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（Savannahs）是高度异质性的乔草混合生态系统，不连续的冠层覆盖所带来的结构变异，会导致土壤性质出现空间分异，具体包括植物有效养分、土壤温度与土壤湿度。多数稀树草原生态系统还以大型脊椎动物植食者为优势类群，这类生物会通过取食、排泄与物理扰动等一系列途径，对土壤性质产生独特的影响。
2. 在养分匮乏、水分受限的生境中，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中异质性关键驱动因子的相对作用与交互效应的研究仍较为匮乏。
3. 本研究于南非克鲁格国家公园（Kruger National Park）的两个对比样地中，探究了植食者与乔木冠层覆盖对0~10cm深度范围内多种土壤性质的独立与交互效应，所测指标包括容重（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。研究采用嵌套实验设计，对成年乔木冠层下方与冠层间隙区域，以及长期植食者围封区（herbivore exclosures）内外的土壤样品开展了分析。
4. 研究结果显示，乔木冠层对土壤碳（C）、氮（N）的全量库与活性库均具有显著正向效应，而对植物有效态磷（P）与土壤温度则表现为负向效应。乔木冠层对土壤水分可利用性的影响虽微弱，但可被清晰检测到。在降水事件发生期间，与开阔生境相比，花岗岩土壤上的乔木会提升水分输入量，而玄武岩土壤上的乔木则会降低水分输入量。
5. 与之相反，本研究关于植食者对土壤化学性质与温度的影响结果存在分歧，但对土壤水分的影响更为明确：围封区外的区域土壤水分保有量更高，尤其在降水事件刚结束后更为显著。土壤容重作为养分与土壤水分动态的共同驱动因子，在乔木冠层下方以及排除植食者的样地中更低。
6. 研究总结：本研究结果表明，在该稀树草原系统的0~10cm表层土壤中，乔木覆盖度的变异是驱动碳、氮、磷动态的主导生物因子；植食者主要通过影响土壤含水量发挥作用；且乔木冠层与植食者对土壤性质的影响多为加性效应，而非协同效应。