Elevated [CO2] changes soil organic matter composition and substrate diversity in an arid ecosystem

Little is known about how elevated atmospheric [CO2] will impact the dynamics of soil organic matter (SOM) in arid ecosystems. Evans et al. (2014) reported greater ecosystem carbon (C) and nitrogen (N) concentrations following 10 years exposure to elevated atmospheric [CO2] at the Nevada Desert Free-Air Carbon dioxide Enrichment (FACE) Facility (NDFF). In this study, we investigated potential mechanisms of SOC and total N accumulation and potential SOM stabilization using high resolution mass spectrometry. Samples were collected from soil profiles to 1 m in depth with 0.2 m increment under the dominant evergreen shrub Larrea tridentate and were air dried at room temperature. SOM was extracted sequentially with solvents with different polarity. The differences in the molecular composition and diversity of SOM in the different extracts were more evident in surface soils and declined with depth, and were consistent with higher SOC and total N concentrations under elevated than ambient [CO2]. Our results support the hypothesis that increased root exudation and/or microbial necromass from stabilization of labile C and N can contribute to SOM and N pools. We found that plant-derived compounds were primary substrates for microbial activity under elevated [CO2] and microbial necromass were the main constituents of stabilized SOM. Our results suggest that arid ecosystems are a potential large C sink under elevated [CO2], given arid ecosystems constitute 47% of the terrestrial land surface, and that labile compounds are transformed to stable SOM via microbial processes. Arid systems are limited by water, and thus may have a different C storage potential under changing climates than other ecosystems that are limited by nitrogen or phosphorus.