Background/Question/Methods

The frequency and severity of drought is increasing in semi-arid, mid-latitude regions over recent decades, and is expected to continue increasing with climate change.  The increase in the number of drought events is altering the structure and function of piñon-juniper woodlands in the southwestern US. To assess drought induced changes on ecosystem function, specifically nitrogen cycling, we took advantage of a large-scale replicated (n=3) precipitation manipulation (50% reduction drought treatment, a 50% increased precipitation treatment, a water removal and reapplication control, and an unmanipulated control) at the Sevilleta Long-Term Ecological Research site in central New Mexico. In the summer of 2008, ion-exchange resin probes were incubated for 6-weeks beneath 5 piñon and 5 juniper trees in each of the 12 plots to assess nitrate and ammonium availability. In addition, 120 soil cores were removed from the same trees and incubated in the laboratory for 30 and 60 days to determine treatment effects on potential nitrogen mineralization.

Results/Conclusions

We found that nitrogen availability increased 92% in the water removal treatments relative to the water addition treatments (F = 5.3, P < 0.01). In addition, there was 29% more nitrogen available under piñon relative to juniper (F = 4.4, p = 0.04).  Preliminary results for the laboratory incubation indicate that mineralization is lower in the drought treatments relative to the rest of the treatments. These data suggest that chronic drought may decrease leaching, mobility, and nitrogen uptake by plants resulting in an increase in available nitrogen. In addition, data suggest that nitrogen availability is dramatically increased under piñon potentially due to adjustments in leaf litter quality.  These findings show that changes in precipitation may alter the nitrogen cycle in this system resulting in a decrease in available, mobile nitrogen in an already nutrient limited ecosystem.