Contrasting Green Manure Species and Return Methods Regulate Soil Organic Carbon Mineralization and Retention in Coastal Saline Soil

Green manure application is widely used to improve soil quality in saline-alkaline soils, yet the combined effects of green manure species and return methods on soil organic carbon (SOC) mineralization and field SOC retention remain poorly understood. A field experiment was conducted in coastal saline soil of Jiangsu Province with five treatments: no green manure (CK), alfalfa mulching (AM), alfalfa incorporation (AF), ryegrass mulching (RM), and ryegrass incorporation (RF). A 60-day laboratory incubation was performed to assess SOC mineralization dynamics, apparent priming effects, labile carbon fractions and soil enzyme activities. Our results showed that compared with CK, green manure incorporation significantly increased SOC mineralization and induced positive priming effects. Alfalfa showed higher mineralization rates and stronger priming effects (peaking at day 5, 403.67–551.61%) than ryegrass which peaked at day 21 (395.02–432.50%). After 60 days of incubation, SOC increased by 0.34–0.54 mg·g−1 in alfalfa treatments, while it decreased by 1.15–2.42 mg·g−1 in ryegrass treatments. In the field, however, SOC content in the 0–10 cm layer was highest in RM (6.88 mg·g−1), which was 76.9% higher than that in CK, whereas AF showed the lowest SOC content among the green manure treatments in the 10–20 cm layer. Soil MBC and ROC were highest in AF (196.18 mg·kg−1 and 1.973 mg·g−1 in the 0–10 cm layer, respectively), while lowest in RM (118.46 mg·kg−1 and 1.623 mg·g−1). Sucrase and cellulase activities were significantly higher in AF than in other treatments, while RM showed the lowest enzyme activities. These results indicated that alfalfa incorporation stimulated microbial activity and carbon turnover, whereas ryegrass mulching suppressed decomposition and favored carbon retention. Ryegrass mulching is recommended as a promising practice for enhancing short-term SOC retention in this study.