Long-Term Water Balance and Nitrate Biogeochemistry in Cultivated Alluvial Landscapes of Central Montana.

Understanding the effects of land use on ecosystem processes is essential for predicting the outcomes of climate change. A profound example is human transformation of the global nitrogen cycle for food production, leading to unprecedented fluxes of industrially fixed N between water, land and atmosphere, with consequences for biological productivity and ecosystem health. Globally, the net flux of industrially fixed fertilizer nitrogen to groundwater has reached at least 15 Tg N y^-1. Due to probable groundwater residence times, the effect of this influx on downstream ecosystems will not be fully realized for decades. In Montana, the effects of land-use change on nitrate in surface water are increasingly recognized. Here we quantify the contribution of cereal and forage production to nitrate in groundwater and surface water in the Judith River Watershed (JRW) of central Montana, where there is a history of high and increasing groundwater nitrate. We use isotopic tracers of nitrate, water source, and residence times to determine the timescale and magnitude of anthropogenic influence on nitrate dynamics in soils, aquifers and surface streams. This pattern in the JRW provides a model system for evaluating the interaction between long-term land management practices and downstream effects on human health and ecosystem function.