The Impact of Soil Heterogeneity on Nitrate Dynamic and Losses in Tile-Drained Arable Fields

Intensive agriculture and the related application of fertilizers are one of the main sources for diffuse nitrate (NO3-N) inputs into surface-and groundwater. It is assumed that especially subsurface tile drainage is an essential source. To examine the impact of heterogeneous soil properties on NO3-N leaching via drains, field experiments in the northern Altmark region in Saxony-Anhalt (Germany) were realized on two neighboring, tile-drained arable fields with adjusted agronomic conditions but different soil physical properties from 1 November 2013 until 31 October 2015. Field campaigns, the implemented measuring technology, and numerical simulations provided the detailed characterization of the subsurface, the flow regime, and the NO3-N dynamic. The results revealed that field I was groundwater influenced and dominated by ponding and surface runoff, whereas at field II, the whole amount of seepage water percolated freely to the drains. Thus, the amount of drained water and discharged NO3-N loads differed significantly between both sites although their comparable size (field I/field II, 17.1 mm and 2.5 kg N ha(-1)/150.2 mm and 40.4 kg N ha(-1)). Furthermore, NO3-N accumulated in the top soil of field I; whereas, this site showed a huge denitrification potential due to long hydraulic residence times. These aspects could not be reproduced at field II. A site-specific N management is absolutely required to reduce further N losses at field II as a requirement to fulfill the objectives of the European water protection policy.