Drought Reduces Release of Plant Matter Into Dissolved Organic Matter Potentially Restraining Ecosystem Recovery

Future climate scenarios indicate increasing drought intensity that threatens ecosystem functioning. However, the behavior of ecosystems during intense drought, such as the 2018 drought in Northern Europe, and their respective response following rewetting is not fully understood. We investigated the effect of drought on four different vegetation types in a temperate climate by analyzing dissolved organic matter (DOM) concentration and composition present in soil leachate, and compared it to two accompanying years. DOM is known to play an important role in ecosystem recovery and holds information on matter flows between plants, soil microorganisms and soil organic matter. Knowledge about DOM opens the possibility to better disentangle the role of plants and microorganisms in ecosystem recovery. We found that the average annual DOM concentration significantly decreased during the 2018 drought year compared to the normal year. This suggests a stimulation of DOM release under normal conditions, which include a summer drought followed by a rewetting period. The rewetting period, which holds high DOM concentrations, was suppressed under more intense drought. Our detailed molecular analysis of DOM using ultrahigh resolution mass spectrometry showed that DOM present at the beginning of the rewetting period resembles plant matter, whereas in later phases the DOM molecular composition was modified by microorganisms. We observed this pattern in all four vegetation types analyzed, although vegetation types differed in DOM concentration and composition. Our results suggest that plant matter drives ecosystem recovery and that increasing drought intensity may lower the potential for ecosystem recovery

Stability and carbon uptake of the soil microbial community is determined by differences between rhizosphere and bulk soil

The interactions between plants and soil microorganisms are fundamental for ecosystem functioning. However, it remains unclear if seasonality of plant growth impacts plant-microbial interactions, such as by inducing shifts in the microbial community composition, their biomass, or changes in the microbial uptake of plant-derived carbon. Here, we investigated the stability of the microbial community and their net assimilation of plant-derived carbon over an entire growing season. Using a C3–C4 vegetation change experiment, and taking advantage of a natural 13C label, we measured the plant-derived carbon in lipid biomarkers of soil microorganisms in rhizosphere and bulk soil in two soils with contrasting textures. We found that temporal stability was higher in bacterial than in fungal biomass, whereas the spatial stability of the fungal biomass was higher than that of bacterial biomass. Moreover, symbiotic AM fungi tended to be more stable in the uptake of plant-derived carbon than bacteria and saprophytic fungi. While soil texture did influence microbial community composition as expected, it had no effect on the microbial plant carbon assimilation and the differences between rhizosphere and bulk soil. In addition, the putative differences in carbon utilization between microbial groups, with the exception of AM fungi, were generally smaller than expected, reflecting opportunistic utilization of energy sources. Our results suggest that microbial uptake of plant carbon is primarily limited by plant carbon allocation rather than by environmental factors such as soil texture and seasonality. This indicates that the ongoing carbon assimilation during the growing season is supported by a functional redundancy within the microbial community, which, in turn, helps sustain ecosystem functioning.ISSN:0038-0717ISSN:1879-342