Directory UMM :Data Elmu:jurnal:A:Applied Soil Ecology:Vol15.Issue3.Nov2000:

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Soil carbon, microbial activity and nitrogen availability in

agroforestry systems on moderately alkaline soils in northern India

B. Kaur

, S.R. Gupta

a,∗

, G. Singh

a_{Department of Botany, Kurukshetra University, Kurukshetra 136119, India} b_{Central Soil Salinity Research Institute, Karnal, India}

Received 6 November 1998; received in revised form 28 April 2000; accepted 28 April 2000

Abstract

The present investigation aimed to analyze the role of agroforestry systems in improving soil organic matter status, microbial activity and nitrogen availability with a view to effective management of the fertility of moderately alkaline soils. The study

site was located at Karnal (29◦

59′

N, 76◦

51′

E, 250 m.s.l.) and the systems were characterized by a rice–berseem crop rotation; agrisilvicultural systems of Acacia, Eucalyptus and Populus along with rice–berseem and single species tree plantations. Soil microbial biomass was measured using the fumigation extraction technique and nitrogen mineralization using the aerobic

incubation method. Microbial biomass carbon was low in rice–berseem crops (96.14mg g−1_{soil) and increased in soils under}

tree plantations (109.12–143.40mg g−1soil) and agrisilvicultural systems (133.80–153.40mg g−1soil). Microbial biomass

was higher by 42% (microbial C) and 13% (microbial N) in tree-based systems as compared to monocropping. Microbial biomass immobilized 2.32–2.57% of the soil carbon and 4.08–4.48% of the soil nitrogen in tree-based systems. Soil carbon increased by 11–52% due to integration of trees along with the crops for 6–7 years. Cropland management practices and tree

species influenced CO2-C production, biomass specific respiratory activity, and nitrogen mineralization rates. In tree-based

systems, soil inorganic N levels were higher by 8–74% and nitrogen mineralization by 12–37% as compared to monocropping. On the basis of increased soil organic matter content, enlarged soil microbial biomass pool and greater soil N availability, agrisilvicultural systems have been found to be ecologically sustainable land-use systems for utilizing moderately alkaline soils. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Carbon dioxide; Microbial biomass; Microbial activity; Nitrogen mineralization; Soil carbon; Soil nitrogen

1. Introduction

Saline and alkaline soils are of widespread oc-currence in arid and semiarid regions of northern India. More than 2.5 million ha of otherwise arable lands in the Indo-Gangetic plains have become un-suitable for cultivation due to soil sodicity (Abrol

∗_{Corresponding address. Tel.:}₊_{91-1744-20196(501);} fax:+91-1744-20277.

E-mail address: kuru@doe.ernet.in (S.R. Gupta).

and Bhumbla, 1971). These soils are character-ized by high pH throughout the soil profile, high exchangeable sodium and low soil organic mat-ter content (Gupta et al., 1984), a sparse cover of natural vegetation (Rana and Parkash, 1987) and low microbial activity (Kaur et al., 1998; Pathak and Rao, 1998). The productive capacity of al-kaline soils has been found to improve by grow-ing plants adapted to sodic soils (Gupta et al., 1990). Reclamation agroforestry systems have been reported to improve biological production and

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ameliorate sodic conditions of soils by increas-ing soil organic matter content and availability of soil inorganic nitrogen (Singh, 1995; Singh et al., 1997).

The soil microbial biomass is a labile pool of organic matter and comprises 1–3% of total soil or-ganic matter (Jenkinson and Ladd, 1981). The soil microbial biomass acts as a source and sink of the plant nutrients (Singh et al., 1989; Smith and Paul, 1990) and regulates the functioning of the soil sys-tem. Plant cover through its effects on the quantity and quality of organic matter inputs influences the levels of soil microbial biomass (Wardle, 1992). The specific respiratory activity of soil microbial biomass has been used to analyze the effects of en-vironmental factors, crop management, and organic inputs on the microbial populations (Anderson and Domsch, 1990, 1993; Campbell et al., 1991). It is sensitive to the changes in the quantity and quality of soil organic matter and ecosystem stability (Insam, 1990).

In saline and alkaline soils, excessive amounts of salts have an adverse effect on biological activity including soil enzyme activity (Frankenberger and Bingham, 1982; Rao and Pathak, 1996), nitrogen min-eralization (McClung and Frankenberger, 1985) and soil microbial biomass (Sarig and Steinberger, 1994; Batra and Manna, 1997; Kaur et al., 1998). The bio-logical activity of alkaline soils has been found to im-prove under a crop, grass or tree cover (Rao and Ghai, 1985). Nitrogen mineralization is an essential func-tion of the soil microbial system (Ellenberg, 1971). Pathak and Rao (1998) reported that added organic matter has a favorable effect on nitrogen mineraliza-tion rate in saline and alkaline soils under laboratory conditions. In Acacia, Eucalyptus and Populus based agroforestry systems, Singh et al. (1997) reported an increase in soil organic carbon and soil available nitrogen.

Soil biological activity in relation to plant growth and land-use practices is poorly understood for salt-affected soils. The present investigation aims to analyze the effects of cropping, forestry and agroforestry on soil organic carbon, total nitrogen, soil microbial biomass and net nitrogen mineraliza-tion and to consider implicamineraliza-tions for the long-term management of the fertility of moderately alkaline soils.

2. Materials and methods 2.1. Study site

The agroforestry systems selected for study were on the Central Soil Salinity Research Institute experi-mental farm at Karnal (29◦_{59 N}′_{, 76}◦₅₁′_{E, 250 m.s.l.).} The climate of the study area is semiarid and mon-soonic and characterized by hot dry summers and cold winters. The mean annual rainfall of the area is about 700 mm, about 80% of which is received dur-ing July and September. Geologically the area consti-tutes a part of the Indo-Gangetic alluvial plains and belongs to the Pleistocene age. The soils are classi-fied as Haplic-Salonetz, very strongly alkaline, loam to clay loam in A and B horizons. They are sandy loam in texture in the surface 0–5 cm layer and clay loam at 5–85 cm. Initially the soils of the study site were highly sodic with moderately low permeability and had a sparse cover of salt-tolerant grasses.

