Modeling the soil moisture and mineral N balance to support advices for better farming practices. A case study in North-East Flanders (Belgium)

In the context of the EU Nitrate directive and supported by the Flemish Land Agency, a project on surface water quality was started in 2012 in the area of the Horstgaterbeek, a stream in North Limburg, Flanders. Due to its characteristics, this is an ideal area to gain more insight in nitrate flows linked with agricultural activities. The area has an important agricultural activity, with many livestock farms and grassland, but also arable crops. It is a flat area with sandy to sandy-loam soils and shallow groundwater tables. In the area, 2 monitoring sites for surface water quality, belonging to the MAP monitoring network of the Flemish Environmental Agency, are situated in small ditches. In one of them, the nitrate content regularly exceeds the standard (50 mg NO3- l-1). In the other site, the nitrate contents remained below the nitrate standard during the last 10 years. The aim of the project was to gain a better insight in the nitrate flows within the catchments of the monitoring sites and to improve surface water quality, through an intensive follow-up of the agricultural activities, in collaboration with the local farmers. After three years, very interesting improvements were achieved in terms of nitrate residues and nitrate losses from agricultural land, partly thanks to an integrated monitoring of both the farming and fertilization practices and the water quality. In order to gain a better insight in the N-dynamics in the agricultural parcels, a mechanistic modeling approach of the mineral-N-balance in the soil was developed. In this approach, the different in- and outputs of nitrogen are estimated. The N-mineralization from soil organic matter is calculated based on the potential mineralization rate and taking into account the temperature and moisture content in the soil. The soil moisture content as well as the water flushing rates are calculated with a soil moisture balance model developed by the Soil Service of Belgium. This model takes into account soil characteristics, changes of the groundwater table, local precipitation and parcel-specific evapotranspiration, crop rotation and soil cover. The modeling approach of the N-dynamics in the soil allowed us to give the concerned farmers a better understanding of the different factors (fertilization, crop uptake, mineralization, leaching,…) affecting the nitrate residues and nitrate losses from their agricultural parcels to the groundwater.