Abstract
Above and belowground compartments in ecosystems are closely coupled on daily to annual timescales. In mature forests, this interlinkage and how it is impacted by drought is still poorly understood. Here, we pulse‐labelled 100‐year‐old trees with 13CO2 within a 15‐year‐long irrigation experiment in a naturally dry pine forest to quantify how drought regime affects the transfer and use of assimilates from trees to the rhizosphere and associated microbial communities. It took 4 days until new 13C‐labelled assimilates were allocated to the rhizosphere. One year later, the 13C signal of the 3‐h long pulse labelling was still detectable in stem and soil respiration, which provides evidence that parts of the assimilates are stored in trees before they are used for metabolic processes in the rhizosphere. Irrigation removing the natural water stress reduced the mean C residence time from canopy uptake until soil respiration from 89 to 40 days. Moreover, irrigation increased the amount of assimilates transferred to and respired in the soil within the first 10 days by 370%. A small precipitation event rewetting surface soils altered this pattern rapidly and reduced the effect size to +35%. Microbial biomass incorporated 46 ± 5% and 31 ± 7% of the C used in the rhizosphere in the dry control and irrigation treatment respectively. Mapping the spatial distribution of soil‐respired 13CO2 around the 10 pulse‐labelled trees showed that tree rhizospheres extended laterally 2.8 times beyond tree canopies, implying that there is a strong overlap of the rhizosphere among adjacent trees. Irrigation increased the rhizosphere area by 60%, which gives evidence of a long‐term acclimation of trees and their rhizosphere to the drought regime. The moisture‐sensitive transfer and use of C in the rhizosphere has consequences for C allocation within trees, soil microbial communities and soil carbon storage.
Highlights
Water is essential for all organisms and the increasing intensity and frequency of drought in large areas of the Northern hemisphere is impacting the diversity and functioning of forest ecosystems
Our hypotheses were that in this naturally dry pine forest (1) the velocity and magnitude of the allocation of recent assimilates to the belowground decreases with decreasing soil moisture, primarily due to a declining C demand of the rhizobiome and that (2) the difference between the irrigated and the natural dry site is reinforced by the 15-year-long tree growth stimulation through irrigation leading to a greater rhizosphere
Our whole tree pulse-labelling experiment in a mature forest documents that the drought regime affects the coupling of the above-and belowground system and the feedback between plant and soil processes at various timescales
Summary
Water is essential for all organisms and the increasing intensity and frequency of drought in large areas of the Northern hemisphere is impacting the diversity and functioning of forest ecosystems. While mild drought frequently increases C allocation belowground (Gaul et al, 2008), more severe and long- lasting drought was observed to lead to a decrease (Hommel et al, 2016) These acclimative allocation and growth adjustments under drought will impact the C transfer from plants to soils, but these indirect legacy effects remain unknown for forests. Our hypotheses were that in this naturally dry pine forest (1) the velocity and magnitude of the allocation of recent assimilates to the belowground decreases with decreasing soil moisture, primarily due to a declining C demand of the rhizobiome and that (2) the difference between the irrigated and the natural dry site is reinforced by the 15-year-long tree growth stimulation through irrigation leading to a greater rhizosphere
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