Data from: Functional attributes of savannah soils: contrasting effects of tree canopies and herbivores on bulk density, nutrients and moisture dynamics

1. Savannahs are highly heterogeneous tree-grass mixtures, and the structural variation imposed by a discontinuous canopy cover results in spatial variation in soil properties such as plant-available nutrients, temperature and soil moisture. Many savannahs are also dominated by large vertebrate herbivores, which impose a different suite of effects on soil properties related to consumption, excretion and physical disturbance. 2. In nutrient-poor, water-limited systems, variation in soil resource availability may play a fundamental role in structuring plant communities, but the relative and interactive effects of key drivers of heterogeneity have received little attention in savannah ecosystems. 3. We investigated the independent and interactive effects of herbivores and tree canopy cover on multiple soil properties up to a depth of 10 cm, including bulk density, soil C pools, total and plant-available nutrients, moisture dynamics and temperature at 2 contrasting sites in Kruger National Park, South Africa. We analyzed samples from locations under and between adult tree canopies and inside and outside long-term herbivore exclosures using a nested design. 4. We found strong positive effects of tree canopies on total and labile pools of soil C and N and negative effects on plant-available P and soil temperature. Tree canopy effects on soil moisture availability were small but clearly detectable. During precipitation events, trees increased moisture inputs on granite soils but decreased moisture inputs on basalt soils compared to open sites. 5. Conversely, we found mixed evidence for herbivore effects on soil chemistry and temperature, but a clearer effect on soil moisture, with areas outside exclosures experiencing higher soil moisture retention, particularly immediately following precipitation events. Soil bulk density, a driver of both nutrient and soil moisture dynamics, was lower under trees and when herbivores were excluded. 6. Synthesis. Our results suggest that variation in tree cover is the dominant biotic driver of C, N, and P dynamics in these savannah systems in the top 10 cm of soil, that herbivores primarily affect soil moisture content, and that canopy and herbivore effects tend to be additive rather than synergistic.

1. 稀树草原（Savannah）是高度异质性的乔草混合生态系统，其不连续冠层覆盖所引发的结构变异，会导致土壤属性出现空间分异，涵盖植物有效养分、土壤温度与土壤含水量。多数稀树草原还以大型脊椎动物植食者为优势类群，这类植食者通过取食、排泄与物理扰动等过程，对土壤属性产生一系列独特影响。
2. 在养分匮乏、水分受限的生态系统中，土壤资源可获得性的异质性可能在植物群落构建中发挥核心作用，但针对稀树草原生态系统中关键异质性驱动因子的相对作用与交互效应的研究仍较为匮乏。
3. 本研究以南非克鲁格国家公园（Kruger National Park）两处生境迥异的样地为研究对象，针对0~10 cm土层的多项土壤属性，探究了植食动物与乔冠层覆盖各自的独立效应及交互效应，所测指标包括土壤容重（bulk density）、土壤碳库（soil C pools）、全量与植物有效养分、水分动态与土壤温度。本研究采用嵌套设计（nested design），分别采集成年乔冠下方、冠层间隙以及长期植食动物排除区（long-term herbivore exclosures）内、外的土壤样品进行分析。
4. 研究结果表明，乔冠层对土壤总碳、总氮以及活性碳、活性氮库具有显著正向调控作用，而对植物有效磷与土壤温度则产生负向影响。乔冠层对土壤水分可获得性的影响幅度较小，但仍可被清晰检测到。相较于开阔无树区域，降雨事件期间乔冠层会提升花岗岩母质土壤（granite soils）的水分输入量，却降低玄武岩母质土壤（basalt soils）的水分输入量。
5. 与之相反，植食动物对土壤化学属性与温度的影响结果存在分化，但对土壤含水量的影响更为明确：排除区外的样地土壤保水性更强，尤其在降雨事件后即刻表现显著。土壤容重作为养分与水分动态的共同调控因子，在乔冠下方以及植食动物被排除的区域中均更低。
6. 综合与结论：本研究结果显示，在上述稀树草原生态系统的0~10 cm土层中，乔冠覆盖度的异质性是调控碳、氮、磷动态的主导生物驱动因子；植食动物主要通过影响土壤含水量发挥调控作用；且乔冠层与植食动物的效应多为加性而非协同效应。