Effects of lowering nitrogen and phosphorus surpluses in agriculture on the quality of groundwater and surface water in the Netherlands

The ecological status of many surface waters in the Netherlands (NL) is poor, due to relatively high discharges of N and P from agriculture, industry and wastewater treatment plants. Agriculture is suggested to be a major source, as discharges from industry and wastewater treatment plants have sharply decreased from the 1980s onwards. Agricultural land covers more than 60% of the total surface area in NL, and most of this land is managed intensively and is intersected by a dense network of ditches (total length not, vert, similar300,000 km), streams and lakes. On average, groundwater levels are shallow to very shallow. It has been suggested that nutrient balances of agricultural land are easy to measure proxies for nutrient discharges from agricultural land, though the relationships between nutrient balances and nutrient discharges into groundwater and surface water are not well-established. Thus, we explored the effects of lowering N and P surpluses in NL agriculture on the quality of groundwater and surface waters. Effects of N surpluses in the range of 40–300 kg ha-1 yr-1, and of P surpluses in the range of 0.4–17.5 kg of P per ha per year were examined using an integrated set of mathematical models and databases. Results indicate that nitrate leaching to groundwater and N and P discharges to surface waters are related to both N and P surpluses, hydrological condition, land use and soil type. On a national scale, decreasing N surplus by 1 kg ha-1, decreased nitrate leaching to groundwater on average by 0.08 kg ha-1 and N leaching to surface waters on average by 0.12 kg ha-1. Decreases of N and P concentrations in surface waters upon lowering surpluses were smaller than the calculated discharges. Decreases in N and P concentrations were much smaller in the coastal zone and Lake IJsselmeer, than in regional waters (ditches and small streams). The small improvement in the quality of surface waters upon lowering surpluses in agriculture is related to the relative importance of other nutrient sources, including wastewater discharges, nutrient imports via rivers, seepage of nutrient-rich ‘old’ groundwater and sediments. The relative contribution of other sources to the loading of surface waters varies between water districts, and as a result N and P concentrations in surface waters vary regionally. For improving the ecological state of surface waters, we recommend a combination of low N and P surpluses in agriculture, with dredging P rich sediments, flushing of ditches, and decreasing discharges from other sources