Soil aggregation and organic matter in a sandy clay loam soil of the Indian Himalayas under different tillage and crop regimes

in agricultural systems, maintenance of soil organic C (SOC) has long been recognized as a strategy to reduce soil degradation. Management practices, such as conservation tillage and legume-based cropping sequences, have the potential to enhance SOC and total soil N (TSN) content and improve soil aggregation. We examined the effects of three tillage systems [conventional tillage (CT), minimum tillage (MT) and zero tillage (ZT)] and three crop rotations [soybean-wheat (S-W), soybean-lentil (S-L) and soybean-pea (S-P)] on SOC and TSN storage and their distribution within aggregate-size fractions in Indian Himalayas, where almost all above-ground crop residues were removed. A field experiment was conducted on a sandy clay loam soil (Typic Haplaquept; Eutric Cambisols) from 1999 to 2003 near Almora, India. Results indicate ZT significantly (P < 0.05) increased SOC and TSN storage over CT in the 0-15 cm, depth by 10.2 and 17.2%, respectively. Plots under S-L and S-P (continuous leguminous cropping) had 10.7 and 13.1% higher SOC content than S-W plots in the surface soil layer (0-15 cm depth). However, both tillage and crop rotation had no impact on the sub-surface (15-30 cm) soil layer or 030 cm soil profile as a whole. On an equivalent initial soil mass basis, SOC storage to about 15 cm after four years was 26.0 Mg ha(-1) in continuous NT plots, but just 23.9 Mg ha-1 in continuous CT. Zero tillage increased bulk density, mean weight diameter and the proportion of macroaggregate fractions (24.75 mm, 0.25-2.0 mm) in soil compared with CT in the surface soil layer, but not in the sub-surface soil layer. In the surface soil layer, ZT significantly (P < 0.05) increased SOC concentration compared with CT in all aggregate-size fractions. Crop rotation had no effect on aggregate associated-SOC in both soil layers. Plots under S-P and S-L rotations had higher SOC concentration only for whole soil (<4.75 mm fraction) than S-W rotation in the 0-15 cm soil depth. Impacts of ZT included a greater proportion of macroaggregate (2.0-4.75 mm size fraction)-associated TSN than MT and CT and higher aggregate-associated TSN in ZT plots than CT within all aggregate-size fractions (except <0.053 mm size fraction) in the surface soil layer (0-15 cm) only. Plots under S-L and S-P rotations increased TSN concentration compared with S-W by about 19 and 21% for whole soil (<4.75 mm), in the 0-15 cm soil. Thus, short-term conservation tillage and continuous leguminous cropping under rainfed conditions improved SOC and TSN storage in the soil Surface in the Indian Himalayas. (C) 2009 Elsevier B.V. All rights reserved.