mineralized N. The amount and temporal

distribution of rainfall strongly influences soil microbial

activity and the fluctuation of mineralizable N in tropical

soils. Amounts of N released from crop residue depend on

microbial immobilization/mineralization as influenced by

crop residue type, placement, and degree of incorporation

into the soil (Aulakh et al., 1991). Nyberg et al. (2002) suggested

that high quality residues, like Sesbania and Crotolaria,

supplying easily accessible C to the microbial community,

can deplete soil oxygen, and create local anaerobic conditions,

stimulating denitrification (Aulakh et al., 2000).

Management practices can influence biological nitrogen

dynamics by their effects on microbial populations. Soils

receiving long-term application of manufactured N fertilizers

often have lower biological activity relative to soils that have

received repeated additions of organic materials (Collins et al.,

1992; Dick et al., 1988; McGill et al., 1986). The diversity of

microbial biomass determines ecological stability and sus-

tained soil productivity in a system; mixed application of

Sesbania + wheat straw, where both low and high quality

resources are available simultaneously, may lead to increased

diversity of the microbial biomass. Low quality resource

application favors slow growing K-strategists over r-strate-

gists for longer duration, however, application of Sesbania may

facilitate rapidly growing r-strategist species on nutrient rich

substrate. In the mixed input treatment presumably both, r-

and K-strategists remain active for longer time periods,

thereby maintaining higher microbial biomass throughout

the annual cycle (Singh et al., 2007; Fontaine et al., 2003). This

could help in build up of SOM and increased cation exchange

capacity and erosion resistance (Young, 1997), minimizing the

risk of losses of the remaining N through leaching, volatiliza-

tion and erosion.

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 1 6 4 – 1 7 5 173

4.2. Available-N in soil and apparent N-recovery

In these agroecosystems the dominance of NH4-N was

probably due to the predominance of diverse ammonifying

microbes. In contrast to nitrifying bacteria these microbes can

be retarded by low water potential present in dryland soils,

resulting in high NH4-N levels (Dommergues et al., 1978).

During the rainy season, due to higher soil moisture content,

the rate of biological activity and decomposition are decreased

due to the creation of anaerobic conditions. These anaerobic

conditions can enhance the rate of denitrification resulting in

a lower rate of nitrification during rice cropping compared to

barley cropping (Fig. 2). Aulakh et al. (2000) also reported

higher denitrification under anaerobic soil conditions created

due to high soil moisture content at nearly-saturation and

flooded soil environment. Moreover, under anaerobic condi-

tions, decomposition depends only on anaerobic bacteria

which are less efficient than aerobic organisms (Campbell,

1978; Patrick, 1982). They can function at much lower energy

levels and require low N, resulting in a more rapid release of

NH4+ ions (Patrick, 1982). After the rainy season, with gradually

drying soils, nitrification is favored resulting in a decreased

NH4-N relative to NO3-N. The other possible factor for this

contrast in the seasonal pattern between NO3-N and NH4-N

was probably due to the marked difference in their mobility.

NH4-N is derived from the mineralization of soil organic N,

added organic materials, or addition of urea or ammonium

fertilizer. In contrast to the NH4-N, there is little tendency for

the NO3-N anion to be adsorbed by the soil colloids, which

commonly possess a net negative charge. Nitrate is thus

susceptible to diffusion and mass transport with soil water

(Cameron and Haynes, 1986). Nitrate losses are often larger in

agricultural ecosystems (Morisot, 1981) and leaching is the

most important channel of N loss from cultivated field soils

other than accounted for in crop uptake (Allison, 1973; Legg

and Meisinger, 1982). Dominance of NH4-N is expected to

reduce N losses from these dryland agroecosystems.

