School of Natural Resources and Environment, University of Florida, Gainesville, FL, USA.
Department of Biological Sciences, Virginia Tech University, Blacksburg, VA, USA.
Glob Chang Biol. 2018 Jul;24(7):2997-3009. doi: 10.1111/gcb.14070. Epub 2018 Feb 24.
Despite the large contribution of rangeland and pasture to global soil organic carbon (SOC) stocks, there is considerable uncertainty about the impact of large herbivore grazing on SOC, especially for understudied subtropical grazing lands. It is well known that root system inputs are the source of most grassland SOC, but the impact of grazing on partitioning of carbon allocation to root tissue production compared to fine root exudation is unclear. Given that different forms of root C have differing implications for SOC synthesis and decomposition, this represents a significant gap in knowledge. Root exudates should contribute to SOC primarily after microbial assimilation, and thus promote microbial contributions to SOC based on stabilization of microbial necromass, whereas root litter deposition contributes directly as plant-derived SOC following microbial decomposition. Here, we used in situ isotope pulse-chase methodology paired with plant and soil sampling to link plant carbon allocation patterns with SOC pools in replicated long-term grazing exclosures in subtropical pasture in Florida, USA. We quantified allocation of carbon to root tissue and measured root exudation across grazed and ungrazed plots and quantified lignin phenols to assess the relative contribution of microbial vs. plant products to total SOC. We found that grazing exclusion was associated with dramatically less overall belowground allocation, with lower root biomass, fine root exudates, and microbial biomass. Concurrently, grazed pasture contained greater total SOC, and a larger fraction of SOC that originated from plant tissue deposition, suggesting that higher root litter deposition under grazing promotes greater SOC. We conclude that grazing effects on SOC depend on root system biomass, a pattern that may generalize to other C4-dominated grasslands, especially in the subtropics. Improved understanding of ecological factors underlying root system biomass may be the key to forecasting SOC and optimizing grazing management to enhance SOC accumulation.
尽管草原和牧场对全球土壤有机碳（SOC）储量的贡献巨大，但对于大型食草动物放牧对 SOC 的影响仍存在很大的不确定性，尤其是对于研究较少的亚热带放牧地。众所周知，根系输入是大多数草地 SOC 的来源，但放牧对碳分配到根系组织生产与细根渗出的影响尚不清楚。鉴于不同形式的根 C 对 SOC 合成和分解的影响不同，这代表了知识上的一个重大空白。根分泌物应该主要在微生物同化后才有助于 SOC 的形成，从而通过稳定微生物残体来促进微生物对 SOC 的贡献，而根凋落物的沉积则直接作为植物来源的 SOC 贡献，随后微生物分解。在这里，我们使用原位同位素脉冲追踪方法，结合植物和土壤采样，将植物的碳分配模式与美国佛罗里达州亚热带牧场长期放牧围栏内的 SOC 库联系起来。我们量化了碳在根系组织中的分配，并测量了放牧和未放牧地块的根渗出量，并量化了木质素酚类物质，以评估微生物和植物产物对总 SOC 的相对贡献。我们发现，放牧排除与地下总分配量明显减少有关，根系生物量、细根渗出物和微生物生物量均较低。同时，放牧牧场的总 SOC 含量更高，且来自植物组织沉积的 SOC 比例更大，这表明放牧下更高的根凋落物沉积促进了更大的 SOC。我们的结论是，放牧对 SOC 的影响取决于根系系统生物量，这种模式可能适用于其他 C4 占主导地位的草地，特别是在亚热带地区。更好地理解根系系统生物量的生态因素可能是预测 SOC 和优化放牧管理以促进 SOC 积累的关键。
