Abstract

Dryland (arid and semi-arid) ecosystems globally provide more than half of livestock production and store roughly one-third of soil organic carbon (SOC). Biogeochemical pools are changing due to shrub encroachment, livestock grazing, and climate change. We assessed how vegetation microsite, grazing, and precipitation interacted to affect SOC and total nitrogen (TN) at a site with long-term grazing manipulations and well-described patterns of shrub encroachment across elevation and mean annual precipitation (MAP) gradients. We analyzed SOC and TN in the context of vegetation cover at ungrazed locations within livestock exclosures, high-intensity grazing locations near water sources, and moderate-intensity grazing locations away from water. SOC was enhanced by MAP (p < 0.0001), but grazing intensity had little effect regardless of MAP (p = 0.12). Shrubs enhanced SOC (300–1279 g C m⁻²) and TN (27–122 g N m⁻²), except at high MAP where the contribution or stabilization of shrub inputs relative to grassland inputs was likely diminished. Cover of perennial herbaceous plants and litter were significant predictors of SOC (r² = 0.63 and 0.34, respectively) and TN (r² = 0.64 and 0.30, respectively). Our results suggest that continued shrub encroachment in drylands can increase SOC storage when grass production remains high, although this response may saturate with higher MAP. In contrast, grazing – at least at the intensities of our sites – has a lesser effect. These effects underscore the need to understand how future climate and grazing may interact to influence dryland biogeochemical cycling.

摘要

全球旱地（干旱和半干旱）生态系统提供了一半以上的牲畜产量，并储存了大约三分之一的土壤有机碳 (SOC)。由于灌木侵占、牲畜放牧和气候变化，生物地球化学池正在发生变化。我们评估了植被微场地、放牧和降水如何相互作用以影响一个场地的 SOC 和总氮 (TN)，该场地具有长期放牧操作以及在海拔和平均年降水量 (MAP) 梯度上的灌木侵占模式。我们在牲畜围栏内的未放牧地点、水源附近的高强度放牧地点和远离水的中等强度放牧地点的植被覆盖背景下分析了 SOC 和 TN。MAP 增强了 SOC ( p < 0.0001)，但放牧强度几乎没有影响 MAP ( p =  0.12)。灌木提高了 SOC (300–1279 g C m ^-2 ) 和 TN (27–122 g N m ^-2 )，但在高 MAP 时，灌木投入相对于草地投入的贡献或稳定性可能会降低。多年生草本植物和枯枝落叶的覆盖是 SOC（分别为r ² = 0.63 和 0.34）和 TN（r ²= 0.64 和 0.30，分别）。我们的研究结果表明，当草产量仍然很高时，旱地持续的灌木侵占可以增加 SOC 的储存，尽管这种响应可能会随着 MAP 的升高而饱和。相比之下，放牧——至少在我们场地的强度下——影响较小。这些影响强调了了解未来气候和放牧如何相互作用以影响旱地生物地球化学循环的必要性。