Effects of variants of target surpluses on the quality of surface water in the Netherlands

Surpluses of N and P indcate the difference between inputs via purchased fertilisers, animal feed and animal manure, and N and P outputs via crop and animal products as well as export of animal manure. Target surpluses of N from 300 to 40 kg per ha per year, depending on soil type and land use, and of P from 40 to 1 kg P2O5 per ha per year (equivalent to 17.5 to 0.4 kg of P per ha per year), were examined.

A sequence of models has been used to explore the effects of variants of target N and P surpluses on the discharge of N and P from agricultural activities to surface waters, and on N and P concentrations in rivers, freshwater lakes, streams, ditches and the marine coastal zone. The results indicate that lowering target N and P surpluses will decrease the discharge of N and P from agricultural activities into surface waters and also the N and P concentrations in these waters. However, the mean decrease in N and P concentrations in surface waters is relatively low (less than proportional), given the relatively high mean decreases in target N and P surpluses in agriculture.

This relatively small improvement has been attributed to such factors as: (i) surpluses of N decrease less on average in areas were surface-water loading is large, (ii) P discharge to surface waters is primarily determined by the soil P status and groundwater level, and only indirectly by P surplus, and (iii) seepage of N- and P-rich seepage water from marine sub-soils, contribute greatly to surface-water loading. Lowering target N and P surpluses combined with additional measures (like dredging P-rich sediments, flushing ditches with water low in nutrients, and decreasing discharges from other sources) is argued to be necessary to improve the ecological state of surface waters.