N-fixing species outperform non-N-fixing species in promoting soil organic carbon stability via enhancing edaphic-litter nitrogen availability in Eucalyptus plantations

Mixed planting is a more favorable approach than monocultures for soil organic carbon (SOC) sequestration; however, due to the diversity in mixed-species assemblages, the underlying mechanisms remain unclear, hindering afforestation schemes that protect SOC stocks. A 13-year mixed-species experiment was conducted in a subtropical region, focusing on a Eucalyptus urophylla × E. grandis pure plantation (PP), a Eucalyptus and nitrogen-fixer Erythrophleum fordii mixed plantation (EE), and Eucalyptus and non-nitrogen-fixer Castanopsis hystrix mixed plantation (EC), by measuring SOC in bulk soil and aggregates, followed by partitioning SOC into active carbon pools (labile carbon pool I [LPI-C] and labile carbon pool II [LPII-C]) and a recalcitrant carbon pool (RP-C). The present study considered various environmental factors, including plants, soil, microflora, and enzyme activities, to analyze the processes and factors underlying SOC sequestration within different mixed plantation modes. Mixed forests improved SOC sequestration by optimizing soil aggregate stability, increasing fertility, relieving phosphorus limitation, and stimulating microbial activity. SOC sequestration in EE soils was driven by both fine root biomass and litter quality, whereas SOC sequestration in EC soils relied on fine root elongation. Only nitrogen-fixing mixtures enhanced SOC stability, favoring long-term carbon storage. The high carbon (C)/nitrogen (N) ratio in the soil and the nitrogen-rich environment promoted an increase in Gram-negative bacteria, achieving stable SOC storage by forming large aggregates. The present study highlights the advantages of mixed-species afforestation in SOC sequestration and reveals the unique value of nitrogen-fixing species in long-term carbon storage. The present findings offer theoretical and practical guidance for scientific afforestation and enhancing the carbon sink function of forest ecosystems.