Evaluating Soil Carbon Balance under Different Organic Fertilizers: Integrating Decomposition Dynamics and Soil Feedback

Organic fertilizer application is a key strategy for reducing soil organic carbon (SOC) loss and achieving sustainable agriculture. In this study, a 91-day laboratory incubation experiment was conducted to evaluate soil carbon balance, defined by cumulative CO 2 emissions, changes in SOC during incubation, and carbon inputs, for soils amended with compost manure (M), raw slurry (S), and digestate (D), with inorganic fertilizer (IF) and no fertilizer (NF) as references. Two composite indices were developed to standardize carbon decomposition rate and retention efficiency. All organic amendments increased SOC ( p < 0.05), whereas CO 2 emissions did not increase linearly with carbon input, following the order S > D > M. In M, both CO 2 emissions and SOC accumulation were suppressed despite high mineral N availability and sustained N 2 O emissions, consistent with anaerobic microsite formation that limited decomposition and resulted in the lowest carbon retention efficiency. In treatment S and D, 81.1% and 98.0% of the applied carbon was either emitted as CO 2 or retained as SOC, respectively. The highest carbon retention efficiency with the lowest carbon input was achieved in D, likely due to the compositional changes induced by prior anaerobic digestion. Variations in nitrate, ammonium, and cumulative N 2 O emissions explained the composite indices more effectively than changes in SOC fractions, indicating that nitrogen dynamics are closely associated with carbon balance when comparing fertilizers with different characteristics. Overall, integrating the decomposition process with soil feedback signals provides a more informative assessment of organic fertilizer performance than relying solely on SOC or CO 2 measurements. • Organic amendments increased soil organic carbon (SOC), but gains did not scale with carbon input. • Digestate achieved the highest carbon balance efficiency with the lowest carbon input. • Compost may form anaerobic microsites, decoupling C and N and limiting SOC accumulation. • Nitrogen dynamics explained carbon outcomes better than SOC fractions or CO 2 emission.