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\uffe2\uff80\uff93soil carbon (C) and nitrogen (N) cycling. We exposed model grassland plant communities with contrasting resource acquisition strategies (a fast\uffe2\uff80\uff90 vs a slow\uffe2\uff80\uff90strategy plant community), to a gradient of drought intensity. The drought gradient ranged from well\uffe2\uff80\uff90watered to severely water\uffe2\uff80\uff90limited conditions. We identified thresholds of plant community productivity (above\uffe2\uff80\uff90ground biomass) at peak drought and 2\uffe2\uff80\uff89months after re\uffe2\uff80\uff90wetting, and measured net ecosystem exchange and ecosystem respiration of C\uffc2\uffa0throughout the drought and recovery phases. At peak drought and 1\uffe2\uff80\uff89week after re\uffe2\uff80\uff90wetting, 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\uffe2\uff80\uff90strategy plant communities were more drought resistant than fast\uffe2\uff80\uff90strategy communities, as the threshold in plant productivity occurred at a higher drought intensity for the slow\uffe2\uff80\uff90 than the fast\uffe2\uff80\uff90strategy community. Shortly after re\uffe2\uff80\uff90wetting, microbial uptake of recent plant\uffe2\uff80\uff90assimilated 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\uffe2\uff80\uff90linear recovery responses, with greater compensatory growth in the fast\uffe2\uff80\uff90strategy community. At peak drought, increasing drought intensity reduced C uptake and increased relative C partitioning to leaves and microbial biomass. Upon re\uffe2\uff80\uff90wetting, plant community strategy mediated drought intensity effects on plant and soil C and N dynamics and plant recovery trajectories. The fast\uffe2\uff80\uff90strategy 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\uffe2\uff80\uff93soil system display non\uffe2\uff80\uff90linear responses to increasing drought intensity both during and after drought, which has implications for plant community recovery trajectories.
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\uffe2\uff80\uff93soil carbon (C) and nitrogen (N) cycling. We exposed model grassland plant communities with contrasting resource acquisition strategies (a fast\uffe2\uff80\uff90 vs a slow\uffe2\uff80\uff90strategy plant community), to a gradient of drought intensity. The drought gradient ranged from well\uffe2\uff80\uff90watered to severely water\uffe2\uff80\uff90limited conditions. We identified thresholds of plant community productivity (above\uffe2\uff80\uff90ground biomass) at peak drought and 2\uffe2\uff80\uff89months after re\uffe2\uff80\uff90wetting, and measured net ecosystem exchange and ecosystem respiration of C\uffc2\uffa0throughout the drought and recovery phases. At peak drought and 1\uffe2\uff80\uff89week after re\uffe2\uff80\uff90wetting, 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\uffe2\uff80\uff90strategy plant communities were more drought resistant than fast\uffe2\uff80\uff90strategy communities, as the threshold in plant productivity occurred at a higher drought intensity for the slow\uffe2\uff80\uff90 than the fast\uffe2\uff80\uff90strategy community. Shortly after re\uffe2\uff80\uff90wetting, microbial uptake of recent plant\uffe2\uff80\uff90assimilated 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\uffe2\uff80\uff90linear recovery responses, with greater compensatory growth in the fast\uffe2\uff80\uff90strategy community. At peak drought, increasing drought intensity reduced C uptake and increased relative C partitioning to leaves and microbial biomass. Upon re\uffe2\uff80\uff90wetting, plant community strategy mediated drought intensity effects on plant and soil C and N dynamics and plant recovery trajectories. The fast\uffe2\uff80\uff90strategy 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\uffe2\uff80\uff93soil system display non\uffe2\uff80\uff90linear responses to increasing drought intensity both during and after drought, which has implications for plant community recovery trajectories.