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

内容摘要

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. 高寒草甸退化显著影响土壤氮循环过程,但不同退化阶段土壤氮库稳定性及δ15N的响应机制尚不明确。本研究以青海省玛沁县高寒草甸为对象,探讨了草甸退化对土壤稳定氮同位素(δ15N)的影响及其驱动因素。退化阶段分为未退化的禾草草甸时期(ND)、矮嵩草+小嵩草草甸时期(SD)、小嵩草草甸草毡表层加厚时期(MD)和黑土滩-杂类草次生裸地时期(HD)。结果表明:随着退化程度加剧,地上生物量显著减少,HD时期生物量仅为ND时期的56.5%。草甸退化显著降低了0～30 cm土层土壤pH、电导率和含水量,并改变了土壤C/N的垂直分布模式。土壤全氮(TN)和微生物生物量氮(MBN)随退化程度加重显著降低,在表层土壤(0～5 cm)中表现最明显,降幅分别达57.8%和70.6%。草甸退化显著影响了δ15N的分布,SD和MD时期δ15N值随土层深度增加而升高,而HD时期则趋向均一化。土壤表层与深层δ15N差值(Δδ15N)随草甸退化呈先降低后升高的趋势,在HD时期接近0,表明此时期氮循环高度开放且土壤剖面氮含量均质化。随机森林模型分析表明,δ15N的主导调控因子因退化时期而异:铵态氮(NH4+-N)主导ND时期,pH主导SD和MD时期,MBN主导HD时期。表层土壤δ15N与pH、电导率、含水量、C/N、TN、MBN和NH4+-N呈显著负相关,表明退化通过改变表层土壤环境影响氮循环过程。综上,高寒草甸退化通过改变地上生物量和土壤理化性质进而显著影响土壤氮库和氮转化效率,Δδ15N是指示氮循环开放程度与退化阶段的有效指标。.