Wetting-induced soil CO 2 emission pulses are driven by interactions among soil temperature, carbon, and nitrogen limitation in the Colorado Desert.

Warming-induced changes in precipitation regimes, coupled with anthropogenically-enhanced nitrogen (N) deposition, are likely to increase the prevalence, duration, and magnitude of soil respiration pulses following wetting via interactions among temperature and carbon (C) and N availability. Quantifying the importance of these interactive controls on soil respiration is a key challenge as pulses can be large terrestrial sources of atmospheric carbon dioxide (CO 2 ) over comparatively short timescales. Using an automated sensor system, we measured soil CO 2 flux dynamics in the Colorado Desert-a system characterized by pronounced transitions from dry-to-wet soil conditions-through a multi-year series of experimental wetting campaigns. Experimental manipulations included combinations of C and N additions across a range of ambient temperatures and across five sites varying in atmospheric N deposition. We found soil CO 2 pulses following wetting were highly predictable from peak instantaneous CO 2 flux measurements. CO 2 pulses consistently increased with temperature, and temperature at time of wetting positively correlated to CO 2 pulse magnitude. Experimentally adding N along the N deposition gradient generated contrasting pulse responses: adding N increased CO 2 pulses in low N deposition sites, whereas adding N decreased CO 2 pulses in high N deposition sites. At a low N deposition site, simultaneous additions of C and N during wetting led to the highest observed soil CO 2 fluxes reported globally at 299.5 μmol CO 2 m -2 s -1 . Our results suggest that soils have the capacity to emit high amounts of CO 2 within small timeframes following infrequent wetting, and pulse sizes reflect a non-linear combination of soil resource and temperature interactions. Importantly, the largest soil CO 2 emissions occurred when multiple resources were amended simultaneously in historically resource-limited desert soils, pointing to regions experiencing simultaneous effects of desertification and urbanization as key locations in future global C balance.