The present study is part of a long-term field ex-periment initiated in 1989 at the exex-perimental farm of the CSSRI, Karnal. The treatments in this experiment were three tree species, viz., Acacia nilotica Delile sub sp. Indica (Benth.) Brenan, Eucalyptus tereticornis Smith, Populus deltoides Bartr. Ex Marsh, in the main plots and four inter-crop treatments, viz., rice–wheat (Triticum aestivum), rice–berseem (Trifolium alexan-drium), pigeonpea (Cajanus cajan)–mustard (Brassica juncea) and no crop in the sub-plots. The 12 treatment combinations were replicated three times in a spilt plot design. Soil carbon for various treatments varied from 0.22–0.28% and the soil pH was 9.26–9.34.

Saplings of Eucalyptus, Acacia and Populus were planted during September 1989 and February 1990 in augerholes. These augerholes were filled with the original soil mixed with gypsum (3 kg) and farmyard manure (8 kg). The size of plots was 12 m×12 m and the distance between rows of trees was 4 m and be-tween trees in a row 2 m from 1989 to 1995. Inter-row space was also amended with gypsum before the first inter crop was planted in the summer of 1990.

During May 1995, one row of trees was removed so as to increase the distance between the rows of trees to 8 m. Rice was grown during June–October followed by a berseem crop during October–May. Inorganic fertilizers and agronomic practices were applied ac-cording to the requirements of the crops (Singh et al.,

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1997). The rice crop was planted using nursery-raised 20 days old seedlings on 28 June 1995 and harvested on 12 October 1995. Nitrogen fertilizer was applied at the rate of 150 kg N ha−1_{and ZnSO}

4was added at

the rate of 50 kg ha−1_{during the crop growing season.}

After preparing the field, berseem was sown on 25 October 1995 with a basal dose of 25 kg N ha−1_{. Green}

herbage of berseem was cut during the last weeks of December, February and March and finally the mature crop was harvested on 29 May 1996. The inter-crops without trees were also grown in a separate field adja-cent to the main experiment with similar management and agronomic practices. In this area, the intercrop sequences were replicated three times. An unweeded control (fallow for 5 years) was also maintained.

In the present study, soil samples were taken from only one inter-crop treatment of the experiment, i.e. rice–berseem. Therefore, the treatments compared in this study were: rice–berseem cropping, rice–berseem with tree species of A. nilotica, E. tereticornis, P. deltoides; Acacia, Eucalyptus and Populus without rice–berseem, fallow with naturally growing plants as a control. At the start of this study during 1995, the soil pH (1:2) varied from 8.72 to 9.17. The soil or-ganic carbon content in different treatment plots var-ied from 0.42 to 0.68% and total soil nitrogen ranged from 0.046 to 0.086%.

2.2. Soil sampling and preparation

To analyze the effects of tree and crop species on soil organic matter, soil microbial biomass and nitrogen mineralization, soil samples were collected during July and December 1995 and March 1996 from the various treatment plots. The soil was sam-pled (0–10 cm depth) using a soil corer (12 cm diam.). For obtaining a representative sample five soil cores were collected along a transect for each of the three replicates of the eight treatments. Soil samples for the three replicates from the respective treatments were also composited for analysing soil carbon and nitro-gen, microbial biomass and nitrogen mineralization.

During May 1996, the effect of soil depth and hor-izontal spatial variability (distance from the rows of trees) was also studied on soil microbial activity and nitrogen mineralization. After removing the ground floor litter and plant residues, the underlying soil was sampled using a soil corer (12 cm diam.) at depth

intervals of 0–7.5, 7.5–15, and 15–30 cm to obtain three pooled samples at 1, 2 and 3 m distance from the trees within the alleys. A total of three compos-ite samples for each depth and distance from the trees was obtained. Samples were transported to the laboratory in polyethylene bags and stored at 4◦_C until analysis. The soil was pre-conditioned before measuring soil microbial biomass by spreading it overnight in a thin layer within folded polyethylene sheets, with moisture content adjusted to 40% of wa-ter holding capacity. The measurement of microbial biomass carbon and nitrogen was completed within a few days of sieving the soil. Subsamples of air dried soil were used for determining soil pH (1:2), organic carbon (Kalembasa and Jenkinson, 1973), and total nitrogen (Bremner, 1965). Soil bulk density was also determined under field conditions.

2.3. Microbial biomass

Microbial biomass C and N were determined us-ing the fumigation extraction methods (Brookes et al., 1985; Vance et al., 1987). The sieved, field moist soil sub-samples (equivalent to 50 g oven dry soil) were fumigated with alcohol free chloroform in vacuum desiccators and stored in the dark for 24 h. After re-moving the fumigant (by repeated de-evacuation of chloroform from the soils), the samples were extracted with 200 ml 0.5 M K2SO4 for 30 min on a shaking

machine. The unfumigated soil samples were extracted similarly at the start of experiment.

The filtered soil extracts of both fumigated and un-fumigated samples were analyzed for organic C using the acid dichromate method (Vance et al., 1987). To-tal nitrogen in K2SO4soil extract was determined by

acid digestion and Kjeldahl distillation (Brookes et al., 1985). Soil microbial biomass carbon (MBC) was es-timated as BC=2.64 EC (Vance et al., 1987), where EC (extractable carbon) is the difference between car-bon extracted from fumigated and unfumigated sam-ples, both expressed in the same measurement unit. Soil microbial biomass nitrogen (MBN) was estimated as BN=EN/0.54 (Brookes et al., 1985), where EN (extractable nitrogen) is the difference between N ex-tracted from fumigated and unfumigated samples.

