How drought modulates carbon-use efficiency and soil carbon formation in rhizosphere, detritusphere, and bulk soils

Carbon use efficiency (CUE) is theorized to be positively associated with the formation of microbially-derived, mineral-associated soil carbon.  Yet few empirical studies have directly tested this relationship. Moreover, it is unclear: (1) how differences between distinct soil microbial communities (for example, differences in competitive interactions and/or growth rate among rhizosphere, detritusphere, and bulk soil communities) may yield different relationships between carbon-use efficiency and soil carbon formation, and (2) how microbial ecophysiology – such as physiological changes induced by drought – may modulate the strength and/or direction of the CUE-soil carbon relationship.

To investigate these questions, we conducted a 12-week ¹³C tracer study to track the movement of two dominant sources of plant carbon – rhizodeposition and root detritus – into soil microbial communities and carbon pools under normal moisture vs drought conditions. Using a continuous ¹³CO₂-labeling system, we grew the Mediterranean annual grass Avena barbata in controlled growth chambers and measured the formation of organic matter from ¹³C-enriched rhizodeposition. As the plants grew, we harvested rhizosphere and bulk soil at three time points (4, 8, and 12 weeks) to capture changes in soil carbon pools and microbial community dynamics. In parallel microcosms, we tracked the formation of soil carbon derived from ¹³C-enriched A. barbata root detritus during 12 weeks of decomposition; harvesting detritusphere and bulk soil at 4,8, and 12 weeks. In all microcosms, we manipulated soil moisture to generate drought (7.8 ± 2.1 % soil moisture) and ‘normal moisture’ (15.1 ± 4.2 % soil moisture) treatments.

In all samples (over 150 observations), we measured CUE via the ¹⁸O-H₂O method, and quantified the formation of different ¹³C-soil organic carbon pools via density fractionation. Here we will present data on how soil moisture influences CUE in rhizosphere, detritusphere, and bulk soil communities, and whether differences in CUE are correlated with the formation of mineral-associated soil organic carbon. These results will help to illustrate whether CUE acts as a lynchpin variable with predictive power for stable soil carbon formation, or whether other microbial traits may require consideration.