The largest proportion of N is found in the soil organic

matter and its availability to plants is dependent on its rate of

mineralization. Rate of N-mineralization largely governed the

availability of N (NH4-N + NO3-N) in the agroecosystem

studied, as evident from the strong relationship between N-

mineralization rate and available-N (r = 0.93). Among the

treatments, available-N level was enhanced in fertilizer,

Sesbania and Sesbania + wheat straw treatments during rice

period, which was in accordance with the enhanced rate of N-

mineralization. Munoz et al. (2003) reported that fertilizer and

manure additions tended to increase soil available-N (parti-

cularly NO3-N). They observed that NO3-N from inorganic

fertilizer was soluble and therefore more susceptible to

downward movement; in comparison, less NO3-N was found

in the lower depths in a manure treatment. Dey and Jain (1997)

reported increased availability of N during the initial period,

due to application of Sesbania, which declined 45 days after

application.

In contrast to fertilizer and Sesbania treatments, low

availability of N in wheat straw treated plots in the early

phase of rice cropping was due to an initial lag of N-

mineralization. During the later phase (barley crop) N became

available due to re-mineralization in the soil resulting in

significantly higher levels of available-N. Ocio et al. (1991)

found no significant immobilization after wheat straw

application. Aulakh et al. (2001) observed a tendency of early

immobilization of straw N which mineralized thereafter later

in the season. They observed that plots receiving Sesbania

showed high mineral N levels for a longer duration, compared

to plots receiving fertilizer where initial low levels and later on

higher levels were found.

The mixed application of Sesbania + wheat straw main-

tained higher levels of available-N throughout the annual

cycle, suggesting that C-rich inputs favor a gradual release of

available-N. Asynchrony occurs when nutrient availability

exceeds plant requirements, often because release occurs at a

time when plant demand is restricted or non-existent, as in

summer fallow or after crop maturity. Apparent N-recovery is

influenced by weather conditions, management practices and

availability of N in soil (Bellido and Bellido, 2001). Mixed

application of Sesbania + wheat straw resulted in higher

apparent N-recovery and N-uptake compared to application

of wheat straw or Sesbania alone, suggesting lower loss of N

through the system in the mixed treatment and thereby

increased utilization of N by the crop. Sesbania mixed with

wheat straw and fertilizer can be effective in replacing

fertilizer, thereby mitigating environmental problems asso-

ciated with its use. However Aulakh et al. (2000) observed

increased apparent N-recovery in Sesbania treatment com-

pared to fertilizer and attributed these results to the rapid

mineralization of Sesbania.

Achieving N synchrony and mitigating the problems of

leaching or denitrification requires a strategy to avoid periods

of nutrient deficiency and excess. The issue of synchrony in

cropping systems is at the root of most environmental hazards

associated with excess N in the atmosphere, terrestrial and

marine ecosystems (Peoples et al., 2004). Few studies have

carefully compared synchrony over a range of cropping

systems. One recent study conducted by Xu et al. (2005)

observed, in a pot experiment with Chinese cabbage as test

crop, that the amount of mineral N was initially highest when

fertilizer was used. Mineral N concentration decreased there-

after to the lowest level in the later stages of cropping. When

untreated rice straw was combined with fertilizer the levels of

mineral N were initially lower but increased later on and were

maintained at a higher level throughout the crop period.

5. Conclusions

This study shows that it is possible to manage the N

immobilization/mineralization process of soil by application

of mixed treatment of Sesbania + wheat straw so as to adjust

the release of available-N in accordance with plant demands.

Lack of synchrony, i.e., asynchrony, occurred in the plots

receiving fertilizer and also with either high or low quality

organic resources. The additional gain in N-mineralization, N-

uptake and apparent N-recovery in the mixed treatment

demonstrated synergy between the two N sources. Consistent

and prolonged availability of N in combined application of

Sesbania + wheat straw indicated a step towards achieving

synchronization which may lead to better nutrient conserva-

tion and reduced losses from the agroecosystem.

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 1 6 4 – 1 7 5174

Acknowledgements

We thank the Head and the Programme Co-ordinator, Centre

of Advanced Study in Botany, Department of Botany, for

providing laboratory facilities. University Grants Commission,

New Delhi, India, provided financial support in form of a major

research project which included a Project Fellowship to Mr. S.

Singh. Dr. H.H. Janzen, Agriculture and Agri-Food Canada and

Dr. Kathrin Franzluebbers, Texas A&M University gave helpful

suggestions on the manuscript, for this we are grateful.

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