Variations in soil organic C stocks, sources, and composition during the development of Larix principis-rupprechtii plantations: Evidence from spectroscopic, microbial, and lignin analyses

Abstract

Afforestation has been recognized as an effective strategy for enhancing Soil Organic C (SOC) and mitigating climate change. Yet, the temporal dynamics of SOC quantity, quality, and origins, along with the mechanisms driving these changes during stand development, remain inadequately understood. To address this knowledge gap, five Larix principis-rupprechtii plantations of varying ages (7, 18, 25, 34, and 44 yr.) were selected to elucidate the temporal variations in SOC characteristics following reforestation, based on ¹³C-nuclear magnetic resonance, phospholipid fatty acid, eco-enzymatic activity, microbial necromass, and lignin phenols analysis, etc. Results showed that with increasing stand ages, SOC, Plant-derived C (PC), microbial-derived C (MNC), hydrolytic eco-enzymatic activities, and microbial biomass initially increased and subsequently decreased, with most parameters peaking in 25 yr. stand. Notably, the increased stand ages (i.e., 34 and 44 yr.) or soil depths (20 − 40 cm) enhanced SOC stability, as well as enlarged MNC but reduced PC contribution to SOC. Besides, the reduced syringyl/vanillyl values and elevated their acid/aldehyde ratios in 34 and 44 yr. stands (or 20 − 40 cm depths) indicated that an increased degree of lignin microbial oxidation and decomposition with increasing stand ages or soil depths. Random forest model revealed that biotic and abiotic factors (i.e., hydrolytic eco-enzymatic activity, bulk density, Total N (TN), and SOC stability), rather than litter properties, were the primary factors controlling the PC. The MNC, however, was mainly influenced by litter biomass, TN, SOC/TN, and hydrolytic eco-enzymatic activity. Partial least squares path models partially corroborated these findings and highlighted the synergistic effect of plant- and microbial-derived C on long-term SOC stocks. Concluding, these findings provided new insights into the mechanisms underlying SOC formation/characteristics during forest development, and offer a theoretical basis for sustainable development of forests in northern China from the perspective of soil C sequestration.