Organic Amendment Quality Regulates Greenhouse Gas Trade-Offs and Short-Term Carbon Retention During Reductive Soil Disinfestation

Background/Objectives: Reductive soil disinfestation (RSD) is increasingly viewed as a viable alternative to chemical fumigation, yet the role of organic amendment quality in regulating greenhouse gas (GHG) fluxes, soil organic carbon (SOC) dynamics, and net climate forcing remains poorly understood. Methodology: In a 30-day anaerobic incubation experiment, we set up three treatments—a flooded control (CK), soil amended with wheat straw (WS, C/N = 55.5), and soil amended with kiwifruit branches (KB, C/N = 110.8)—each replicated three times under identical conditions. Results: Both WS and KB additions strongly stimulated CO2 and CH4 production, while suppressing N2O emissions by over 85% relative to CK. The WS treatment exhibited a substantially higher global warming potential (GWP, 823.52 t ha−1) than KB (636.23 t ha−1), with CH4 accounting for more than 99% of total GWP. Although WS surpassed KB in short-term carbon sequestration efficiency (25.79% vs. 20.11%) and showed greater hydrolytic enzyme activities (βG, CBH, and XYL), the two organic amendments diverged clearly in carbon fraction distribution: Cmic was 17.9% higher under WS, whereas Cmin was 16.2% higher under KB. When factoring in the CO2 equivalent benefit derived from carbon sequestration, the net GWP (NGWP) indicated that both RSD treatments remained net GHG sources. Notably, KB yielded a markedly lower NGWP (611.60 t CO2-eq ha−1) than WS (798.19 t CO2-eq ha−1), highlighting a fundamental trade-off: WS favored rapid SOC accumulation at the expense of elevated methane emissions, whereas KB achieved a smaller climatic footprint despite more moderate carbon retention. Conclusions: These findings underscore that selecting organic amendments for field RSD requires balancing the competing goals of carbon sequestration and GHG mitigation.