Measurement and Simulation of Two-dimensional Movement of Nitrate as Influenced by Landscape Positions.

Investigation of the factors controlling the fate and transport of nitrate-nitrogen (NO₃-N) within agricultural landscapes is an important aspect of the strategies employed in precision agricultural systems and site-specific management practices. The objectives of this study are: (i) to verify the mechanisms by which topographical positions along a particular transect of the landscape influence the vertical movement of NO₃^- in particular, and the lateral and downward penetration of solute in general; (ii) to examine the roles of NO₃-N fertilizer rates on biomass production, residual soil nitrate, and the potential for NO₃-N leaching; (iii) to employ a mechanistic model (HYDRUS-2D/3D) to simulate two-dimensional movement of water and solute within the landscape under a semi-arid condition. The experimental site was established on a non-tilled, hummocky landscape with 2-5% slope. The study was designed as a factorial experiment comprising of landscape position and NO₃-N fertilizer rates. The plot was delineated into three discrete landscape segments as: upper (UPP), middle (MID) and lower (LOW) topographic positions along pre-selected transects within the plot. Each slope position represents a main plot, which was sub-divided into subplots. Nitrate fertilizer was added to the subplots in form of potassium nitrate (13.5-0-46.2) at the rates of 0, 90 and 135 kg N ha^-1. A microplot demarcated within each subplot received ¹⁵N labelled fertilizer as KNO₃ at the rate similar to the corresponding subplot, and bromide (KBr) at the rate of 200 kg Br^- ha^-1. The field was seeded to canola (Brassica campestris L.) in May 2007. Soil samples were collected in October 2007 and analyzed for NO₃^-, Br^- and ¹⁵NO₃-N. The results of vertical and lateral distribution of NO₃-N and Br^- will be presented and the influence of landscape position and rates of N fertilizer on these measurements will be discussed.