Submitted to: Biogeochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2007
Publication Date: 10/15/2007
Citation: Bhat, S., Hatfield, K., Jacobs, J., Lowrance, R.R., Williams, R.G. 2007. Surface runoff contribution of nitrogen during storm events in a forested watershed: Application of the Riparian Ecosystem Management Model (REMM). Biogeochemistry. 85(3):253-262. Interpretive Summary: Litterfall from trees is the most important source of nutrient recycling in unfertilized forests. In most forests, leaf litter is about 80% of total litterfall. After leaves have reached the forest floor the first rainfall events will leach nutrients, especially nitrogen, from leaves. This leaching can lead to increased nitrogen in soil water and streamflow. Unlike most water quality models, the Riparian Ecosystem Management Model (REMM) can simulate important processes such as litter leaching. REMM was used to model the riparian zone of a small stream at Ft. Benning, Georgia. Model output was similar to observed hydrology at the site after the model was calibrated. REMM effectively captured the trends in litter production, the nutrients in litter, and the increases in stream flow concentrations of nitrogen due to fresh litterfall. These results show the importance of the flush of nitrogen from litterfall and demonstrate that REMM can simulate these occurrences. This is important in simulating water quality in both forested and agricultural watersheds because of the increase in nitrogen loads and concentrations associated with the leaching of fresh leaf litter. Even though the effects will be most obvious in nutrient poor conditions such as the Ft. Benning site, it will occur in all forested watersheds and in agricultural watersheds with riparian forests.
Technical Abstract: Litterfall is usually the most important flux for the return of most nutrients to soils in forested ecosystems. Leaf litter comprises approximately 80% of the total litterfall. After leaves have fallen, the first precipitation events result in nitrogen leaching and a corresponding increase in nitrogen levels in the soil and stream water. This immediate release is of particular interest in low nutrient systems. In these systems, nutrient release prior to litter decomposition may be a significant source of stream water chemistry variation. To explore the nitrogen dynamics in the riparian area during litterfall, the Riparian Ecosystem Management Model developed by USDA-ARS was used to model a second-order watershed in Fort Benning, Georgia. The watershed was completely forested and had received no fertilizer input so it was nutrient-poor. For calibrated simulation results for a four-year period, the modeled stream flow had a Nash-Sutcliffe efficiency of 80%. The model effectively captured the trends of litter mass and litter nitrogen production in the riparian area and subsequent higher streamflow concentrations during those periods. Six storm events between October 2002 and May 2003 were analyzed to evaluate the effects of precipitation in leaching the nitrogen from freshly fallen leaves. The simulated total Kjeldahl N (ammonium & organic N) mass during the events closely matched the observed values. This is important in simulating water quality in both forested and agricultural watersheds because of the increase in nitrogen loads and concentrations associated with the leaching of fresh leaf litter. Even though the effects will be most obvious in nutrient poor conditions such as the Ft. Benning site, it will occur in all forested watersheds and in agricultural watersheds with riparian forests. Unless water quality models can simulate these processes, they will not correctly represent the contributions from forest lands or riparian buffers.