Identification and characterization of land use driven nitrogen fluxes using stable isotopes and reactive hydrologic modeling

Abstract:

The Najinhe watershed is a topographically diverse, mixed agricultural and urban region in northeastern China that provides opportunities for identification of the impact of land use on nitrogen cycling. In addition to agricultural soil amendments, seasonal variation in atmospheric flow introduces dry and wet deposition from urban and desert sources. Both agricultural amendments and atmospheric sources are significant non-point inputs of reactive N, at estimated annual rates of 450 kg/ha and 30 kg/ha respectively in the nearby North China Plain.
Both historic and current land use has influenced the biological processing of nitrogen in a particular area. Soil conditions, including moisture, texture, and organic content, control the capacity of a parcel for processing reactive nitrogen. Compounds derived from natural and anthropogenic sources exhibit characteristic stable isotopes of nitrogen and oxygen that serve as tracers of origin as well as integrators of biological processes. Analysis of bulk soils (including both organic and inorganic N contents) in the system shows δ¹⁵N ranging from 1.3 – 8.6 ‰ suggesting varying influence of anthropogenic inputs, fertilizers, soil organic nitrogen, and atmospheric sources based on land use.
A distributed hydrologic model coupled with one focusing on reactive transport is able to help determine locations with the highest impact on the dissolved N in this system. Spatial statistical methods are employed to determine the biogeochemical influence of model locations whereas δ¹⁸O on soil NO₃^- and δ¹⁵N measurements on NO₃^- and NH₄⁺ in surface water and soil extracts are used to calibrate and validate model predictions based on measured precipitation and streamflow values. Sources are integrated using a Bayesian mixing model to determine likely fate and transport parameters for various N inputs to the watershed. 
The application of the coupled hydrologic and transport models to a landscape scale catchment suggests integration and expansion to larger watersheds on the basin scale is possible. Identification of sensitive parcels on multiple spatial scales can direct targeted land management efforts to mitigate ecological and health effects of reactive N in surface waters.