Stand density influences soil organic carbon stocks and fractions by mediating soil biochemical properties in Mongolian pine plantations

PurposeClimate warming has increased interests in establishing plantations to enhance carbon (C) sequestration. As a fundamental factor, planting density can profoundly influence tree growth and mortality. Although the effect of stand density on tree biomass C stocks is well known, how it affects soil organic C (SOC) stocks and stability remains largely uncertain. Here, we explored mechanisms behind stand density effects on SOC stocks and fractions.MethodsWe leveraged 25 of 40-year-old Mongolian pine (Pinus sylvestris var. mongolica) stands with stand densities ranging from 350 to 1500 trees per hectare, and measured tree biomass C stocks, litter quantity and quality, soil nitrogen (N) and phosphorous (P) concentrations, microbial biomass, C-degrading enzymes, and SOC stocks and fractions.ResultsEcosystem C stocks logarithmically increased with increasing stand densities, and this C accretion was ascribed to tree biomass C sequestration. Mineral-associated organic C (MAOC) stocks in the 0-10 cm soil layer decreased with increasing stand densities, which was related to the decline of soil cellobiohydrolase activity and total P concentration. The decrease of litter C:N ratio was associated with the increase of particulate organic C (POC) stocks in the 10-20 cm soil layer with increasing stand densities. Moreover, stand density had positive effects on POC and MAOC stocks in the 20-40 cm but not in the 40-100 cm soil layers.ConclusionsThese findings highlight that stand density alters the vertical distribution of SOC stocks and suggest that soil biochemical properties and litter quality mediate the dynamics of SOC stocks and fractions along the stand density.