| Humans have doubled natural inputs of mineral nitrogen to terrestrial Earth, and these inputs are accelerating. Greater than 1/3 of this human-derived nitrogen reaches surface and ground waters, creating significant environmental problems such as the hypoxic zones in the Chesapeake Bay and Gulf of Mexico and the pollution of drinking waters. However, human-derived mineral nitrogen is necessary for the maintenance of human health; synthetic ammonia-based fertilizers account for approximately 40% of global human protein consumption.;The vast majority of human-derived mineral nitrogen travels through the soil prior to reaching surface and ground waters. Within the soil, mineral nitrogen is transformed and transported by a variety of water-dependent mechanisms. I explicitly linked biogeochemical and hydrological nitrogen cycling mechanisms at different scales and in different ecosystems. I used a variety of approaches including meta-analysis, laboratory experimentation and field observation.;At the global scale, I demonstrate that within-site spatial variation in soil solution nitrate, dissolved organic nitrogen and saturated hydraulic conductivity are similarly related to soil clay content. Clay content explained greater than 1/3 of within-site spatial variation in nitrate, dissolved organic nitrogen and saturated hydraulic conductivity. These relationships suggest that soil hydrology, as mediated by clay content, may be a significant mechanism affecting variation in soil solution nitrogen. Moreover, these data show that the heterogeneity of an important resource, soil solution N, is a predictable function of clay content.;Across an artificially drained agroecosystem landscape I examined biogeochemical and hydrological controls on the magnitude and timing of nitrous oxide flux from the soil. I collected soil columns from three landscape positions that vary in hydrological and biogeochemical properties. Across all landscape positions, there was a positive linear relationship between total soil nitrogen and the log of cumulative nitrous oxide emissions (r2 = 0.47; p = 0.0132). Within individual soil columns, nitrous oxide flux was a Gaussian function of water filled pore space and matric potential during drainage. These data demonstrate that biogeochemical properties control the absolute magnitude of nitrous oxide flux while hydrological properties control the timing of nitrous oxide flux. The Guassian relationship between nitrous oxide flux and matric potential reveal that water filled pore size is the hydrological property controlling the relative magnitude of soil nitrous oxide flux across all soils; using these data I identified that maximum nitrous oxide flux occurs when pores >40 microm have drained.;Within a northeastern United States deciduous forest, I monitored soil solution ammonium and nitrate concentrations as well as volumetric soil water content across two environmental gradients: a 30 meter catenary hillslope and 300 meter silt-to-sand soil texture gradient. Across the gradients, soil solution nitrate and ammonium increased downslope and with sand content. Immobilization of nitrate and ammonium into insoluble organic nitrogen compounds could not explain this pattern. In contrast, nitrogen mineralization could help to explain this pattern; in situ net ammonification rates were positively correlated with sand content. |
