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

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，不连续冠层覆盖所引发的结构变异，会导致土壤性质出现空间分异，涵盖植物有效养分、土壤温度与土壤含水量等指标。多数稀树草原生态系统还以大型脊椎动物草食动物为优势类群，这类动物可通过采食、排泄与物理扰动等多种途径，对土壤性质产生一系列独特影响。
2. 在养分贫瘠且受水分限制的生态系统中，土壤资源可利用性的变异可能在植物群落构建过程中发挥基础性作用，但目前针对稀树草原生态系统，异质性关键驱动因子的相对作用与交互效应仍缺乏足够研究。
3. 本研究以南非克鲁格国家公园（Kruger National Park）的两处不同样地为研究对象，探究了草食动物与乔木冠层对0~10cm深度范围内多项土壤性质的独立与交互效应，测定指标包括土壤容重（bulk density）、土壤碳库（soil C pools）、全量与植物有效态养分、水分动态以及土壤温度。研究采用嵌套设计（nested design），对成年乔木冠层下、冠层间以及长期草食动物排除区（herbivore exclosures）内外的土壤样品进行了系统分析。
4. 研究结果显示，乔木冠层对土壤碳、氮的全量库与活性库（labile pools）具有显著正向调控效应，而对植物有效态磷与土壤温度则产生负向影响。乔木冠层对土壤水分可利用性的影响幅度较小，但仍可被清晰检测到。在降水事件期间，与开阔对照样地相比，乔木冠层覆盖下的花岗岩母质土壤水分输入量有所增加，而玄武岩母质土壤的水分输入量则出现显著下降。
5. 与之相反，草食动物对土壤化学性质与温度的影响结果存在分化，但对土壤水分的影响更为明确：草食动物排除区外的样地土壤水分保有量更高，尤其是在降水事件刚结束后的时段。土壤容重作为养分循环与水分动态的共同驱动因子，在乔木冠层下以及草食动物被排除的样地中显著更低。
6. 综合分析结论：本研究结果表明，在本次研究涉及的稀树草原生态系统中，0~10cm土层的碳、氮、磷动态主要由乔木覆盖度的变异这一核心生物驱动因子主导；草食动物主要通过调控土壤含水量发挥生态作用；且乔木冠层与草食动物对土壤性质的影响多为加和效应，而非协同交互效应。