Data from: Soil carbon response to woody plant encroachment: Importance of spatial heterogeneity and deep soil storage

1. Recent global trends of increasing woody plant abundance in grass-dominated ecosystems may substantially enhance soil organic carbon (SOC) storage and could represent a strong carbon (C) sink in the terrestrial environment. However, few studies have quantitatively addressed the influence of spatial heterogeneity of vegetation and soil properties on SOC storage at the landscape scale. In addition, most studies assessing SOC response to woody encroachment consider only surface soils, and have not explicitly assessed the extent to which deeper portions of the soil profile may be sequestering C.
2. We quantified the direction, magnitude, and pattern of spatial heterogeneity of SOC in the upper 1.2 m of the profile following woody encroachment via spatially-specific intensive soil sampling across a landscape in a subtropical savanna in the Rio Grande Plains, USA, that has undergone woody proliferation during the past century.
3. Increased SOC accumulation following woody encroachment was observed to considerable depth, albeit at reduced magnitudes in deeper portions of the profile. Overall, woody clusters and groves accumulated 12.87 and 18.67 Mg C ha-1 more SOC compared to grasslands to a depth of 1.2 m.
4. Woody encroachment significantly altered the pattern of spatial heterogeneity of SOC to a depth of 5 cm, with marginal effect at 5-15 cm, and no significant impact on soils below 15 cm. Fine root density explained greater variability of SOC in the upper 15 cm, while a combination of fine root density and soil clay content accounted for more of the variation in SOC in soils below 15 cm across this landscape.
5. Synthesis: Substantial SOC sequestration can occur in deeper portions of the soil profile following woody encroachment. Furthermore, vegetation patterns and soil properties influenced the spatial heterogeneity and uncertainty of SOC in this landscape, highlighting the need for spatially specific sampling that can characterize this variability and enable scaling and modeling. Given the geographic extent of woody encroachment on a global scale, this undocumented deep soil C sequestration suggests this vegetation change may play a more significant role in regional and global C sequestration than previously thought.

1. 当前全球范围内，草本主导生态系统（grass-dominated ecosystems）中木本植物丰度持续上升的趋势，可能显著提升土壤有机碳（soil organic carbon, SOC）储量，并有望成为陆地环境中强劲的碳汇（carbon sink）。然而，鲜有研究在景观尺度（landscape scale）上定量探讨植被与土壤属性的空间异质性（spatial heterogeneity）对土壤有机碳储量的影响。此外，绝大多数评估木本入侵（woody encroachment）对土壤有机碳影响的研究仅关注表层土壤，并未明确探究土壤剖面（soil profile）深层的固碳潜力。
2. 本研究以美国里奥格兰德平原（Rio Grande Plains）一处过去一个世纪以来发生木本植物扩张（woody proliferation）的亚热带稀树草原（subtropical savanna）景观为研究对象，通过空间精细化的高密度土壤采样，定量解析了木本入侵后剖面上层1.2 m土层土壤有机碳空间异质性的方向、强度与分布模式。
3. 研究发现，木本入侵后土壤有机碳的累积可延伸至较深土层，尽管在剖面深层的累积幅度有所降低。整体而言，至1.2 m土层深度，木本簇丛与灌丛的土壤有机碳储量较草原生态系统分别高出12.87 Mg C ha⁻¹与18.67 Mg C ha⁻¹。
4. 木本入侵显著改变了0~5 cm土层的土壤有机碳空间异质性分布模式，对5~15 cm土层仅存在边际效应（marginal effect），而对15 cm以下土层无显著作用。在本研究景观中，0~15 cm土层的土壤有机碳变异主要由细根密度（fine root density）解释，而15 cm以下土层的土壤有机碳变异则同时由细根密度与土壤黏粒含量（soil clay content）共同主导。
5. 综合分析表明，木本入侵后土壤剖面深层可发生显著的土壤有机碳固存。此外，植被格局与土壤属性共同影响了本研究景观中土壤有机碳的空间异质性与不确定性，这凸显了开展空间精细化采样的必要性——此类采样可精准表征该变异特征，为尺度推绎与模型模拟提供支撑。鉴于全球范围内木本入侵的地理分布范围，这一此前未被记录的深层土壤碳固存现象表明，这类植被变化在区域乃至全球碳固存过程中所发挥的作用，可能远超此前的认知。