Carbon dioxide evolution rates were measured us-ing the alkali absorption method. The moist soil sam-ples (45% WHC) were placed in 100 ml beakers and

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incubated in 2 l air tight jars at room temperature (27–35◦_{C) for 30 days. The CO}

2produced from the

soil was absorbed in 0.5 N NaOH and determined titri-metrically with 0.1 N HCl using phenolphthalein as indicator (Gupta and Singh, 1981). The CO2evolution

rates were determined at an interval of 2–5 days. Car-bon mineralization rates were determined from CO2-C

evolved from the soil samples. Specific respiratory ac-tivity of soil microbial biomass carbon was estimated by dividing the CO2-C produced (i.e. mineralizable

carbon) by the value of soil microbial biomass fol-lowing Campbell et al. (1991). Mineralizable carbon was estimated from the quantity of CO2-C produced

during 10 days incubation of the soil under laboratory conditions.

2.4. Soil inorganic N and nitrogen mineralization rates

Inorganic N levels were determined using the Kjeldahl distillation method. Sieved, moist soil sub-samples (equivalent to 50 g oven dry weight) were extracted with 0.5 M K2SO4(4 K2SO4: 1 soil).

Inorganic N levels were determined in the soil fil-trate by MgO and Devardas alloy distillation. Similar sub-samples of soil were incubated under laboratory conditions at ambient temperature. After periods of 10, 20 and 40 days the incubated soil samples were extracted with 0.5 M K2SO4. Net nitrogen

mineral-ization rates were determined by subtracting initial soil mineral N from final mineral N at 40 days. 2.5. Statistics

Data on soil carbon, nitrogen, microbial biomass and nitrogen mineralization rates were analyzed using one way analysis of variance (ANOVA). Least signif-icant difference (LSD) values at the 5% levels of sig-nificance (p<0.05) and Duncan’s Multiple Range Test values (DMRT) were calculated following Gomez and Gomez (1984).

3. Results

3.1. Soil carbon and nitrogen

Variations in soil carbon and nitrogen with soil depth and horizontal spatial variability (distance from

rows of trees) are shown in Table 1. Soil carbon and nitrogen concentration at 0–7.5 cm depth were higher than at 7.5–15 and 15–30 cm. Soil carbon and nitrogen concentration varied significantly with soil depth (p<0.01) and distance from the trees (p<0.01) in all the treatments. Organic carbon and nitrogen accretion in the soil was significantly higher when trees were associated with agricultural crops. Max-imum build up of organic carbon in soil occurred under Acacia based systems followed by Populus and Eucalyptus based systems. The nitrogen content increased by 8–46% (0–7.5 cm soil depth) due to integration of trees with crops; the increase being maximum in the case of Acacia based systems. Av-eraged across the distance from the trees at 0–7.5 cm soil depth, carbon and nitrogen concentrations were found to be higher in the case of Acacia based sys-tems (0.57–0.63% C; 0.068–0.077% N) and Populus based systems (0.41–0.59% C; 0.047–0.073% N) as compared to Eucalyptus based systems (0.38–0.54% C; 0.039–0.054% N). It is evident that the ameliora-tive effect of trees was more conspicuous at 1 and 2 m distance from trees and in upper soil layers (0–7.5 cm soil depth). There were no significant differences due to tree species in organic carbon and total nitrogen at 3 m distance from the trees (Table 1).

3.2. Microbial biomass

The levels of soil microbial biomass carbon and nitrogen in rice–berseem cropping, tree plantations and agroforestry systems during July 1995–March 1996 are shown in Fig. 1. Soil microbial biomass C (90.56–168.0mg g−1 soil) and biomass N (13.16–

35.59mg N g−1 soil) were maximum during the

summer month of March 1996. The soil microbial biomass showed a marked decrease in July as com-pared to values in March and December (p<0.01). During the major growth phase of crop plants and the trees in July, the nitrogen released by microbial biomass could be readily taken up by the plants, thereby causing a decrease in soil microbial biomass. It is evident that there was low variability in soil mi-crobial biomass within the treatments as the analysis has been made on pooled soil samples for the various treatments. This could also be attributed to selective microbial adaptations to alkaline conditions of the soil.

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Table 1

Soil carbon and nitrogen concentrations at different soil depths and distances from rows of tree (1, 2 and 3 m) in different systemsa

Treatments Soil carbon (%) Soil nitrogen (%)

0–7.5 (cm) 7.5–15 (cm) 15–30 (cm) 0–7.5 (cm) 7.5–15 (cm) 15–30 (cm)

