Seasonal variation in N availability and nitrification rates due to precipitation variability and elevation

Background/Question/Methods

There are many factors that influence N-cycling within an ecosystem. Within this wide range of influences, recent studies have reemphasized the variability of nitrogen availability at small temporal and spatial scales. Seasonal variation, including temperature and plant community composition, is an important driver in the rates of N-mineralization and nitrification. To test this hypothesis, we experimentally altered the timing and size of rainfall events and reduced ambient rainfall during the growing season for xeric, low-elevation sites and mesic, high-elevation sites at the Great Basin Experimental Range in Ephraim Canyon, UT, USA. The experimental treatments were (1) ambient rain, (2) 30% reduction in ambient rain, (3) 70% reduction in ambient rain, (4) reapplication of ambient rain weekly, and (5) reapplication of ambient rain every 3 weeks. During this 3 year-long experiment, we monitored N-mineralization, nitrification, plant biomass and community-type, soil-moisture content and temperature. We also used Century Ecosystem Model to project long-term effects not experimentally feasible. Here we report the results of this experiment.

Results/Conclusions

We found that that availability of inorganic nitrogen increased from June to August. In addition, as the growing season progressed, the rate of nitrification increased so that the highest nitrate-availability was in August. This indicates a positive temperature correlation with nitrification. During this same period we observed a decrease in soil moisture from saturated soils at the beginning of the growing season to water potentials below wilting point during late July and August. As a result, this indicates that within this system, moisture-availability is not as much of a control as was previously thought. However, modeling results suggest that drought is a stronger control over N-availability that precipitation timing. Our results indicate that seasonal changes such as temperature are important drivers of N-availability in both high-elevation and low-elevation sites.