Estimating nitrogen leaching fractions to ground and surface water on agricultural farms from long-term monitoring data

The Nitrates Directive (91/676/EEC) obliges all EU Member States to protect groundwater and surface water against pollution caused by nitrates from agricultural sources. To meet this objective, the Netherlands has developed standards for the use of manure and inorganic fertilisers. Empirical models are used to evaluate these standards to ensure that they are consistent with the objectives of the Nitrates Directive. This study has evaluated field data over a period of 30 years to assess leaching fractions of nitrogen surplus for different soil types and land use (arable vs grassland).   The results serve as input for the empirical models.

The aim of this study was to calculate the part of the nitrogen surplus on arable land and grassland that leaches into ground and surface water (nitrogen leaching fraction). This is done for four regions characterised by different soil types (sand, loess, clay and peat). The sand region is herein divided in different groundwater depth regime classes (GRC’s) which are an indicator for the soil drainage condition. 

The type of soil and its usage impact specific soil microorganisms. These microorganisms are able to break down nitrate. The more denitrification takes place, the less of the nitrogen surplus, in the form of nitrate, reaches ground and surface water, resulting in a reduced leaching fraction. 

This study utilised monitoring data from the Minerals Policy Monitoring Programme (LMM). This long term monitoring programme monitors the agricultural practice and water quality on agricultural farms in the Netherlands since 1991 onwards. All farms in this study were randomly sampled and selected (about 750 farms over the whole period).

Nitrogen surplus was derived by subtracting nitrogen outputs from input at the farm level. Precipitation surpluses were used to calculate nitrogen loads from nitrate concentrations per soil region and land use (arable or grassland). This was done using year specific long-term median precipitation surplus based on fractions of crop types, soil types and GRC’s. 

The nitrogen leaching fraction is highest in dry sandy soils, followed by loess, clay and peat. Leaching fractions were found to be significantly higher on arable land than on grassland. The findings of this study closely align with prior research on leaching fractions from 1991 to 2014   even though input data was completely renewed.

The most remarkable change in input data from the former version was visible on Dutch soil and GRC maps: soil types and GRC’s have shifted over the monitoring period. Shallow peat soils located on sands have shifted, due to oxidation, to a more sandy soil, whereas groundwater tables have fallen. 

This method is, as far as known, unique because of the use of a large sample of random, shallow water quality measurements, aggregating to a long term leaching fraction, without the use of complex model instruments. This measurement-based method can be a helpful tool to derive environmentally sound N use standards to meet with the objectives of the Nitrate Directive.