Data from: Facilitation by leguminous shrubs increases along a precipitation gradient

Combining nutrient dynamics (plant nutrient uptake and soil fertility) can help uncover mechanisms of shrub-grass interactions and assess the validity of the stress-gradient hypothesis, which predicts that facilitation between plants increases in stressful environments. However, how facilitation via shrub-mediated nutrient increases varies with precipitation is poorly resolved.
We first synthesized a global dataset from 66 studies and evaluated how shrubs affected soil organic carbon (C) and nitrogen (N) in grasslands along a precipitation gradient. We then made new measurements in a single grassland type encroached by leguminous shrubs from the same genus (Caragana) to constrain the variations caused by combining different grassland types and shrubs traits in the meta-analysis. Specifically, we investigated how shrubs mediated N dynamics and how shrub-grass interactions varied along a precipitation gradient (147-342 mm) in a temperate steppe of China.
At the global scale, leguminous shrub-mediated effects on soil nutrients increased with precipitation, while no relationship was found for non-leguminous shrub. For the field experiment, greater N and lower δ15N in Caragana compared to non-leguminous shrub (reference shrub, Salsola collina) suggested active N-fixation in Caragana. We found that Caragana enhanced N concentration and leaf quality (low C:N ratio) in neighbouring plants more on mesic sites than on xeric sites. Thus, facilitation increased via higher soil N and with decreasing environmental stress, at least along this relatively arid precipitation gradient.
Our results highlight the importance of precipitation in determining the nutritional facilitation to neighbouring grasses from encroaching leguminous shrubs. Conceptual frameworks for plant facilitation may therefore need to include shrub characteristics (N-fixers vs. non-fixers) and positive effects of higher precipitation on this type of facilitation to characterize plant interactions along stress gradients.

结合养分动态（植物养分吸收与土壤肥力）研究，有助于揭示灌草互作的内在机制，并评估胁迫梯度假说（stress-gradient hypothesis）的合理性——该假说预测，植物间的促进作用会在胁迫环境中增强。然而，由灌木介导的养分提升所带来的促进作用如何随降水变化，目前仍未得到充分阐明。
我们首先整合了来自66项研究的全球数据集，评估了沿降水梯度分布的草原中，灌木对土壤有机碳（C）与氮（N）的影响。随后，我们在一处被同属豆科灌木（Caragana，锦鸡儿属）入侵的典型草原开展了新的野外测定，以约束荟萃分析（meta-analysis）中因整合不同草原类型与灌木性状所带来的变异。具体而言，我们在中国温带草原中，探究了沿147-342 mm降水梯度的灌木介导氮动态过程，以及灌草互作的变化规律。
在全球尺度上，豆科灌木对土壤养分的介导效应随降水增加而增强，而非豆科灌木则未呈现此种关联。野外实验结果显示，与非豆科灌木（参考灌木：猪毛菜Salsola collina）相比，锦鸡儿属植物的氮含量更高、δ¹⁵N值更低，表明其具备活跃的固氮能力。我们发现，相较于旱生生境，锦鸡儿属植物在中生环境中更能提升邻近植物的氮浓度与叶片质量（低C:N比）。因此，至少在该相对干旱的降水梯度范围内，促进作用会随土壤氮含量升高、环境胁迫减弱而增强。
本研究结果凸显了降水在调控入侵豆科灌木对邻近草本植物产生的养分促进作用中的关键意义。因此，用于阐释植物促进作用的概念框架，需纳入灌木特性（固氮型vs非固氮型）以及高降水对该类促进作用的正向效应，以精准刻画胁迫梯度下的植物互作规律。