High stand density promotes soil organic carbon sequestration in Robinia pseudoacacia plantations in the hilly and gully region of the Loess Plateau in China

Planting density has an effect on the accumulation and stabilization of soil organic carbon (SOC) in plantations. However, the mechanism by which SOC dynamics and sequestration respond to stand density is not well understood in Robinia pseudoacacia plantations. Here, we analyzed R. pseudoacacia plantations in the loess hillygully region of northern Shaanxi Province, China with three stand densities: 750 trees ha(-1) (low-density), 1125 trees ha(-1) (medium-density) and 1550 trees ha(-1) (high-density). We determined the effects of stand density on SOC, SOC fractions, microbial carbon (C) use efficiency (CUE), and microbial C metabolism function. In particular, soil microbial metabolism function was investigated with the Biolog-ECO microplate. Carbohydrates and amino acids were the main available C sources for soil microbes in R. pseudoacacia plantations. Microbial biomass C (MBC), mineral-associated organic C (MOC), and dissolved organic C (DOC) content were the highest in high-density plantations, whereas easily oxidizable C (EOC) content, the MBC/SOC ratio, the DOC/SOC ratio, and the EOC/SOC ratio were the lowest in high-density plantations. Compared with low- and medium-density plantations, the average SOC content in high-density plantations increased by 61% and 52%, respectively. Across all stands, the MOC/SOC ratio ranged from 32% to 65%, whereas labile C accounted for only a minor portion of SOC (20 +/- 8%). A substantially higher CUE and lower microbial metabolic activity were found in high-density plantations. Pearson correlation analysis showed that SOC content was significantly positively correlated with above-ground litter biomass and CUE but was significantly negatively correlated with microbial metabolic activity and liable C/SOC ratios. Mantel tests showed that soil nitrogen (N) level and the microbial biomass C/N ratio were the main factors affecting SOC quantity and stability changes. These results suggested that R. pseudoacacia plantations under high density increased SOC sequestration by increasing C input via aboveground litter and decreased C loss by improving microbial CUE and suppressing microbial metabolic activity. Thus, increasing stand density may be an effective tool for improving SOC sequestration in R. pseudoacacia plantations.