Influence of C and N availability on repeated wetting-induced pulses of soil respiration in a southern California desert

Background/Question/Methods

            Wetting-induced pulses of soil respiration (R_soil) dominate annual emissions in arid environments, and climate change-induced shifts in precipitation patterns make understanding regulation of these pulses increasingly important.  We investigated the influence of carbon (C) and nitrogen (N) availability on R_soil pulse responses in two desert sites, representing high and low anthropogenic N deposition.   We hypothesized that C and N availability would influence both the timing and the magnitude of the pulse response.  Collars placed under the canopy of the dominant vegetation, Larrea tridentata, received a 3 cm artificial rain event with one of the following treatments: water only, N (45 kg N ha^-1 NH₄NO₃), C (90 g/L dextrose) and both N and C.  Additional collars served as a control (no addition) and to monitor soil temperature and moisture after water addition.  R_soil was measured before treatment additions and then at 5, 15, 30 and 60 minutes after.  On subsequent days, R_soilwas measured before and one hour after another 3 cm wetting event to follow the length of the pulse response for each treatment, with and without the influence of soil moisture.

Results/Conclusions

            R_soil at the low N deposition site (Low) ranged from 0 to 19.52 µmol m^-2 s^-1 across all treatments.  The Low site demonstrated peak flux rates of 7±5, 10±2, 17±3, 13±5  (mean ± standard deviation) within 5 minutes of treatment additions for water, nitrogen, dextrose and both, respectively.  The high N deposition site (High) demonstrated R_soil from 0.18 to 42.16 µmol m^-2 s^-1.  Unlike the Low site, peak fluxes at High occurred at different times depending on treatment.   Peaks were 10±3 at 15 minutes for water only and 8±2 for the N treatment.  The two treatments that included dextrose peaked 48 hours after addition at 17±11 for C alone and 27±13 for both C and N.  At both High and Low, C addition resulted in fluxes that were significantly greater than water alone (p<0.05), but N had no effect.  On the first day of study, the Low site demonstrated higher fluxes, but peak fluxes were significantly greater at the High site (p<0.05).  Vegetation at the High site was larger with a deeper litter layer than the Low site, consistent with the effects of N deposition.  These results highlight the importance of a potential legacy effect from vegetation productivity and soil C and N status on influencing both the magnitude and the timing of wetting-induced pulses of soil respiration.