Different responses of soil microbial respiration to nitrogen addition between surface and deep soil in a temperate steppe in Inner Mongolia

Over the past few decades, terrestrial ecosystems have experienced rising atmospheric nitrogen (N) deposition, which further impacts the global carbon (C) budget through soil microbial respiration (MR). However, the effects of N deposition on MR are rarely characterized in deep soil (depth > 10 cm) rather than in surface soil (0–10 cm). This study attempted to elucidate how N deposition regulates MR along the soil profile and its underlying mechanism. We collected soil samples and determined MR across three soil layers (shallow, medium, and deep) from a decade-long and five-level N addition experiment in a temperate steppe in Inner Mongolia. We further used structural equation modeling to explore how long-term N addition regulates MR through various biotic (plant attributes and microbial community structure) and abiotic (soil properties) factors across the three soil layers. The overall response of MR to N addition varied with soil depth, shifting from stimulation in the shallow soil layer (standardized total effect of 0.36) to inhibition in the medium and deep soil layers (-0.34 and − 0.31). The identified direct and indirect pathways by which N addition regulates MR significantly differed across soil layers. Our results found that the N addition effect on soil C decomposition varied across different soil layers and involved distinct mechanisms in the temperate grassland. As soil depth increases, the suppressive effect of N deposition on MR provides evidence that increasing N deposition may contribute to C accrual in deep soil in grassland ecosystems.