Data from: Land use in mountain grasslands alters drought response and recovery of carbon allocation and plant-microbial interactions

1. Mountain grasslands have recently been exposed to substantial changes in land-use and climate and in the near future will likely face an increased frequency of extreme droughts. To date is not known how the drought responses of carbon (C) allocation, a key process in the C cycle, are affected by land-use changes in mountain grassland. 2. We performed an experimental summer drought on an abandoned grassland and a traditionally managed hay meadow and traced the fate of recent assimilates through the plant-soil continuum. We applied two 13CO2 pulses, at peak drought and in the recovery phase shortly after rewetting. 3. Drought decreased total C uptake in both grassland types and led to a loss of aboveground carbohydrate storage pools. The belowground C allocation to root sucrose was enhanced by drought, especially in the meadow, which also held larger root carbohydrate storage pools. 4. The microbial community of the abandoned grassland comprised more saprotrophic fungal and Gram (+) bacterial markers compared to the meadow. Drought increased the newly introduced AM and saprotrophic fungi:bacteria ratio in both grassland types. At peak drought the 13C transfer into AM fungi, saprotrophic fungi and Gram (-) bacteria was more strongly reduced in the meadow than in the abandoned grassland, which contrasted the patterns of the root carbohydrate pools. 5. In both grassland types the C allocation largely recovered after rewetting. Slowest recovery was found for AM fungi and their 13C uptake. In contrast, all bacterial markers quickly recovered C uptake. In the meadow, where plant nitrate uptake was enhanced after drought, C uptake was even higher than in control plots. 6. Synthesis. Our results suggest that resistance and resilience (i.e. recovery) of plant C dynamics and plant-microbial interactions are negatively related, i.e. high resistance is followed by slow recovery and vice versa. The abandoned grassland was more resistant to drought than the meadow and possibly had a stronger link to AM fungi that could have provided better access to water through the hyphal network. In contrast, meadow communities strongly reduced C allocation to storage and C transfer to the microbial community in the drought phase, but in the recovery phase invested C resources in the bacterial communities to gain more nutrients for regrowth. We conclude that management of mountain grasslands increases their resilience to drought.

1. 山地草原近期正经历土地利用与气候的显著变化，且未来极可能面临极端干旱事件频次增加的威胁。截至目前，学界尚未明确碳循环关键过程之一的碳（C）分配对干旱的响应如何受山地草原土地利用变化的调控。
2. 本研究针对弃耕草原与传统管理的刈草甸实施夏季干旱胁迫实验，并通过植物-土壤连续体追踪近期同化物的去向。我们分别在干旱峰值期及复水后不久的恢复阶段，施加两次13CO2脉冲标记。
3. 干旱胁迫降低了两种草原类型的总碳吸收量，并导致地上碳水化合物储存库出现损耗。同时，干旱提升了地下碳向根系蔗糖的分配比例，这一效应在刈草甸中尤为显著；且刈草甸的根系碳水化合物储存库规模本身更大。
4. 与刈草甸相比，弃耕草原的微生物群落中含有更多腐生真菌与革兰氏阳性（Gram-positive）细菌标志物。干旱胁迫提升了两种草原类型中新引入的丛枝菌根（Arbuscular Mycorrhizal, AM）真菌与腐生真菌的真菌-细菌比值。在干旱峰值期，刈草甸中13C向AM真菌、腐生真菌以及革兰氏阴性（Gram-negative）细菌的转移量降幅较弃耕草原更为显著，这与根系碳水化合物储存库的变化模式形成鲜明对比。
5. 两种草原类型的碳分配在复水后均基本恢复。其中AM真菌及其13C吸收量的恢复速度最慢。与之相反，所有细菌标志物的碳吸收量均快速恢复。在刈草甸中，干旱后植物的硝酸盐吸收量有所提升，其碳吸收量甚至高于对照样地。
6. 综合分析表明，植物碳动态与植物-微生物互作的抗性和恢复力（即恢复能力）呈负相关关系，即抗性越高则恢复速度越慢，反之亦然。弃耕草原的干旱抗性较刈草甸更强，且其与AM真菌的联系可能更为紧密，可通过菌丝网络获取更充足的水分。与之相反，刈草甸群落于干旱阶段大幅降低了碳向储存库的分配以及向微生物群落的碳转移，但在恢复阶段则将碳资源投入细菌群落，以获取更多养分用于恢复生长。本研究最终认为，山地草原的人为管理可提升其应对干旱的恢复能力。