Climate change is expected to impact precipitation regimes; for instance models specifically reveal reduced precipitation in North America. This study looks at the impacts of changes in precipitation quantity and nutrient levels on soil respiration in the Chihuahuan Desert in New Mexico. Specifically, soil respiration is measured to examine the effects on belowground activity of changing precipitation levels in past and current years, and the effects of nitrogen fertilization. Soil respiration was expected to increase with rainfall in previous years (the legacy effect) and in the current year due to increased root growth and microbial activity. Nitrogen addition was expected to increase soil respiration rates due to increased root growth, which also creates substrate for increased microbial activity. Lastly, soil respiration rates in plots irrigated once were expected to merge with rates in control plots. Rainout shelters and irrigation systems were used to change rainfall quantity by -80%, -50%, 0, +50% and +80% in the 2007-2008 growing season, and then treatments either remained the same or reversed for the 2009 growing season at -80%, 0, or +80%. Soil respiration was measured in each of the 22 precipitation treatments with four replicated 2.5x2.5-m plots. Plots were also measured in treatments with consistently +80%, 0, or -80% precipitation, and with or without nitrogen addition, with six repetitions each. Ten 1x1 m plots were created; where 5 plots were irrigated once with 10-mm of precipitation, and 5 remained as controls. Soil respiration rates were measured for 16 days after irrigation to examine how soil respiration changes over time.  

Results/Conclusions

The results of the rainfall experiment revealed the expected changes in soil respiration (R²=0.6179); soil respiration increased with precipitation in the previous years (p=0.0400) and in the current year (p=0.0004). Nitrogen had no effect on soil respiration rates, conflicting with the hypothesized results. Negligible nutrient effect may be due to stomatal limitations on photosynthesis when nitrogen is available in excess. The results of the time response experiment revealed that the difference in soil respiration between irrigated and non-irrigated plots decreased over time, but increased after natural precipitation events. This increased difference between irrigated and control plots after rainfall may be due to displacement of CO₂ with soil water saturation. These results give us insight on how belowground activity and a specific carbon flux between the soil and the atmosphere may change with the changes predicted in climate models.