Control 0.65c _0.26cd _0.16de _0.083ab _0.036ab _0.023b

Rice–berseem 0.47d _0.28bc _0.15de _0.055c _0.037ab _0.018b

1 m Distance

Acacia 0.68bc 0.30b 0.21bc 0.083ab 0.037ab 0.021ab

Acacia+rice–berseem 0.77a 0.36a 0.27ab 0.090c 0.043a 0.033a

Eucalyptus 0.48d 0.19e 0.14e 0.058c 0.029ab 0.017b

Eucalyptus+rice–berseem 0.67bc 0.22d 0.18cde 0.073b 0.035ab 0.019b

Populus 0.50d 0.20de 0.19cd 0.060c 0.031b 0.018b

Populus+rice–berseem 0.71b 0.36a 0.28a 0.088a 0.042a 0.025ab

LSD(p<0.05) 0.034 0.038 0.033 0.010 0.011 0.008

2 m Distance

Acacia 0.57b _0.20ab _0.18ab _0.065ab _0.025abc _0.018a

Acacia+rice–berseem 0.64a _0.22a _0.14bc _0.077a _0.030a _0.022a

Eucalyptus 0.38c _0.15c _0.12e _0.035bc _0.015abc _0.014a

Eucalyptus+rice–berseem 0.55b _0.19bc _0.15abc _0.050bc _0.021bc _0.017a

Populus 0.42c _0.16bc _0.14bc _0.049bc _0.019bc _0.015a

Populus+rice–berseem 0.58b _0.21a _0.19a _0.070a _0.028ab _0.021a

LSD(p<0.05) 0.029 0.029 0.028 0.014 0.009 0.010

3 m Distance

Acacia 0.48a _0.13a _0.11a _0.057a _0.019ab _0.016ab

Acacia+rice–berseem 0.50a _0.15a _0.13a _0.064a _0.023a _0.019a

Eucalyptus 0.29b _0.14a _0.11a _0.026b _0.012ab _0.011b

Eucalyptus+rice–berseem 0.41b _0.12a _0.10a _0.041b _0.016ab _0.014ab

Populus 0.32c _0.12a _0.10a _0.032b _0.015ab _0.012ab

Populus+rice–berseem 0.49a _0.14a _0.12a _0.061a _0.020a _0.017ab

LSD(p<0.05) 0.044 0.029 0.028 0.012 0.007 0.006

a_{For each column, values not marked with the same letter are significantly different (DMRT; p}_<_0.05).

Averaged across the sampling dates, microbial biomass C and N varied from 76.1 to 153.4mg C g−1

soil and 11.1 to 32.6mg N g−1 soil, respectively

(Table 2). The tree species had a significant effect on the levels of microbial carbon and nitrogen. The

Table 2

Soil microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) averaged across the seasons and their relationships with total soil carbon and nitrogen in different management systemsa

Treatments MBC (mg g−1 soil) MBN (mg g−1soil) Microbial C:N MBC Soil C (%) MBN Soil N (%)

Control 76.13f _11.06e _6.88a _1.21c _1.51d

Rice–berseem 96.14e _18.61cd _5.16cd _2.29cd _4.04ab

Acacia 143.40b _31.45a _4.56d _2.31b _4.08ab

Acacia+rice–berseem 153.40a _32.57a _4.71d _2.26d _3.79bc

Eucalyptus 109.12d _15.78de _6.92a _2.32ab _3.36c

Eucalyptus+rice–berseem 133.80c _21.78bc _6.14ab _2.23b _3.69bc

Populus 131.10c _21.70c _6.04bc _2.57a _4.48a

Populus+rice–berseem 150.13a _25.62b _5.86bc _2.24b _3.28c

LSD(p<0.05) 4.86 3.38 0.731 0.194 0.525

a_{For each column, values not marked with the same letter are significantly different (DMRT; p}_<_0.05).

integration of trees with crops had a favorable effect on soil microbial biomass as it showed an increase of 7, 23 and 15% in the cases of Acacia, Eucalyp-tus and Populus based systems, respectively. On the average, microbial biomass was found to increase by

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Fig. 1. Seasonal variation in soil microbial biomass Carbon (a) and Nitrogen (b) in different treatments (C=control; RB=rice–berseem; An=Acacia nilotica; An+RB=Acacia nilotica+rice–berseem; Et=Eucalyptus tereticornis; Et+RB=Eucalyptus tereticornis+rice–berseem; Pd=Populus deltoides; Pd+RB=Populus deltoides+rice–berseem).

42% (microbial C) and 13% (microbial N) in the case of tree-based systems as compared to monocropping. The ratio of microbial C:microbial N varied from 6.92 to 5.86 in Eucalyptus and Populus based systems, whereas in Acacia based systems the microbial C:N was in the range of 4.56–4.71.

The proportions of microbial carbon and nitrogen to total soil C and N were higher in the case of mono-cultures indicating higher C and N immobilization in microbial biomass (Table 2). The ratio of microbial C to total soil carbon was comparatively higher in mono-cultures of Eucalyptus and Populus (2.32–2.57%). The proportion of microbial N to total soil N varied from 4.08 to 4.48% in Acacia and Populus plantation.

The effect of soil depth on biomass carbon was sig-nificant, reflecting the steep decline in soil organic matter content in sub-surface layers of soil (Table 3).

Distance from trees also had a significant effect on mi-crobial biomass C and N at 0–7.5 cm and 7.5–15 cm soil depths (p<0.01). Significantly higher amounts of microbial biomass carbon and nitrogen were found in soil at 1 m from the trees than at 2 and 3 m. Aver-aged across the distance from the trees, the microbial biomass carbon and nitrogen were found to be higher under systems with Acacia (125 –136mg C g−1 soil;

25–27mg N g−1 soil) and Populus (95–133mg C g−1

soil; 17–23mg N g−1 soil) (Table 3). In general, the

microbial biomass carbon was significantly higher in agroforestry as compared to tree plantations. Micro-bial biomass nitrogen did not differ significantly be-tween soils under trees alone and agroforestry systems (Table 3).

