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.