Changes in soil aggregate-associated organic carbon, enzymatic activity, and biological pools under conservation agriculture based practices in rice–wheat system

Abstract

Crop residue burning or removal from fields has detrimental impacts on crop productivity and soil quality due to decreased carbon (C) sustainability under intensive rice–wheat system (RWS). The conservation agriculture (CA) based on reduced tillage coupled with crop residue retention and raising leguminous green manure (GM) (Sesbania aculeata) during the intervening period between wheat harvest and rice establishment offers opportunities for the restoration of C sustainability. We studied the impacts of tillage intensity, crop residue management, and GM on soil organic C dynamics within the macro- and micro-aggregates after 6 years of RWS. Four main plot treatments in rice including combinations of wheat straw and GM were conventional till puddled transplanted rice with no wheat straw (CTR_W0), CTR with 25% wheat stubbles retained (CTR_W25), CTR_W0 plus GM (CTR_W0 + GM), and CTR_W25 plus GM (CTR_W25 + GM). Three sub-plot treatments in subsequent wheat were CT with rice straw removed (CTW_R0), zero tillage with rice straw removed (ZTW_R0), and the ZT with 100% rice straw retained as surface mulch (ZTW_R100). The ZTW_R100 significantly (p< 0.05) increased the soil moisture retention at field capacity (FC) (Ψm = 0.33bar) and available moisture content (AWC) (Ψm = 0.33–15 bar), compared with CTW_R0. The ZTW_R100 treatment also significantly increased the dehydrogenase (DHA), cellulase, and β-glucosidase activities by ~54, 56, and 45% and the easily extractable glomalin (EEG)- and total glomalin (TG)-related soil protein concentration by ~30.5 and 9.4%, respectively, compared with the CTW_R0 treatment. The four C fractions of variable oxidizability increased significantly within both soil macro- and micro-aggregates under CTR_W25 + GM and ZTW_R100 over their respective control treatments (CTR_W0 and CTW_R0). Crop residue retention plus GM significantly improved the proportion of total water-stable aggregates (WSA), mean weight diameter (MWD), and aggregate ratio (AR), compared with the treatments involving residue removal and no GM. The principal component analysis (PCA) elucidated FC, TG in macro-aggregates, non-labile C in micro- and macro-aggregates, WSA, and the permanent wilting point (PWP) as the most dominant indicators for assessing soil quality under CA based sustainable practices in RWS.