Dual-Track Assembly of Mineral-Associated Organic Carbon

Substituting chemical nitrogen (N) fertilizers with organic materials is a vital strategy for restoring degraded agricultural soils. However, the exact biophysical pathways governing mineral-associated organic carbon (MAOC) accumulation under varying organic amendments remain poorly decoupled. In a 5-year field experiment on a subtropical red soil, we evaluated the substitution of 20% chemical N fertilizer with equivalent N-P-K nutrient inputs sourced from biochar (CB), cow dung (CD), and maize straw (MS). Utilizing temporal carbon fractionation and 13C isotopic tracing, we identified a comprehensive "Dual-Track" soil carbon assembly mechanism. The results demonstrated a striking isotopic paradox: while 13C tracing revealed that CD drove the highest biological integration of newly derived carbon (fOM) into the MAOC pool, it incurred massive respiratory losses. Conversely, CB maximized absolute soil organic carbon (SOC) accumulation (+133.0%) despite a lower fOM. Furthermore, even with the lowest absolute mass of carbon input, the CB treatment exhibited a normalized Carbon Sequestration Efficiency (CSEMAOC) that outperformed CD and MS by orders of magnitude. Our SEM explicitly detangled these competing pathways, confirming that while in vivo microbial turnover (MBC -> MAOC, β = 0.776) dictates the biological biosynthesis funnel, an independent physical encapsulation bypass (POC -> MAOC, β = 0.460) fundamentally drives CB's superior matrix retention. Crucially, the gross carbon input mass was negatively correlated with structural particulate accrual (β = -0.757), dismantling conventional mass-balance hypotheses. We conclude that for degraded arable soils nearing carbon deficiency, prioritizing the intrinsic biochemical recalcitrance of amendments over sheer biodegradable mass provision activates a highly efficient physical "microbial-bypass" trajectory, thereby maximizing long-term carbon stabilization without suffering intense respiratory penalties.