At 0–7.5 cm soil depth, the C:N ratio of the mi-crobial biomass was 5.10–6.20. The proportion of

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Table 3

Variations in soil microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) with soil depths in different treatmentsa

Treatments MBC (mg C g−1 soil) MBN (mg N g−1 soil) MC:MN

0–7.5 (cm soil depth)

Control 51.54f 8.51d 6.06a

Rice–berseem 97.20d 17.20bc 5.65a

Acacia 125.47b 24.61a 5.10a

Acacia+rice–berseem 136.21a 26.55c 5.13a

Eucalyptus 87.21e 14.07c 6.20a

Eucalyptus+rice–berseem 106.76c 17.72c 6.02a

Populus 94.84de 16.57bc 5.72a

Populus+rice–berseem 133.06ab 22.51ab 5.91a

LSD(p<0.05) 7.42 4.76 1.35

7.5–15 (cm soil depth)

Control 36.33e _5.78c _6.28a

Rice–berseem 40.57de _7.44abc _5.45a

Acacia 50.08b _9.72ab _5.15a

Acacia+rice–berseem 57.56a _10.65a _5.40a

Eucalyptus 37.68e 5.71c 6.60a

Eucalyptus+rice–berseem 43.98cd 6.99bc 6.29a

Populus 37.54e 6.85bc 5.48a

Populus+rice–berseem 48.13bc 8.00abc 6.02a

LSD(p<0.05) 4.25 2.87 2.16

15–30 (cm soil depth)

Control 10.63c _1.52b _6.99a

Rice–berseem 19.20ab _3.38a _5.68a

Acacia 22.55a _3.89a _5.80a

Acacia+rice–berseem 22.38a _3.93a _5.69a

Eucalyptus 17.01b _2.45ab _6.94a

Eucalyptus+rice–berseem 17.22b _2.63ab _6.55a

Populus 15.64b _2.59ab _6.04a

Populus+rice–berseem 21.77a _3.56a _6.11a

LSD(p<0.05) 3.42 1.25 1.74

a_{For each column, values not marked with same letter are significantly different (DMRT; p}_<_0.05).

microbial carbon to total soil carbon varied from 1.98 to 2.59% (trees with or without crops) and from 1.45 to 2.06% (rice–berseem cropping). Microbial N ac-counted for 2.61–3.60% of the total soil nitrogen in the surface layers of soil (0–15 cm). With increasing depth, the proportion of microbial C and N in the to-tal soil organic pool decreased appreciably, the values being 1.15–1.42% for C and 1.42–1.60% for N.

The proportion of microbial C and N to soil organic C and N was found to be greater in tree-based systems. Immobilization of nitrogen was 2.61–3.60% (0–15 cm soil depth) in soils receiving organic inputs in the form of litterfall and fine roots from the trees. In the case of rice–berseem cropping or the fallow, relatively less nitrogen was immobilized in microbial biomass in the soil.

3.3. Carbon dioxide evolution and biomass specific activity

The CO2-C production from the soil varied with

tree species, soil depth and distance from trees. CO2-C production decreased significantly with

in-crease in soil depth (p<0.01). For the various treat-ments, carbon dioxide evolution rates for the three soil depths (mg CO2-C g−1 soil per day) were:

10.75–21.73 (0–7.5 cm); 2.92–5.78 (7.5–15 cm) and 1.94–3.00 (15–30 cm). The CO2 evolution rates

were significantly higher at 1 m from the rows of trees than at 2 m for the soil from 0–7.5 cm depth (p<0.05). With increase in soil depth, the effect of distance from the trees on CO2-C production was not

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Table 4

Specific soil microbial activity (mg CO2-C produced mg−1 MBC per day) in soils of different management systems

Treatments Distance (m) mg CO2-C produced mg−1MBC per day LSD(p<0.05)

0–7.5 (cm) 7.5–15 (cm) 15–30 (cm)

Control – 208.57 85.60 218.25 35.98

Rice–berseem – 111.94 71.97 39.55 40.36

Acacia 1 122.90 52.81 98.29 14.43

2 105.85 67.44 113.40 17.18

Acacia+rice–berseem 1 125.31 59.17 108.10 29.07

2 123.59 93.31 120.03 17.20

Eucalyptus 1 118.91 57.96 108.24 42.78

2 106.23 67.87 117.29 27.03

Eucalyptus+rice–berseem 1 118.95 56.81 115.21 16.78

2 96.63 64.96 123.61 33.58

Populus 1 114.61 65.23 112.30 19.76

2 100.70 67.67 132.38 43.72

Populus+rice–berseem 1 119.43 72.25 100.07 11.03

2 103.62 93.93 119.47 18.80

LSD(p<0.05) 1 14.74 19.84 41.13

2 13.32 22.39 36.23 –

Microbial biomass specific respiratory activity did not vary significantly with tree species (Table 4). Spe-cific respiratory activity was comparatively greater at 0–7.5 and 15–30 cm than at 7.5–15 cm. Biomass spe-cific respiratory activity was higher in the control soil than in the tree plantations and agroforestry systems. 3.4. Soil inorganic N and nitrogen mineralization

There were significant variations in levels of soil inorganic nitrogen due to treatments (p<0.01, Fig. 2a). During all three seasons, soil inorganic N (15.89–17.37mg N g−1 soil) was maximum in the

case of Acacia+rice–berseem. The levels of soil in-organic N were 41–74, 8–30 and 32–42% higher, respectively, for Acacia, Eucalyptus and Populus based agroforestry systems than for rice–berseem. Soil inorganic N showed appreciable increases in the agroforestry systems as compared with the tree plantations. It is interesting to note that this increase was greater in the case of the Eucalyptus+crop sys-tem (17–47%) than in the Acacia+crop (7–13%) or Populus+crop system (6–16%).

There were significant variations in nitrogen min-eralization due to tree species, soil depth and seasons (p<0.01) (Table 5). Nitrogen mineralization was max-imum in July (6.69–31.03mg g−1soil) and minimum

in December (1.55–4.26mg g−1soil) (Fig. 2b).

Maxi-mum nitrogen mineralization during the rainy season corresponded with the active growth period of plants. In winter months, the presence of dead roots in soil cores together with low temperature caused low rates of N-mineralization. Nitrogen mineralization rates for different agroforestry systems(mg g−1soil) were:

Aca-cia (4.26–31.03), Populus (3.84–26.04) and Eucalyp-tus (3.13–22.87). In the agroforestry systems nitrogen mineralization increased by 12–37% as compared to monocultures.

