Effects of alpine meadow degradation on soil stable nitrogen isotope and its drivers

The degradation of alpine meadow profoundly alters soil nitrogen (N) cycling. However, the stability of soil N pools and the response mechanisms of δ¹⁵N across different degradation stages remain unclear. In an alpine meadow of Maqin County, Qinghai Province, China, we investigated the effects of degradation on soil stable nitrogen isotopes (δ¹⁵N) and their driving factors. Four degradation stages were identified: non-degraded grass meadow (ND), Kobresia humilis + Kobresia pygmaea meadow (SD), thickened turf layer of K. pygmaea meadow (MD), and "black-soil beach" with secondary bare land (HD). The results showed that aboveground biomass significantly decreased with increasing degradation intensity, with biomass in the HD stage reduced to 56.5% of that in the ND stage. Degradation significantly reduced soil pH, electrical conductivity, and soil moisture in the 0-30 cm layer, and altered the vertical distribution pattern of soil C/N. Soil total nitrogen (TN) and microbial biomass nitrogen (MBN) declined markedly with increasing degradation, particularly in the surface soil (0-5 cm), with reductions of 57.8% and 70.6%, respectively. Meadow degradation significantly influenced the distribution of δ¹⁵N. In the SD and MD stages, δ¹⁵N values increased with soil depth, whereas in the HD stage they became homogenized. The difference in δ¹⁵N between surface and deep soil layers (Δδ¹⁵N) first decreased and then increased with degradation, approaching zero in the HD stage, indicating a highly open N cycle and homogenization of soil N across the profile. Random forest analysis revealed that the dominant drivers of δ¹⁵N varied among degradation stages. Ammonium (NH₄⁺-N) dominated in the ND stage, soil pH in the SD and MD stages, and MBN in the HD stage. Surface soil δ¹⁵N was significantly negatively correlated with pH, electrical conductivity, soil moisture, C/N, TN, MBN, and NH₄⁺-N, suggesting that degradation influenced N cycling primarily by altering surface soil environmental condition. In summary, alpine meadow degradation significantly affected soil N pools and N transformation efficiency by altering aboveground biomass and soil physicochemical properties. Δδ¹⁵N could serve as an effective indicator of the openness of ecosystem N cycling and degradation stage.