Pathways of soil organic carbon stabilization driven by long-term straw returning practices in a rainfed agroecosystem

Abstract

Enhancing soil organic carbon (SOC) stability is a central strategy for achieving carbon sequestration, emission reduction, and sustainable soil fertility improvement in rainfed agroecosystems. Straw returning is a widely adopted practice to increase SOC stocks, yet the microbially mediated mechanisms by which different straw return modes regulate SOC stabilization remain poorly constrained, particularly across the Loess Plateau of China. Here, we leveraged a 30-year field experiment in a rainfed maize system to compare three straw return strategies under equivalent carbon input: straw mulching (S1), shredded straw incorporation (S2), and livestock digestion return (S3). We integrated physical aggregate fractionation with multi-biomarker analyses (lignin phenols, amino sugars, and phospholipid fatty acids) and extracellular enzyme assays to elucidate the divergent pathways of SOC stabilization. Our results demonstrated distinct carbon stabilization pathways among the three treatments. S1 followed a physical protection pathway, characterized by the highest fungal-to-bacterial ratio and the preservation of plant-derived lignin within macroaggregates, but resulted in a limited SOC increase (+15.56 %) and low carbon conversion efficiency (+17.45 %). S2 represented an intermediate fungal decomposition pathway; although lignin phenol content increased significantly across all aggregate fractions, high ligninolytic enzyme activities consumed substantial carbon, yielding a moderate SOC gain of + 19.44 %. In contrast, S3 achieved synergistic stabilization via microbial transformation. The low C/N ratio of manure alleviated nutrient limitation, fostering a bacterial- and actinobacterial-dominated community (reaching 84.96 nmol·g⁻¹ PLFA in microaggregates) and upregulating hydrolytic enzyme activities by 73.93 %–193.65 %. This shift promoted the formation of microaggregates (+21.18 %) and silt–clay particles (+40.55 %), with microbial necromass contributing > 40 % to stable SOC pools. Consequently, S3 maximized SOC storage with a 40.00 % increase and achieved the highest carbon conversion efficiency (+29.15 %). These findings highlight that livestock-mediated straw return optimizes the continuum of bacterial proliferation, microaggregate assembly, and microbial necromass accrual, representing an effective strategy for enhancing both the magnitude and long-term stability of SOC in dryland agroecosystems on the Loess Plateau.