On the basis of seasonal N mineralization rates, to-tal annual nitrogen release has been calculated. Toto-tal N release was highest in the Acacia+rice–berseem system (312.90 kg N ha−1_{) followed by Acacia alone}

(297.14 kg N ha−1_{) and the other systems studied}

(182.54–268.58 kg ha−1_{). Annual release of}

nitro-gen was 13–95% higher under tree-based systems as compared to monocropping. Total annual nitrogen mineralization accounted for 24–31% of total soil N in tree/tree-cropping systems, 23% in rice–berseem cropping and 6.36% in the control treatment.

Due to higher biological activity, N mineraliza-tion rates were highest at 0–7.5 cm and decreased with increasing soil depth (Table 5). The rates were highest under Acacia (38.17–40.60) and Pop-ulus (28.31–39.12). The mineral nitrogen (NH4+-N

and NO3−-N) concentrations under the various

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Fig. 2. Variations in (a) initial levels of soil inorganic N and (b) nitrogen mineralization rates in soils of different treatments (C=control; RB=rice–berseem; An=Acacia nilotica; An+RB=Acacia nilotica+rice–berseem; Et=Eucalyptus tereticornis; Et+RB=Eucalyptus tereticornis+rice–berseem; Pd=Populus deltoides; Pd+RB=Populus deltoides+rice–berseem).

Table 5

Variations in nitrogen mineralization rates at various soil depths in control, cropping system, tree plantation and agroforestry systems N-mineralization (mg N g−1 soil) LSD(p<0.05)

Treatments 0–7.5 (cm) 7.5–15 (cm) 15–30 (cm)

Control 7.39 3.02 1.71 0.87

Rice–berseem 23.02 12.96 4.16 0.78

Acacia 38.17 15.28 5.68 1.34

Acacia+rice–berseem 40.60 16.91 5.89 2.06

Eucalyptus 22.06 10.67 4.77 2.41

Eucalyptus+rice–berseem 30.99 14.74 5.54 1.03

Populus 28.31 13.81 5.65 0.96

Populus+rice–berseem 39.42 15.27 5.69 1.19

LSD(p<0.05) 1.38 1.03 0.96 –

Table 6

Variations in levels of NH4+-N and NO3−-N (mg g−1 soil) at two distances from rows of trees (averaged across the soil depths) in cropping, forestry and agroforestry systems

Treatments NH4+-N NO3−-N NO3−-N/NH4+-N

1 (m) 3 (m) LSD∗ _{1 (m)} _{3 (m)} _LSD∗ _{1 (m)} _{3 (m)} _LSD∗

Control 2.22 – – 7.53 – – 3.39 – –

Rice–berseem 2.19 – – 7.71 – – 3.52 – –

Acacia 7.51 5.30 1.46 3.75 5.36 2.07 0.49 1.01 0.17

Acacia+rice–berseem 9.22 6.22 1.87 4.15 6.00 2.29 0.45 0.96 0.13

Eucalyptus 6.98 4.63 2.99 3.00 4.23 1.61 0.42 0.91 0.24

Eucalyptus+rice–berseem 7.66 5.01 1.95 3.25 4.49 1.86 0.42 0.89 0.19

Populus 7.08 4.34 2.35 3.43 4.95 2.51 0.48 1.14 0.33

Populus+rice–berseem 8.99 5.02 2.25 4.00 5.76 1.99 0.44 1.15 0.43

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greater amounts of ammonium were present and the NO3−-N/NH4+-N ratios were significantly lower

closer to the trees (p<0.05). The consistently lower NO3−-N/NH4+-N ratio near the trees (0.42–0.49)

than far from trees (0.89–1.11) in all the treatments indicated an efficient uptake of nitrate-N by tree roots.

4. Discussion

The integration of trees with crops on moderately alkali soils improved soil carbon and total nitrogen status. This was due to organic matter inputs in the form of litter fall and fine roots from the tree species. The level of increase in soil organic matter was influ-enced by the tree species as there were marked varia-tions in the quantity and quality of litterfall among the tree species (Kaur, 1998). A. nilotica alone or in com-bination with crops caused greater improvement of soil organic carbon content closely followed by Pop-ulus and Eucalyptus based systems. While studying the organic matter dynamics in these systems, Kaur (1998) found that the litter accumulation on the ground floor was higher in Acacia based systems (3465 kg dry matter ha−1_{per year) as compared to agroforestry}

systems with Populus (2722 kg dry matter ha−1 _per

year) and Eucalyptus (2107 kg dry matter ha−1 _per

year). Besides litterfall, the fine roots also contributed to soil organic matter, which accounted for 27–23% of the total dry matter input to the soil. Fine root biomass in the surface soil layers close to the trees varied from 1.0 to 2.4 Mg ha−1_{per year. Various}

stud-ies on sodic soils have shown appreciable build up of organic carbon status of alkali soils when planted to trees (Singh and Gill, 1992; Singh et al., 1994; Bhojvaid et al., 1996). In the control plot, high or-ganic matter content could be due to undisturbed soil conditions and greater stability of soil organic mat-ter as contributed by the litmat-ter and roots of the native vegetation.

Soil microbial biomass and its activity increased due to the ameliorative effects of trees and organic matter inputs. According to Rao and Pathak (1996), carbon is an important factor influencing microbial activity in alkali soils. In the agroforestry systems, microbial C and N increased by 10–60 and 17–43%, respec-tively. It seems likely that litter quality regulated the

levels of soil microbial biomass in tree-based systems. The leaf litter of Acacia and Populus were character-ized by medium N concentration (1.52–2.51%) and low lignin content (6.13–9.0%), favoring the growth of organisms and resulting in higher soil micro-bial biomass (Kaur, 1998).

