Woody plant encroachment has transformed land-cover patterns in savannas and grasslands throughout the world, with implications for changes in soil organic carbon (SOC) dynamics. However, our understanding of the effects of woody encroachment on SOC is very limited. In a study of pairs of neighbouring, woody encroached and grassland plots along a rainfall gradient (300–1500 mm mean annual precipitation, MAP) in South African savannas, we quantified the changes in SOC and nitrogen pools to a depth of 100 cm, in three soil physical fractions (coarse particulate organic matter, macroaggregates and silt and clay) and bulk soil. The 0–10 cm soil layer of the 1500 mm MAP site lost 26% of SOC stocks due to shrub encroachment. Contrastingly, the 300 and 350 mm MAP sites gained 24% and 27% total SOC, respectively, due to shrub encroachment. In the soil fractions, the largest changes in SOC, ranging within 2.64–464%, were in the coarse particulate organic matter (unprotected organic matter > 250 μm), and the lowest in the silt and clay fraction (6–32%) across all sites and depths. Soil nitrogen increased across all encroached sites compared with open grasslands, regardless of MAP. For every 1-unit increase in MAP and soil fraction size there was a change of –0.06 and 0.1 SOC (gC/kg soil) in encroached grasslands, respectively. We conclude that shrub-encroached grasslands potentially sequester more C in soils of arid regions than in humid regions. At three of our sites, we were unable to differentiate SOC between encroached and open-grassland soils from bulk soil but could do so from soil fraction analysis. Thus, soil fraction analysis is a more effective way to assess changes in soil C than bulk soil.

木本植物的入侵已经改变了全世界稀树草原和草原的土地覆盖方式，对土壤有机碳（SOC）动态变化具有影响。但是，我们对木本侵害对SOC的影响的了解非常有限。在对南非热带稀树草原沿降雨梯度（300-1500 mm年平均降水量，MAP）的邻国，木质入侵地和草原地带的成对研究中，我们量化了土壤有机碳和氮库在100 cm深度的变化。三个土壤物理部分（粗颗粒有机物，大集料以及淤泥和粘土）和大块土壤。1500 mm MAP站点的0-10 cm土层由于灌木侵蚀而损失了SOC的26％。相反，由于灌木侵蚀，300和350 mm MAP站点的总SOC分别增加了24％和27％。在土壤组分中，SOC的变化最大，在2.64-464％范围内，是在粗颗粒有机物（未保护的有机物> 250μm）中，而在粉砂和粘土组分中，变化最小（6-32％）所有地点和深度。与空旷的草原相比，所有土壤侵蚀地区的土壤氮素都增加了，而与MAP无关。在被侵蚀的草原中，每增加1单位MAP和土壤分数，其变化分别为–0.06和0.1 SOC（gC / kg土壤）。我们得出的结论是，灌木丛侵蚀的草原比干旱地区的土壤更可能吸收更多的碳。在我们的三个站点中，我们无法从块状土壤中区分出侵蚀土壤和开放草地土壤中的SOC，但可以通过土壤成分分析来区分。因此，