Alpine peatland degradation enhances soil nitrogen losses and alters plant nitrogen uptake strategies: evidence from nitrogen isotopes

Abstract Background Nitrogen plays a critical role in sustaining ecosystem functions in peatlands; however, the degradation of approximately 12% of global peatlands substantially alters nitrogen cycling. Although the abundance of stable nitrogen isotopes (δ 15 N) has been widely used to trace nitrogen processes, their patterns and implications across degradation gradients are not well understood. This study examined changes in δ 15 N and their relationships with nitrogen content and environmental factors along a degradation gradient in alpine peatlands, including flooded wetlands, wet meadows, moderately degraded meadows, and severely degraded meadows. Results Soil δ 15 N increased from flooded wetlands to wet meadows and moderately degraded meadows, likely due to increased nitrogen release as the peatlands dried. However, soil δ 15 N declined from moderately to severely degraded meadows, possibly reflecting reduced microbial activity and limited nitrogen transformation under extreme degradation. Across all sites, roots were depleted in 15 N relative to soil, with increasingly negative Δδ 15 N root–soil values in more degraded sites, likely driven by shifts in plant community composition and changes in nitrogen uptake strategies. Random forest analysis revealed that the soil water content, phosphorus, and nitrogen availability were the primary factors influencing the soil and plant δ 15 N values, as did 15 N fractionation during plant nitrogen uptake along the degradation gradient. Conclusions Peatland degradation leads to greater soil δ 15 N and increased 15 N depletion from soil to plants, indicating a shift toward more open ecosystem nitrogen dynamics and altered plant nitrogen uptake strategies associated with greater nitrogen losses. These findings provide new insights into the impact of peatland degradation on nitrogen dynamics and demonstrate the effectiveness of δ 15 N as a tool for monitoring changes in nitrogen cycling and availability across degradation levels. Graphical Abstract