How does watering change the journey of tree carbon into soil?

Details

Aboveground and belowground parts of ecosystems are closely linked over timescales from days to years, but this connection and the effects of drought on it remain poorly understood in mature forests.

To measure how drought affects the transfer and use of tree assimilates in the rhizosphere and its microbial communities, researchers pulse-labelled 100-year-old trees with 13CO2 within a 15-year irrigation experiment in a naturally dry pine forest. The rhizosphere is the soil region influenced by roots.

New 13C-labelled assimilates took 4 days to reach the rhizosphere. A year after the 3-hour pulse, its 13C signal was still detectable in stem and soil respiration. This provides evidence that trees store some assimilates before they are used in rhizosphere metabolic processes.

Irrigation that removed natural water stress reduced the mean carbon residence time from canopy uptake to soil respiration from 89 to 40 days. It increased the amount of assimilates transferred to and respired in the soil within the first 10 days by 370%. A small precipitation event that rewetted surface soils rapidly changed this pattern, reducing the effect to +35%.

Microbial biomass incorporated 46 ± 5% of the carbon used in the rhizosphere under the dry control, compared with 31 ± 7% under irrigation. Mapping soil-respired 13CO2 around the 10 pulse-labelled trees showed that their rhizospheres extended laterally 2.8 times beyond their canopies, implying strong overlap between neighbouring trees' rhizospheres. Irrigation increased rhizosphere area by 60%, providing evidence of long-term acclimation of trees and their rhizospheres to the drought regime.

The researchers concluded that moisture-sensitive carbon transfer and use in the rhizosphere has consequences for carbon allocation within trees, soil microbial communities and soil carbon storage.