Effect of Stand Density on Soil Microbial Carbon and Nitrogen Use Efficiency in Robinia pseudoacacia Plantations in the Loess Hilly‐Gully Region, China

Abstract

Soil microbial carbon (CUE) and nitrogen (NUE) use efficiency are critical for soil carbon (C) sequestration and nitrogen (N) availability. However, how microbial CUE and NUE respond to stand density is unclear. Furthermore, it remains unclear whether the response of microbial CUE and NUE to stand density could be mediated by soil depth. This study investigated the impact of stand density (low‐density, 625~1000; medium‐density, 1001~1625; high‐density, 1626~2100 trees ha −1 ) on microbial CUE and NUE in topsoil (0–10 cm) and subsoil (10–20 cm) of Robinia pseudoacacia plantations in the loess hilly‐gully region, China. In the 0–10 cm soil layer, microbial CUE decreased with increasing stand density. Specifically, microbial CUE under low‐density stands was significantly higher than that under medium‐ and high‐density stands, with increases of 4.9% and 7.1%, respectively. Microbial NUE in low‐density stands was significantly markedly lower than that in medium‐ and high‐density stands, with decreases of 19.9% and 27.8%, respectively. In the 10–20 cm layer, microbial CUE in medium‐density stands was 4.8% and 6.4% higher than that in low‐ and high‐density stands, respectively. In contrast, in the 10–20 cm layer, microbial NUE in medium‐density stands was 24.5% and 32.1% lower than that in low‐ and high‐density stands, respectively. Hierarchical partitioning analysis revealed that the relative importance of environmental factors on microbial CUE and NUE differed markedly across soil depths. Specifically, enzymatic C:N ratio and soil organic C:total N ratio explained the largest proportions of variation in microbial CUE and NUE at the 0–10 cm (19.2%) and 10–20 cm (19.5%), respectively. Results from multiple linear regression indicated that microbial metabolic limitation was the main driver of microbial CUE and NUE at the 0–10 cm layer; however, microbial biomass C:N ratio was the primary driver of microbial CUE and NUE at 10–20 cm soil layers. Overall, these results highlighted that stand density regulates microbial nutrient use efficiency by driving variations in nutrient limitation caused by the stoichiometric imbalances across soil depths.