AUTHOR=Karlowsky Stefan , Augusti Angela , Ingrisch Johannes , Akanda Mohammad Kamal Uddin , Bahn Michael , Gleixner Gerd 

TITLE=Drought-Induced Accumulation of Root Exudates Supports Post-drought Recovery of Microbes in Mountain Grassland

JOURNAL=Frontiers in Plant Science

VOLUME=Volume 9 - 2018

YEAR=2018

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2018.01593

DOI=10.3389/fpls.2018.01593

ISSN=1664-462X

ABSTRACT=Droughts strongly affect carbon and nitrogen cycling in grasslands, with consequences for ecosystem productivity. Therefore, we investigated how experimental grassland communities interact with groups of soil microorganisms. In particular, we explored the mechanisms of the drought-induced decoupling of plant photosynthesis and microbial carbon cycling and its recovery after rewetting. Our aim was to better understand how root exudation during drought is linked to pulses of soil microbial activity and changes in plant nitrogen uptake after rewetting.
We set up a mesocosm experiment on a meadow site and used shelters to simulate drought. We performed two 13C-CO2 pulse labelings, the first at peak drought and the second in the recovery phase, and traced the flow of assimilates into carbohydrates of plants, and water extractable organic carbon and microorganisms from soil. Total microbial tracer uptake in the main metabolism was estimated by chloroform fumigation extraction, while lipid biomarkers were used to assess differences between microbial groups.
Drought led to a reduction of above- versus belowground plant growth and to an increase of 13C tracer contents in carbohydrates, particularly in roots. Newly assimilated 13C tracer unexpectedly accumulated in water extractable soil organic carbon, indicating that root exudation continued during drought. In contrast, drought strongly reduced the amount of 13C tracer assimilated into soil microorganisms. This reduction was more severe in the growth related lipid biomarkers than in metabolic compounds, suggesting a slowdown of microbial processes at peak drought. Shortly after rewetting, the tracer accumulation in belowground plant carbohydrates and in water extractable soil organic carbon disappeared. Interestingly, this was paralleled by a quick recovery of the carbon uptake into metabolic and growth related compounds from rhizospheric microorganisms, which was probably related to the higher nitrogen supply to plant shoots. 
We conclude that the decoupling of plant photosynthesis and soil microbial carbon cycling during drought is due to reduced carbon uptake and metabolic turnover of rhizospheric soil microorganisms. Moreover, our study suggests that the maintenance of root exudation during drought is connected to a fast re-initiation of soil microbial activity after rewetting, supporting plant recovery through increased nitrogen availability.