Nutrient management impacts on organic carbon pool in soils under different cropping systems in the Indo-Gangetic Plains in South Asia
Pritpal Singh, Bijay Sıngh, Bhupinder S. Farmaha
Abstract
Nutrient management impacts the dynamics of organic carbon (C), C sequestration and various C pools in soils and sustainability of cropping systems through biomass input and organic matter addition through organic manures. We reviewed long-term field experiments on different cropping systems in the Indo-Gangetic Plains of South Asia to study the effect of balanced and imbalanced application of mineral fertilizers applied either alone or conjointly with organic manures on different soil organic C pools. Application of mineral fertilizers not only enhanced crop productivity, but also C input and accumulation in the total organic C (TOC) pool with a significant impact on C fractions of differential lability or oxidizability. Balanced use of nitrogen, phosphorus, and potassium through fertilizers alone, or conjointly with organic manures or crop residues enhanced plant-mediated C input, TOC pool, and stocks, amount of biomass C required to maintain TOC levels, net C sequestration rate, and C fractions such as permanganate oxidizable C, water-extractable organic C, microbial biomass C, and C fractions of variable oxidizability. The amount of C input (plant mediated + exogenous) added was 0.88–12.2 Mg C ha−1 year−1 in the 0–15 cm plough layer soils under rice–wheat, 1.37–6.68 Mg C ha−1 year−1 under other rice-based systems (rice–berseem/rice–wheat–jute/rice–mustard–sesame/rice–fallow–rice) and 0.51–8.07 Mg C ha−1 year−1 in soils under non-rice-based cropping systems. In general, C sequestration rate was high in soils with low initial TOC content and high silt + clay fraction. Balanced application of mineral fertilizers conjointly with organic manures enhanced the percentage of macro-aggregates (> 0.25 mm), with simultaneous decrease in the percentage of micro-aggregates (< 0.25 mm). The stable or passive C pool (less labile + recalcitrant C) was the largest pool, comprising of about 50.5–80.3% of TOC in soils under different rice–wheat systems; almost similar (44.2–80.8% of TOC) to that for maize–wheat, but higher compared with those under cotton–wheat (44.1–61.9% of TOC) cropping system. The amount of C input required to maintain TOC stocks at steady state varied between 2.30 and 4.59 Mg C ha−1 year−1 for soils under rice-based systems, as compared with 1.10 and 3.47 Mg C ha−1 year−1 in soils under non-rice-based cropping systems. On an overall basis, balanced application of mineral fertilizers conjointly with organic manures exhibited an overwhelming role in enhancing C accumulation in recalcitrant C pool, while enlarging the labile C pool for increased crop productivity due to increased nutrient cycling.