Leucaena leucocephala plantations can increase soil organic carbon storage by increasing labile fractions without reducing stability in dry-hot valley savannas

Abstract Background Soil organic carbon (SOC) stocks increase under nitrogen-fixing plantations. However, the mechanisms underlying this increase in SOC stocks and the stability in savannas converted to Leucaena leucocephala plantations remain unknown. This study explored the dynamic changes in SOC stocks in bulk and labile soil fractions, carbon stability, and their driving factors in L. leucocephala plantations across different layers in the top 50 cm of soil in the dry-hot valley of the Jinsha River. Results L. leucocephala plantations significantly increased the bulk SOC stocks and some labile fractions and significantly altered vertical patterns of SOC stocks. Stand age had a strong positive effect on SOC stocks, particularly in the surface soil layer (0–10 cm). SOC stocks and contribution from litter carbon inputs decreased significantly with soil depth. Moreover, the relative proportion of new C in bulk soil, readily oxidizable organic carbon (ROC), and dissolved organic carbon (DOC) stocks and their ratios to SOC stocks were significantly and positively correlated with SOC stocks. Carbon mineralization efficiency exhibited an initial increase followed by a decrease during the growth of L. leucocephala and was primarily regulated by soil C:N and the proportion of hot-water extractable organic carbon. Conclusions L. leucocephala plantations increased SOC stocks by increasing labile carbon fractions, with no significant reduction in SOC stability as stand age increased. This study highlights the pivotal role and underlying mechanisms of nitrogen-fixing plantations in achieving and sustaining persistent SOC sequestration in dry-hot valley savannas.