The maintenance of a large microbial biomass pool has been reported to be dependent on soil organic matter (Powlson et al., 1987). The availability of car-bonaceous materials and substrates such as sugars, amino acids and organic acids to the soil from the roots is important for supplying maintenance energy for microbial populations (Bowen and Rovira, 1991). The importance of root exudates in maintaining a larger microbial biomass closer to the trees has also been reported by Browaldh (1997). The rapid de-cline in microbial biomass with depth in the soil and distance from the trees in the present study could be attributed to the quantity and quality of organic inputs.

An increased proportion of microbial carbon and nitrogen in the total soil organic carbon pool of agro-forestry systems indicates greater potential nutrient availability to the plants. Carbondioxide evolution rates and biomass specific respiratory activity were influenced by organic carbon build-up in the soil under different management systems. Sarig and Stein-berger (1994) reported that microbes invest more energy in microbial metabolism to overcome salt stress at high soil pH. At moderate soil pH, it may be the quantity and quality soil organic matter which have a dominating influence on carbon mineraliza-tion, specific microbial activity and soil nitrogen availability.

Higher N availability under agroforestry systems could be attributed to high organic inputs from the trees. For certain tropical agroforestry systems, Haggar et al. (1993) showed that higher rates of ni-trogen mineralization resulted from the build up of readily mineralizable organic N in the soil over 7 years of tree mulch application. In this study, maxi-mum rates of N-mineralization under A. nilotica may be attributed to high foliar N and rapid circulation of N through the litterfall. The ratio of NO3−-N and

NH4+-N was low closer to trees indicating an efficient

uptake of nitrate by trees. Higher rates of N mineral-ization close to the trees may be due to the rhizospheric effects.

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5. Conclusions

1. The integration of trees with crops improved the productivity and fertility of moderately alkaline soils. The agroforestry systems improved soil con-ditions by increasing total soil organic matter, soil nitrogen availability and biological activity after 6–7 years of tree growth.

2. Organic matter inputs from trees in the form of lit-terfall and fine roots contributed to increased soil organic matter content. The organic matter inputs into the soil for different tree species was in the order: Acacia>Populus>Eucalyptus. Soil microbial biomass and nitrogen mineralization rates showed appreciable increase in agroforestry systems. An increased proportion of microbial carbon and ni-trogen in the total soil organic pool indicate higher nutrient availability to the plants.

3. On the basis of soil microbial parameters, agro-forestry has been found to be a viable land-use option for utilizing moderately alkaline soils for meeting the needs of food, fodder and fuelwood as well as for maintaining soil fertility.

Acknowledgements

Financial assistance in the form of Senior Research Fellowship to B. Kaur from the Council of Scien-tific and Industrial Research, New Delhi is gratefully acknowledged.

References

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Table 3

Variations in soil microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) with soil depths in different treatmentsa

Treatments MBC (mg C g−1 soil) MBN (mg N g−1 soil) MC:MN

0–7.5 (cm soil depth)

Control 51.54f 8.51d 6.06a

Rice–berseem 97.20d 17.20bc 5.65a

Acacia 125.47b 24.61a 5.10a

Acacia+rice–berseem 136.21a 26.55c 5.13a

Eucalyptus 87.21e 14.07c 6.20a

Eucalyptus+rice–berseem 106.76c 17.72c 6.02a

Populus 94.84de 16.57bc 5.72a

Populus+rice–berseem 133.06ab 22.51ab 5.91a

LSD(p<0.05) 7.42 4.76 1.35

7.5–15 (cm soil depth)

Control 36.33e _5.78c _6.28a

Rice–berseem 40.57de _7.44abc _5.45a

Acacia 50.08b _9.72ab _5.15a

Acacia+rice–berseem 57.56a _10.65a _5.40a

Eucalyptus 37.68e 5.71c 6.60a

Eucalyptus+rice–berseem 43.98cd 6.99bc 6.29a

Populus 37.54e 6.85bc 5.48a

Populus+rice–berseem 48.13bc 8.00abc 6.02a

LSD(p<0.05) 4.25 2.87 2.16

15–30 (cm soil depth)

Control 10.63c _1.52b _6.99a

Rice–berseem 19.20ab _3.38a _5.68a

Acacia 22.55a _3.89a _5.80a

Acacia+rice–berseem 22.38a _3.93a _5.69a

Eucalyptus 17.01b _2.45ab _6.94a

Eucalyptus+rice–berseem 17.22b _2.63ab _6.55a

Populus 15.64b _2.59ab _6.04a

Populus+rice–berseem 21.77a _3.56a _6.11a

LSD(p<0.05) 3.42 1.25 1.74

a_{For each column, values not marked with same letter are significantly different (DMRT; p<0.05).}

3.3. Carbon dioxide evolution and biomass specific activity

The CO2-C production from the soil varied with

tree species, soil depth and distance from trees.

CO2-C production decreased significantly with

in-crease in soil depth (p<0.01). For the various

treat-ments, carbon dioxide evolution rates for the three

soil depths (mg CO2-C g−1 soil per day) were:

10.75–21.73 (0–7.5 cm); 2.92–5.78 (7.5–15 cm) and

1.94–3.00 (15–30 cm). The CO2 evolution rates

were significantly higher at 1 m from the rows of trees than at 2 m for the soil from 0–7.5 cm depth

(p<0.05). With increase in soil depth, the effect of

distance from the trees on CO2-C production was not

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Treatments Distance (m) mg CO2-C produced mg−1MBC per day LSD(p<0.05)

0–7.5 (cm) 7.5–15 (cm) 15–30 (cm)

