292 B. Kaur et al. Applied Soil Ecology 15 2000 283–294
greater amounts of ammonium were present and the NO
3 −
-NNH
4 +
-N ratios were significantly lower closer to the trees p0.05. The consistently lower
NO
3 −
-NNH
4 +
-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 litter 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 micro-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 NO
3 −
-N and NH
4 +
-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.
B. Kaur et al. Applied Soil Ecology 15 2000 283–294 293
5. Conclusions
