DOI: 10.5061/dryad.5qfttdzbb
title: Drought intensity alters productivity, carbon allocation, and plant nitrogen uptake in fast versus slow grassland communities

description: 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.
description: <strong>Experimental
 set-up</strong> An outdoor mesocosm experiment was established
 at the Botanical Garden, University of Innsbruck, Austria
 (47°16'04.1"N
 11°22'46.3"E), which included five sets of mesocosms (7 L
 plastic pots, 21 cm Ø, 25 cm height, n = 210). Two sets were destructively
 harvested throughout the experiment at: peak drought (set 1), early
 recovery (set 2), and the final three sets were used for non-destructive
 measurements throughout the experiment and to monitor recovery (sets 3-5).
 The drought treatment followed a gradient design, which optimizes the use
 of experimental units to determine thresholds and non-linear
 responses (Kreyling et
 al., 2018). We created the
 drought intensity gradient by determining the soil water content (SWC,
 g<sub>water</sub> g<sub>fresh</sub>
 soil-1) at field capacity and then established a
 gradient of increasing drought intensity (increasing soil water deficit
 compared to field capacity, SWD) ranging from well-watered controls (20%
 SWD) to no water added in the drought period (98% SWD). This corresponded
 with a realised SWD of 18%
 to 95% and a realized SWC of 0.266 to 0.020 (g<sub>water</sub>
 g<sub>fresh</sub> soil-1). The experimental drought took place
 between July 21 and August 12-15, 2020, when a rainout-shelter was
 installed over all mesocosms (2.5 m aluminium frame covered with light and
 UV-B permeable plastic, Lumisol clear AF, Folitec, Westerburg, Germany).
 The drought intensity treatment (SWD) was maintained by weighing and
 watering the mesocosms 5-7 times per week to achieve their calculated
 target weight. The drought ended when mesocosms were re-wetted to their
 control weight (20% SWD) over three days.