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 0–30 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 (2–4.75 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.