Applied Soil Ecology 15 2000 283–294
Soil carbon, microbial activity and nitrogen availability in agroforestry systems on moderately alkaline soils in northern India
B. Kaur
a
, S.R. Gupta
a,∗
, G. Singh
b
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 96.14 mg g
− 1
soil and increased in soils under tree plantations 109.12–143.40 mg g
− 1
soil and agrisilvicultural systems 133.80–153.40 mg g
− 1
soil. 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 CO
2
-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.: +91-1744-20196501; fax: +91-1744-20277.
E-mail address: kurudoe.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
0929-139300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 9 - 1 3 9 3 0 0 0 0 0 7 9 - 2
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
