Green manuring and crop residue management: Effect on soil organic carbon stock, aggregation, and system productivity in the foothills of Eastern Himalaya (India)

Improving soil quality, especially the organic carbon (C) accumulation and soil aggregation through economically and environmentally sustainable alternatives, is the need for hill agriculture. A five-year (2012–13 to 2016–17) field experiment was conducted with four green manure treatments [green gram (Vigna radiata); cowpea (Vigna unguiculata); sesbania (Sesbania aculeata) along with non-green manuring] and three cropping systems [groundnut (Arachis hypogaea) –pea (Pisum sativum); maize (Zea mays) –pea, and maize + groundnut – pea] involving two levels of residue management practices [residue removal and residue retention]. The experiment was laid out in a split–split plot design and replicated three times. After five years of experimentation, the results showed that among green manure treatments, Sesbania had a significant effect (p < 0.05) on total soil organic C, C pool, and surface (0–0.15 m) soil aggregation. The increase in C pool (p < 0.05) in Sesbania-treated soil was 43.2% in the very labile C, followed by 40.8% in the labile C relative to those without green-manure. In addition, the profile (0–0.45 m depth) weighted average organic C stocks in Sesbania-treated soils were higher in active (+29.3%), passive (+20.8%), and total (TOC: + 24.1%) forms than in non-green manure soils. Sesbania-treated soils also exhibited a higher C-Lability index, water stable macro-aggregates, and a higher aggregate ratio (AR) than non-green manure soils. Among the three cropping systems studied, inter-cropping of maize and groundnut followed by peas increased (p < 0.05) the stock of C pools, water-stable aggregates, and ARs in soil relative to the other two cropping systems. Residue retention also increased C pools, C-Lability Index, and microbial biomass C in soil compared to residue removal. Therefore, the incorporation of Sesbania into maize+groundnut-pea cropping systems with residue retention is a recommended option to improve soil quality and productivity of maize cropping systems in the Indian Himalayan region.