Differential impacts of nitrogen addition on rhizosphere and bulk‐soil carbon sequestration in an alpine shrubland

Abstract

Abstract Due to complex root–soil interactions, the responses of carbon (C) dynamics in the rhizosphere to elevated nitrogen (N) deposition may be different from those in bulk soil. However, the potentially different response of C dynamics in the rhizosphere and bulk soils and their contributions to soil C sequestration under N deposition is still not elucidated. We conducted an N addition experiment in an alpine shrubland dominated by Sibiraea angustata located on the eastern Qinghai‐Tibet Plateau (QTP). We measured the soil organic C (SOC) contents and density fractions in the rhizosphere and bulk soils in the top 15 cm of mineral soil and then employed a numerical model based on the rhizosphere extent to evaluate how the rhizosphere modulates soil C sequestration under N addition. We also measured the microbial gene abundance and C‐acquisition enzyme activities to assess microbial community responses to N addition. The results showed that nitrogen addition had opposite effects on the rhizosphere and bulk‐soil C stocks. Specifically, N addition decreased the rhizosphere SOC content by increasing bacterial abundance, β‐glucosidase activity, and thus accelerating the loss of free light fraction C (FLF‐C). However, N addition increased the bulk‐soil C content, which was corresponding with the reduced oxidase activities and the accelerated accumulation of heavy fraction C (HF‐C) under N addition. Numerical model analysis showed that the decrease induced by N addition in rhizosphere SOC stock ranged from 0.11 to 3.01 kg C/m2 as root exudation diffusion distance extended from 0.5 to 2 mm, while the corresponding increase in the bulk‐soil C stock ranged from 1.91 to 4.08 kg C/m2. By synthesizing the dynamics of the SOC stocks in these two soil compartments under N addition, the SOC stock at the ecosystem level exhibited an increase in range of 0.73–2.44 kg C/m2. Synthesis. Our results suggest that alpine shrublands on the eastern QTP have great potential for soil C sequestration under N deposition, and the magnitude of the sequestration would depend closely on the responses of rhizosphere microbial C processes and the rhizosphere extent. Our results highlight the importance of integrating rhizosphere processes into land surface models to accurately predict ecosystem functions in the background of elevated N deposition.