Control – 208.57 85.60 218.25 35.98

Rice–berseem – 111.94 71.97 39.55 40.36

Acacia 1 122.90 52.81 98.29 14.43

2 105.85 67.44 113.40 17.18

Acacia+rice–berseem 1 125.31 59.17 108.10 29.07

2 123.59 93.31 120.03 17.20

Eucalyptus 1 118.91 57.96 108.24 42.78

2 106.23 67.87 117.29 27.03

Eucalyptus+rice–berseem 1 118.95 56.81 115.21 16.78

2 96.63 64.96 123.61 33.58

Populus 1 114.61 65.23 112.30 19.76

2 100.70 67.67 132.38 43.72

Populus+rice–berseem 1 119.43 72.25 100.07 11.03

2 103.62 93.93 119.47 18.80

LSD(p<0.05) 1 14.74 19.84 41.13

2 13.32 22.39 36.23 –

3.4. Soil inorganic N and nitrogen mineralization

There were significant variations in levels of

soil inorganic nitrogen due to treatments (p<0.01,

Fig. 2a). During all three seasons, soil inorganic N

(15.89–17.37mg N g−1 _{soil) was maximum in the}

case of Acacia+rice–berseem. The levels of soil

in-organic N were 41–74, 8–30 and 32–42% higher, respectively, for Acacia, Eucalyptus and Populus based agroforestry systems than for rice–berseem. Soil inorganic N showed appreciable increases in the agroforestry systems as compared with the tree plantations. It is interesting to note that this increase was greater in the case of the Eucalyptus+crop sys-tem (17–47%) than in the Acacia+crop (7–13%) or

Populus+crop system (6–16%).

There were significant variations in nitrogen min-eralization due to tree species, soil depth and seasons

(p<0.01) (Table 5). Nitrogen mineralization was

max-imum in July (6.69–31.03mg g−1_{soil) and minimum}

in December (1.55–4.26mg g−1_{soil) (Fig. 2b).}

different agroforestry systems(mg g−1_{soil) were:}

Aca-cia (4.26–31.03), Populus (3.84–26.04) and Eucalyp-tus (3.13–22.87). In the agroforestry systems nitrogen

mineralization increased by 12–37% as compared to monocultures.

On the basis of seasonal N mineralization rates, to-tal annual nitrogen release has been calculated. Toto-tal

N release was highest in the Acacia+rice–berseem

system (312.90 kg N ha−1_{) followed by Acacia alone}

(297.14 kg N ha−1_{) and the other systems studied}

(182.54–268.58 kg ha−1_{). Annual release of}

Due to higher biological activity, N mineraliza-tion rates were highest at 0–7.5 cm and decreased with increasing soil depth (Table 5). The rates were highest under Acacia (38.17–40.60) and

Pop-ulus (28.31–39.12). The mineral nitrogen (NH4+-N

and NO3−-N) concentrations under the various

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Table 5

Variations in nitrogen mineralization rates at various soil depths in control, cropping system, tree plantation and agroforestry systems

N-mineralization (mg N g−1 soil) LSD(p<0.05)

Treatments 0–7.5 (cm) 7.5–15 (cm) 15–30 (cm)

Control 7.39 3.02 1.71 0.87

Rice–berseem 23.02 12.96 4.16 0.78

Acacia 38.17 15.28 5.68 1.34

Acacia+rice–berseem 40.60 16.91 5.89 2.06

Eucalyptus 22.06 10.67 4.77 2.41

Eucalyptus+rice–berseem 30.99 14.74 5.54 1.03

Populus 28.31 13.81 5.65 0.96

Populus+rice–berseem 39.42 15.27 5.69 1.19

LSD(p<0.05) 1.38 1.03 0.96 –

Table 6

Variations in levels of NH4+_{-N and NO3}−_{-N (}

mg g−1 soil) at two distances from rows of trees (averaged across the soil depths) in

cropping, forestry and agroforestry systems

Treatments NH4+

-N NO3−

-N/NH4+

-N

1 (m) 3 (m) LSD∗ _{1 (m)} _{3 (m)} _LSD∗ _{1 (m)} _{3 (m)} _LSD∗

Control 2.22 – – 7.53 – – 3.39 – –

Rice–berseem 2.19 – – 7.71 – – 3.52 – –

Acacia 7.51 5.30 1.46 3.75 5.36 2.07 0.49 1.01 0.17

Acacia+rice–berseem 9.22 6.22 1.87 4.15 6.00 2.29 0.45 0.96 0.13

Eucalyptus 6.98 4.63 2.99 3.00 4.23 1.61 0.42 0.91 0.24

Eucalyptus+rice–berseem 7.66 5.01 1.95 3.25 4.49 1.86 0.42 0.89 0.19

Populus 7.08 4.34 2.35 3.43 4.95 2.51 0.48 1.14 0.33

Populus+rice–berseem 8.99 5.02 2.25 4.00 5.76 1.99 0.44 1.15 0.43

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NO3−-N/NH4+-N ratios were significantly lower

closer to the trees (p<0.05). The consistently lower

NO3−-N/NH4+-N ratio near the trees (0.42–0.49)

than far from trees (0.89–1.11) in all the treatments indicated an efficient uptake of nitrate-N by tree roots.

4. Discussion

Pop-ulus and Eucalyptus based systems. While studying

the organic matter dynamics in these systems, Kaur (1998) found that the litter accumulation on the ground floor was higher in Acacia based systems (3465 kg

dry matter ha−1_{per year) as compared to agroforestry}

systems with Populus (2722 kg dry matter ha−1 _per

year) and Eucalyptus (2107 kg dry matter ha−1 _per

varied from 1.0 to 2.4 Mg ha−1_{per year. Various}

The leaf litter of Acacia and Populus were character-ized by medium N concentration (1.52–2.51%) and low lignin content (6.13–9.0%), favoring the growth of organisms and resulting in higher soil micro-bial biomass (Kaur, 1998).

of N through the litterfall. The ratio of NO3−-N and