Impact of drought on nitrogen concentrations in leaching water from agricultural areas in the Netherlands

Abstract

Since the introduction of the EU Nitrates Directive (91/676/EEC), nitrogen concentrations have gradually declined. The recent summer droughts (2018-2020) in the Netherlands, have caused an increase in nitrogen concentrations in water leaching from agricultural soils, exceeding standards. It is expected that with increasing numbers of climate extremes, summer droughts will occur more often in the Netherlands. In order to develop possible strategies, it is important to better understand the underlying mechanisms and consequent impact of droughts on water quality.  In rural areas of the Netherlands the water quality of shallow groundwater and surface water (leaching water) is strongly influenced by agricultural land use. The EU Nitrates Directive aims to protect waters against pollution caused by nitrates from agricultural sources. In the Netherlands, leaching of nitrogen to shallow groundwater and surface water is monitored for over 30 years by the Dutch Mineral Policy Monitoring Programme (LMM). Within the LMM, the Netherlands is divided in 4 main and 14 subregions based on soil type. The water sampling at participating farms, enables to analyse the effect of farming practices on water quality in the different LMM regions.  Nutrients applied during the growing season can leach to shallow groundwater and surface water in the following autumn and winter. Summer drought inhibits the uptake of nutrients by crop, hampers the process of denitrification and leads to thickening of the soil moist and upper groundwater, leading to accumulation of excess nitrates in the soil. These excess nitrates can potentially increase nitrogen concentrations in leaching water in winter. Not every soil region responds similarly to a drought period. The aim of this study is to investigate why certain regions experience a stronger impact of drought on nitrogen leaching than others.  We used monthly spatial Standardized Precipitation Evaporation Index data provided by the Royal Meteorological Institute (KNMI) over the period 1990-2022 to identify where (region) and when (year) summer droughts occurred. The LMM dataset was used to analyse the change in groundwater levels, nitrogen concentrations in leaching water and soil nitrogen surplus following a summer drought. Furthermore, we investigated the role of soil type and land use on the change in nitrogen concentrations in leaching water caused by drought. Our findings reveal that the magnitude of the increase in nitrogen concentration in leaching water following a summer drought is determined by the duration and intensity of the drought. Furthermore, soil type and agricultural practices influenced the variation of the impact by droughts between regions. These results can be used to identify areas that are more sensitive to impacts of droughts on water quality based on their soil and land use characteristics and thus support the development of adaptation strategies by farmers, water authorities and national government.