284 B. Kaur et al. Applied Soil Ecology 15 2000 283–294
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 implications 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 experimental 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.,
B. Kaur et al. Applied Soil Ecology 15 2000 283–294 285
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
4
was 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 K
2
SO
4
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 K
2
SO
4
soil 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=EN0.54 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
286 B. Kaur et al. Applied Soil Ecology 15 2000 283–294
incubated in 2 l air tight jars at room temperature 27–35
◦
C for 30 days. The CO
2
produced 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 CO
2
evolution rates were determined at an interval of 2–5 days. Car-
bon mineralization rates were determined from CO
2
-C evolved from the soil samples. Specific respiratory ac-
tivity of soil microbial biomass carbon was estimated by dividing the CO
2
-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 CO
2
-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 K
2
SO
4
4 K
2
SO
4
: 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 K
2
SO
4
. 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 p0.05 and Duncan’s Multiple Range Test values DMRT were calculated following Gomez and
Gomez 1984.
3. Results