Export Coefficient Modelling of Nutrient Neutrality to Protect Aquatic Habitats in the River Wensum Catchment, UK

The pressure of nutrient pollution derived from wastewater treatment works and agricultural runoff is a reason for the decline in the ecological health of aquatic habitats. Projected residential development in catchments creates further nutrient loading that can be offset by nutrient management solutions that maintain 'nutrient neutrality' either onsite or elsewhere within the same catchment. This study developed an export coefficient model in conjunction with detailed farm business data to explore a nature-based solution to nutrient neutrality involving seven scenarios of crop conversion to mixed woodland or grazing grass in an area of intensive arable cultivation in the groundwater-fed Blackwater sub-catchment of the River Wensum, UK. When compared with the monitored riverine export of nutrients, the calculated nitrogen (N) and phosphorus (P) inputs under current land use showed that subsurface denitrification is removing 48-78% of the leached N and that P is accumulating in the field soils. The addition of 235 residential homes planned for 2018-2038 in the Blackwater will generate an additional nutrient load of 190 kg N a-1 and 4.9 kg P a-1. In six of the seven scenarios, the modelled fractions of crop conversion (0.02-0.21) resulted in the required reduction in P loading and more than sufficient reduction in N loading (196-1874 kg a-1 for mixed woodland and 287-2103 kg a-1 for grazing grass), with the additional reduction in N load above the requirement for nutrient neutrality potentially contributing to further improvement in water quality. The cost of land conversion is modelled in terms of crop gross margins and nutrient credits generated in the form of 0.1 kg units of N or P. For the range of scenarios considered, the annual cost per credit ranged from GBP 0.78-11.50 for N for mixed woodland (GBP 0.74-7.85 for N for grazing grass) and from GBP 160-782 for P for both scenarios. It is concluded that crop conversion is a viable option to achieve nutrient neutrality in arable catchments in eastern England when considered together with other nutrient management solutions.