Drought intensity alters productivity, carbon allocation, and plant nitrogen uptake in fast versus slow grassland communities

Grasslands face more frequent and extreme droughts, yet their responses to increasing drought intensity are poorly understood. Increasing drought intensity likely triggers abrupt shifts (thresholds) in grassland ecosystem functioning which can implicate recovery trajectories. Here, we determined how drought intensity affects plant productivity, and plant-soil carbon (C) and nitrogen (N) cycling. We exposed model grassland plant communities with contrasting resource acquisition strategies (a fast- vs a slow-strategy plant community), to a gradient of drought intensity. The drought gradient ranged from well-watered to severely water-limited conditions. We identified thresholds of plant community productivity (above-ground biomass) at peak drought and two months after re-wetting, and measured net ecosystem exchange and ecosystem respiration of carbon throughout the drought and recovery phases. At peak drought and one week after re-wetting, we traced recently acquired C from plants to the soil and into microbial biomass and fatty acids using 13C pulse labelling, and measured plant and soil N. At peak drought, slow-strategy plant communities were more drought resistant than fast-strategy communities, as the threshold in plant productivity occurred at a higher drought intensity for the slow- than the fast-strategy community. Shortly after re-wetting, microbial uptake of recent plant-assimilated C increased with increasing past drought intensity, coinciding with an increase in soil N availability and leaf N. Threshold responses to drought intensity at peak drought translated into non-linear recovery responses, with greater compensatory growth in the fast-strategy community. At peak drought, increasing drought intensity reduced C uptake and increased relative C partitioning to leaves and microbial biomass. Upon re-wetting, plant community strategy mediated drought intensity effects on plant and soil C and N dynamics and plant recovery trajectories. The fast strategy community recovered quickly, with higher leaf N than the slow community, while the slow community increased C allocation to microbial biomass. Synthesis: Our findings highlight that C and N dynamics in the plant-soil system display non-linear responses to increasing drought intensity both during and after drought, which has implications for trajectories of plant community recovery.

草原生态系统正面临愈发频繁且极端的干旱事件，然而人们对其随干旱强度加剧的响应机制仍知之甚少。干旱强度的提升可能会触发草原生态系统功能的突变（阈值），进而影响生态系统的恢复轨迹。本研究明确了干旱强度如何影响植物生产力，以及植物-土壤碳（C）、氮（N）循环过程。我们将具有不同资源获取策略的模拟草原植物群落——快速策略群落与慢速策略群落——置于干旱强度梯度环境中培养，该梯度涵盖了从水分充足到严重缺水的全部水分条件。我们分别在干旱峰值时期以及复湿两个月后，确定了植物群落生产力（地上生物量）的阈值，并在整个干旱与恢复阶段监测了生态系统碳净交换量与生态系统呼吸速率。在干旱峰值时期以及复湿一周后，我们通过13C脉冲标记（13C pulse labelling）追踪了植物近期固定的碳从植株向土壤、再到微生物生物量与脂肪酸的转移路径，并测定了植物与土壤中的氮含量。干旱峰值时期，慢速策略植物群落的抗旱性高于快速策略群落：慢速策略群落的植物生产力阈值对应的干旱强度高于快速策略群落。复湿后不久，近期来自植物同化物的碳的微生物摄取量随既往干旱强度的提升而增加，这与土壤氮有效性及叶片氮含量的上升相吻合。干旱峰值时期对干旱强度的阈值响应会转化为非线性的恢复响应，其中快速策略群落展现出更强的补偿性生长。在干旱峰值时期，干旱强度的提升会降低碳吸收量，并增加碳向叶片与微生物生物量的相对分配比例。复湿后，植物群落策略会调控干旱强度对植物与土壤碳氮动态以及植物恢复轨迹的影响。快速策略群落恢复更快，其叶片氮含量高于慢速策略群落；而慢速策略群落则会增加碳向微生物生物量的分配。综合分析表明：本研究结果揭示，植物-土壤系统中的碳氮动态在干旱期间与干旱后均会随干旱强度加剧呈现非线性响应，这对植物群落的恢复轨迹具有重要启示意义。