Wetting-induced soil CO2 emission pulses are driven by interactions among soil temperature, carbon, and nitrogen limitation in the Colorado Desert

Abstract

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 (CO₂ ) over comparatively short timescales. Using an automated sensor system, we measured soil CO₂ 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 CO₂ pulses following wetting were highly predictable from peak instantaneous CO₂ flux measurements. CO₂ pulses consistently increased with temperature, and temperature at time of wetting positively correlated to CO₂ pulse magnitude. Experimentally adding N along the N deposition gradient generated contrasting pulse responses: adding N increased CO₂ pulses in low N deposition sites, whereas adding N decreased CO₂ 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 CO₂ fluxes reported globally at 299.5 μmol CO₂ m^-2 s^-1 . Our results suggest that soils have the capacity to emit high amounts of CO₂ within small timeframes following infrequent wetting, and pulse sizes reflect a non-linear combination of soil resource and temperature interactions. Importantly, the largest soil CO₂ 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.

Keywords: CO2 flux; N deposition; desert; soil C and N; soil wetting; temperature.