Unraveling mechanisms of carbon enrichment via straw and biochar application to enhance soil fertility and improve maize yield

Abstract

While straw and its derived biochar are established as effective soil amendments for carbon (C) sequestration, the divergent underlying mechanisms governing C enrichment through these materials and their corresponding agronomic implications remain poorly understood. In this four-year field study, stable C fractions such as microbial necromass carbon (MNC), humic C and aggregate-associated C were analyzed using biomarker techniques and fluorescence spectroscopy. Compared with no amendment, the application of straw and biochar significantly increased soil organic carbon (SOC) content and four-year average grain yield by 5.63 %–23.21 % and 10.52 %–18.47 %, respectively. Under equal C input rate, biochar enhanced stable C fractions more effectively than straw, including humic C (21.25 % increase), MNC (5.08 % increase), and aggregate-associated C. Regarding the molecular composition of humic acid (HA), biochar enhanced HA aromatization, whereas straw promoted its aliphatization. Furthermore, MNC was pivotal to SOC enrichment in straw-returned soil, while biochar application exhibited superior potential in promoting soil aggregation and achieving yield enhancement. These findings reveal contrasting mechanisms underlying straw-and biochar-induced SOC enrichment pathways: straw primarily enhances the pivotal contribution of microbial necromass to SOC, whereas biochar preferentially facilitates recalcitrant SOC formation via biochemical process and aggregate-mediated physical protection of SOC. Notably, the co-application of straw with a small amount of biochar demonstrated synergistic effects on both SOC enrichment (including MNC and macroaggregate-associated C) and maize yield enhancement. Therefore, optimized straw or biochar management strategies offer practical guidelines for enhancing SOC sequestration and improving crop productivity.