Background and aims Woody plant encroachment is a phenomenon of global concern in drylands due to demonstrated reductions in livestock carrying capacity. However, shrubs are known to contribute to the development of patches of enhanced fertility that might offset any negative effects of increasing grazing. We measured soil physical and chemical characteristics within shrub and open patches across a gradient in livestock grazing to explore how the relative effect of shrubs might change with increasing grazing-induced disturbance. Methods Soil carbon, nitrogen phosphorus and bulk density were measured within 92 shrub patches and their paired interspaces at five sites ranging from long-grazed to long-ungrazed in a semiarid grassland encroached by the N-fixing shrub Caragana microphylla . We used a combination of linear and structural equation modelling to test whether shrubs might buffer any negative effects of overgrazing on soils. Results Shrub soils were more porous, and had more organic carbon, nitrogen and phosphorus than interspace soils. Within both microsites, however, soil bulk density increased, and soil organic carbon and nutrients declined, with increasing grazing intensity. Grazing reduced interspace plant cover and height and exacerbated the negative effects of bulk density on soil carbon, whereas shrubs had the opposite effect. The relative importance of shrubs for soil carbon and nutrients increased with increasing grazing intensity but collapsed under extreme overgrazing. Conclusions These findings highlight the effect of grazing in promoting shrub dominance, which can also prevent grassland degradation. However, any positive effects of grazing collapsed when sites were severely overgrazed.

中文翻译：

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肥沃的岛屿效应在极端过度放牧下崩溃：来自灌木侵占草原的证据

背景和目标 由于牲畜承载能力的下降，木本植物侵占是旱地的一种全球关注现象。然而，众所周知，灌木有助于形成肥力增强的斑块，这可能会抵消增加放牧的任何负面影响。我们测量了牲畜放牧梯度中灌木和开放斑块内的土壤物理和化学特征，以探索灌木的相对影响如何随着放牧引起的干扰的增加而变化。方法在固氮灌木小叶锦鸡儿侵占的半干旱草地中，从长期放牧到长期未放牧的5个地点，测量了92个灌木斑块及其成对间隙的土壤碳、氮磷和容重。我们结合使用线性和结构方程模型来测试灌木是否可以缓冲过度放牧对土壤的任何负面影响。结果灌木土壤比间隙土壤孔隙更多，有机碳、氮和磷含量更高。然而，在两个微型场地内，随着放牧强度的增加，土壤容重增加，土壤有机碳和养分下降。放牧减少了植物间隙和高度，并加剧了容重对土壤碳的负面影响，而灌木则具有相反的影响。灌木对土壤碳和养分的相对重要性随着放牧强度的增加而增加，但在极端过度放牧下崩溃。结论 这些发现强调了放牧在促进灌木优势地位方面的作用，这也可以防止草地退化。然而，