Net Ecosystem Production and Nitrogen Balance in Irrigated and Dryland Agricultural Sites in the Central Grassland Region of the U.S.

Humans have dramatically influenced the amount of carbon and nitrogen stored in terrestrial ecosystems of the Central Grassland region of the U.S. Cultivation management practices have had the largest impact across the region. Early cultivation management practices resulted in net N export from the region, likely accounting for as much 1080 kg N ha-1 averaged for cultivated fields, or approximately 20% of the N stored in surface soils of the region. During this same time period, losses of C likely exceeded those of N, resulting in narrowed C:N. During the last 5 decades, however, with the introduction of fertilization and the increase in N-fixing crops, humans are adding more N to cultivated ecosystems than is removed in crop products. We estimate that, on average, 25 kg ha-1 y-1 is added to cultivated systems of the region. It is unclear what the regional trends in C storage are, given the high rates of production but also very high rates of C export and decomposition. If ecosystems are not aggrading and are in balance with respect to organic matter content, the excess N additions may be lost via volatile ammonia, nitric or nitrous oxide gaseous efflux, through surface runoff, or by nitrate leaching. Many of these loss vectors have the potential to alter regional to global scale biogeochemical cycling.