Shrub encroachment changes the patterns of nutrition allocation in the below- and aboveground soil. However, influence of shrub encroachment on microbial carbon (C) and nitrogen (N) limitations remains unclear. Using the extracellular enzyme stoichiometry model, microbial nutrition limitations in bulk and rhizosphere soils at various soil layers were investigated at non-shrub alpine grasslands (GL) and shrub-encroached alpine grasslands including Spiraea alpina lands (SA), Caragana microphylla lands (CM) and Potentilla fruticosa lands (PF) on the Qinghai-Tibetan Plateau. We determined C-acquisition (β-1,4-glucosidase (BG); β-D-fibrinosidase (CBH)), N-acquisition (β-1,4-N-acetylglucosaminidase (NAG); leucine aminopeptidase (LAP)) and phosphorus (P)-acquisition (acid phosphatase (AP)) enzyme activities. The contents of soil organic carbon (SOC) in top- and subsoils significantly increased following shrub encroachment. Interestingly, (LAP + NAG) activities in subsoil increased following shrub encroachment. E_C:N in subsoil decreased following shrub encroachment. Microbial C and N limitations were found in shrub-encroached and non-shrub alpine grasslands. Furthermore, microbial C and N limitations in bulk topsoil layers decreased following shrub encroachment. Microbial N limitations in subsoil decreased following shrub encroachment. This result indicates that shrub encroachment mitigated microbial C and N limitations. The limitations were gradually mitigated following shrub encroachment, which led to the decrease of the decomposition rate of organic carbon by microorganisms, indicating shrub encroachment might potentially contribute to SOC storage. In addition, the structural equation modeling (SEM) showed that increases of SOC and NH₄⁺–N in top- and subsoils under shrub encroachment could mitigate microbial C and N limitations, respectively. This study provides available information on the environmental variables affecting the stoichiometry of extracellular enzymes following shrub encroachment, and the theoretical basis for the study of C and N cycling in alpine grasslands.

Graphical abstract

中文翻译：

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灌木侵占缓解微生物碳氮限制：青藏高原高寒草地细胞外酶化学计量

灌木侵占改变了地下和地上土壤的营养分配模式。然而，灌木侵占对微生物碳（C）和氮（N）限制的影响仍不清楚。利用细胞外酶化学计量模型，研究了非灌木高山草原（GL）和灌木侵占的高山草原（包括高山绣线菊地（SA）、小叶锦鸡儿地（CM））不同土层的体土和根际土壤的微生物营养限制。和 金露梅登陆（PF）青藏高原。我们确定了 C 获取（β-1,4-葡萄糖苷酶 (BG)；β-D-纤维蛋白糖苷酶 (CBH)）、N 获取（β-1,4-N-乙酰氨基葡萄糖苷酶 (NAG)；亮氨酸氨肽酶 (LAP)）和磷 (P) 获取（酸性磷酸酶 (AP)）酶活性。灌木侵占后，表土和底土中的土壤有机碳（SOC）含量显着增加。有趣的是，灌木侵占后，底土中的 (LAP + NAG) 活性增加。灌木侵占后，底土中的E _C:N减少。在灌木侵占和非灌木高山草原中发现了微生物碳和氮的限制。此外，灌木侵占后，表土层中微生物碳和氮的限制减少。灌木侵占后，底土中微生物氮的限制减少。这一结果表明灌木侵蚀减轻了微生物碳和氮的限制。灌木侵占后，这种限制逐渐减轻，导致微生物对有机碳的分解速率降低，表明灌木侵占可能有助于 SOC 储存。此外，结构方程模型（SEM）表明SOC和NH ₄ ^+的增加– 灌木侵蚀下的表土和底土中的氮可以分别减轻微生物碳和氮的限制。本研究为灌木侵占后影响细胞外酶化学计量的环境变量提供了可用信息，并为高寒草原碳氮循环研究提供了理论基础。

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