Reductions in daily soil temperature variability increase soil microbial biomass C and decrease soil N availability in the Chihuahuan Desert: Potential implications for ecosystem C and N fluxes

Maximum and minimum soil temperatures affect belowground processes. In the past 50 years in arid regions, measured reductions in the daily temperature range of air (DTR _air) most likely generated similar reductions in the unmeasured daily temperature range of soil (DTR _soil). However, the role of DTR _soil in regulating microbial and plant processes has not been well described. We experimentally reduced DTR _soil in the Chihuahuan Desert at Big Bend National Park over 3 years. We used shade cloth that effectively decreased DTR _soil by decreasing daily maximum temperature and increasing nighttime minimum temperature. A reduction in DTR _soil generated on average a twofold increase in soil microbial biomass carbon, a 42% increase in soil CO ₂ efflux and a 16% reduction in soil NO ₃ ^--N availability; soil available NH ₄ ⁺-N was reduced by 18% in the third year only. Reductions in DTR _soil increased soil moisture up to 15% a few days after a substantial rainfall. Increased soil moisture contributed to higher soil CO ₂ efflux, but not microbial biomass carbon, which was significantly correlated with DTR _soil. Net photosynthetic rates at saturating light (A _sat) in Larrea tridentata were not affected by reductions in DTR _soil over the 3 year period. Arid ecosystems may become greater sources of C to the atmosphere with reduced DTR _soil, resulting in a positive feedback to rising global temperatures, if increased C loss is not eventually balanced by increased C uptake. Ultimately, ecosystem models of N and C fluxes will need to account for these temperature-driven processes.