Towards a mechanistic understanding of microbial and nonmicrobial mediated topsoil organic carbon sequestration efficiency in a rice-wheat cropping system

Abstract

Paddy soils constitute the largest anthropogenic wetlands on earth, where microbial and non-microbial pathways jointly determine the fate of carbon from organic amendments. However, the specific mechanisms underlying these processes remain poorly understood. Here, we examined the influence of microbial anabolism versus catabolism and iron (Fe)-mediated soil organic carbon (SOC) protection versus decomposition on the topsoil carbon sequestration efficiency (CSE) of exogenous organic amendments in a rice-wheat cropping system subjected to nine years of no fertilizer (CT), chemical fertilizer (CF), 50% CF plus pig manure compost (CFM), 100% CF plus straw (CFS), 50% CF plus pig manure compost and straw (CFMS), and 87.5% CF plus pig manure compost (OICF) amendments. The CSE under CT and CFM was approximately 1.5- to 2.4-fold higher than that under the other four treatments. A structural equation model indicated that Fe-mediated free radical reactions and exogenous carbon inputs were negatively correlated with CSE, whereas soil microorganisms were positively correlated with CSE. Microbial biomass carbon and Fe-bound organic carbon were higher while the metabolic quotient, the ratio of microbial carbon-use efficiency to nitrogen-use efficiency and the percentage of Fe-bound organic carbon to SOC were lower under CFM and CFMS than under CF, CFS and OICF. The concentrations of ferrous Fe, oxalate-extractable Fe oxides, hydrogen peroxide and hydroxyl radicals were lower, but the activity of catalase was higher under CFM and CFMS than under CF, CFS and OICF. Therefore, partial substitution of chemical fertilizer with compost enhances topsoil CSE in three ways: directly through the input of refractory organic carbon, indirectly through improving the microbial substrate-use efficiency, and to a lesser extent through promoting the formation of organo-Fe complexes and accelerating the decay of hydrogen peroxide, thereby possibly serving as a management tool for attenuating terrestrial carbon-climate feedbacks.