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

Abstract

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￢ﾀﾓsoil carbon (C) and nitrogen (N) cycling. We exposed model grassland plant communities with contrasting resource acquisition strategies (a fast￢ﾀﾐ vs a slow￢ﾀﾐstrategy plant community), to a gradient of drought intensity. The drought gradient ranged from well￢ﾀﾐwatered to severely water￢ﾀﾐlimited conditions. We identified thresholds of plant community productivity (above￢ﾀﾐground biomass) at peak drought and 2￢ﾀﾉmonths after re￢ﾀﾐwetting, and measured net ecosystem exchange and ecosystem respiration of Cￂﾠthroughout the drought and recovery phases. At peak drought and 1￢ﾀﾉweek after re￢ﾀﾐ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￢ﾀﾐstrategy plant communities were more drought resistant than fast￢ﾀﾐstrategy communities, as the threshold in plant productivity occurred at a higher drought intensity for the slow￢ﾀﾐ than the fast￢ﾀﾐstrategy community. Shortly after re￢ﾀﾐwetting, microbial uptake of recent plant￢ﾀﾐ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￢ﾀﾐlinear recovery responses, with greater compensatory growth in the fast￢ﾀﾐstrategy community. At peak drought, increasing drought intensity reduced C uptake and increased relative C partitioning to leaves and microbial biomass. Upon re￢ﾀﾐwetting, plant community strategy mediated drought intensity effects on plant and soil C and N dynamics and plant recovery trajectories. The fast￢ﾀﾐ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￢ﾀﾓsoil system display non￢ﾀﾐlinear responses to increasing drought intensity both during and after drought, which has implications for plant community recovery trajectories.