Functional groups and mineralization kinetics of soil organic matter under contrasting hydro-thermal regimes under conservation agriculture-based rice-wheat system in eastern Indo-Gangetic Plains

Soil organic carbon (SOC) sequestration is important to counteract anthropogenic climate change at the global level. Studying the effect of conservation agriculture (CA) on SOC dynamics in the presence of two distinctively different hydro-thermal regimes across the year in rice- wheat (RW) systems in eastern Indo-Gangetic Plains (E- IGP) is of topical interest. The stabilization mechanism of soil organic matter (SOM), and its effect on C mineralization kinetics in these conditions is not well understood. We collected soil samples from six combinations of CA and conventional farming in an ongoing experiment at CIMMYTBorlaug Institute for South Asia, Bihar, India. CA enhanced SOC, specifically labile C, while decreasing mineral N, as a result of encapsulation/assimilation in aggregates/microbial biomass. Humic acids registered characteristic Fourier transform infrared (FTIR) peaks at 3200- 3600 cm(-1), 2920-2930 cm(-1), 1645- 1655 cm(-1) and 1220- 1240 cm(-1), and displayed lesser degree of humification, aromaticity and redox status under CA. SOC and N mineralization were studied in two different hydro-thermal regimes pertaining to rice (submergence, 35 degrees C; SM35) and wheat (field capacity, 25 degrees C; FC25) growing periods of E- IGP. SM35 displayed temperature-mediated higher decay of SOC. Decay constants of C mineralization were lesser under CA compared with CT. CA promoted higher SOM stability, evidenced by lower decay rates of SOC and N, attributed to (1) better protection of SOM in well-aggregated soil structure and (2) an excess supply of fresh crop residues ensuring higher rate of SOM formation than its decay. Practising CA in E- IGP is imperative towards C-neutral agriculture, especially in the impending global warming scenario.