Leguminous Caragana korshinskii evidently enhances microbial necromass carbon accumulation in dryland soils

The planting of nitrogen (N)-fixing leguminous plants is a common measure to restore degraded vegetation and sequester soil organic carbon (SOC) in globally vast arid areas where N deficiency is a limiting factor for plants. However, the effect of leguminous plants on microbial synthesized SOC accumulation remains a knowledge gap. We investigated the effects of planting shrubs (leguminous Caragana korshinskii and non leguminous Salix psammophila) in abandoned croplands on microbial substance accumulation in soil compared with natural succession (dominated by non leguminous semi-shrub Artemisia desertorum) in an arid area. Seventeen years after plantation, C. korshinskii increased SOC stock in the top 20 cm across canopy and gap areas by 34-80%, but S. psammophila did not affect SOC compared with abandonment (A. desertorum). This variation in the SOC stock across three vegetation types was not consistent with change patterns of aboveground litter accumulation and root biomass, but was consistent with higher living microbial biomass and non living microbial necromass contents in the soil and their proportions in the SOC under leguminous C. korshinskii than non leguminous A. desertorum and S. psammophila. These results indicated that the increased SOC under the leguminous relative to non leguminous species was mainly ascribed to the increased accumulation of microbial synthesized substances under the legume. Microbial necromass N represented 56-59% of total soil N under C. korshinskii, but this proportion was only 25-39% under non leguminous species, responding to much greater plant N input and soil N availability under the legume. Therefore, we suggest that symbiotic N fixation by leguminous species increased the production of microbial biomass and potentially decreased the reuse of N from microbial necromass compared with non leguminous species. As such, legumes can efficiently increase the net accumulation of microbial necromass carbon and thus the SOC content in dryland soils.