Water quality response to water and nitrogen movement through a semi-arid dryland agroecosystem in Montana, USA

Abstract

Humans have increased reactive nitrogen (N) on the planet by an order of magnitude over the past 150 years. Most of this reactive N is used for fertilizer to feed a growing population, but loss of N from cultivated soils threatens agricultural and environmental sustainability. Nitrate accumulated in soil from fertilization or decomposition of soil organic N (SON) may be lost via leaching, which can reduce soil fertility and compromise water quality. Nitrate concentrations commonly exceed human drinking standards in groundwater resources around the globe. In the Judith River Watershed (JRW) in central Montana, nitrate has been detected above the standard since the 1960s. This dissertation contributes to a more holistic understanding of the fate and transport of N in the JRW. An interdisciplinary team engaged with farmers in a participatory research project, making observations in soils, groundwater, and streams to characterize water and N movement. At the landform scale, 5 to 9 cm yr -1 of the 38 cm yr -1 mean annual precipitation moves through soil to recharge groundwater and leaches 11 to 18 kg ha -1 yr -1 of nitrate-N from soil. These leaching rates are approximately 20-30% of fertilizer rates but likely reflect inmixing of nitrate from SON decomposition. Soil modeling analyses suggested that water and N losses were dominated by intense precipitation periods on wetter soils, such that more than half of simulated deep percolation and leaching occurred in two of 14 model years. Simulations further suggest that thinner soils (<25 cm fine-textured materials) experience water and nitrate loss rates five to 16 times higher than thicker soils (>100 cm). Soil sampling demonstrated that increased soil water during fallow periods facilitates conversion of SON to nitrate. Soils are then primed for water and N loss with subsequent precipitation, resulting in disproportionately high leaching rates during and following fallow periods. Isotopic evidence from fallow periods further suggests that nitrate is lost to the atmosphere via denitrification, a gas phase loss combining with leaching losses to compromise the goal of delivering N to crops. These findings suggest that reduction of fallow increases N use efficiency and reduces nitrate loss to groundwater.