Climate models predict that precipitation patterns will change in the coming decades, and in the U.S. Great Plains, the frequency and duration of summer droughts is predicted to increase. Because water is the most frequently limiting resource in semi-arid systems, increased drought will have significant effects on ecosystem dynamics. In addition to an expected decrease in production, changes in biogeochemical cycling may create new feedbacks and irreversible community shifts. Specifically, changes in water and nitrogen availability may cause linked carbon and nitrogen processes to become asynchronous, changing retention and loss patterns that control ecosystem function. In this study, I used 9-year drought manipulations of 25% and 50% reduced rainfall at the Shortgrass Steppe Long Term Ecological Research site to ask how primary production, species composition, and biogeochemical pools are differentially affected by long-term drought. To quantify nitrogen dynamics throughout the growing season, I measured in situ N2O flux using trace gas flux chambers, and inorganic nitrogen pools using ion-exchange membranes. I also measured above and below-ground net primary productivity, CO2 flux, and soil organic carbon to capture changes in carbon dynamics.
Results/Conclusions
I hypothesized that plots subjected to more severe drought would have higher levels of inorganic nitrogen and greater N2O flux as a result of decreased nitrogen retention in drought systems. However, inorganic nitrogen pools and N2O flux were not significantly different among control and drought treatments. Further, very low N2O flux values suggest that this is not a significant nitrogen flux in the shortgrass steppe. Primary production and plant species composition were significantly different in treatments, but the 50% reduction was more similar to the 25% reduction than the control. These results suggest that after 8 years of drought, biogeochemical fluxes may have equilibrated to decreased production levels, resulting in less accumulation of inorganic nitrogen pools. This finding is contrary to short-term studies that suggest that drought causes carbon and nitrogen cycles to become asynchronous. In the context of global change, improved knowledge of ecosystem response to long-term, enduring stress is essential. These results have implications for accurately predicting the response of the semiarid ecosystems to climate change, and for elucidating controls on linkages between carbon and nitrogen cycling.
