NUTRIENTS

Reducing Nitrogen and Phosphorus Losses from Agriculture to Rivers, Estuaries, Seas and Oceans

Because loads of nitrogen (N) and phosphorus (P) are reduced in wetlands, estuaries and shelves through sedimentation / sink, sustaining or restoring wetlands as an off-site sink for agricultural nutrients (10, 29, 238, 271, 283, 293), or installing floodplains / wetlands as filters for agricultural fertilizers / nutrients runoff (71), can serve as a valuable measure to counteract nutrient pollution of marine waters.

The pathways of nutrient losses in agriculture are: leaching, runoff and erosion of nitrogen (N) and phosphorus (P), ammonia volatilisation and inappropriate farm waste disposal. Possible losses start on the farm (from fields, grasslands, forests, animals, stables), i.e. already in the hinterland, sometimes far away from coastal or marine waters. Consequently, all measures or activities to reduce possible nitrogen (N) and phosphorus (P) losses also have to start at farm level.

Though high losses of nitrogen (N) may originate from intensive horticulture and fruit-growing (e.g. strawberries, citrus) especially (12, 285), in common agriculture the most obvious polluters are, it appears, concentrated animal units (dairy cows, pigs, poultry), which produce a surplus of animal manure and slurry (4, 133, 170). On such farms farmers are confronted with the conflicting object of combining concentrated animal populations with reduced contamination of water systems. Farms have to come to a balanced recycling within their agro-ecosystem, i.e. to achieve a balance between intensity of animal production and crop hectarage. Livestock densities have to be set in accordance with the crop demand of nitrogen (N) and phosphorus (P).

Where such a balance cannot be achieved animal production has to be reduced and the structure of an agricultural region has to be changed (4, 143, 286). In particular concerning the reduction of ammonia from livestock farms, detailed recommendations are given by the Helcom Declaration, such as (4).

The overall excretion of nitrogen (N) by livestock should be reduced by use of: high quality and well-composed fodder, i.e. optimized amino acid composition, a balanced carbohydrate/protein ratio or enzymatically improved digestibility of the fodder, advanced feeding systems, i.e. phase feeding and integrated feed evaluations.

Irrespective of the type of farm, all farmers have the possibility to control possible losses of nitrogen (N) and phosphorus (P) through the introduction of an improved nutrient management practice (12, 38, 139, 335, 380) and by applying the principle of 'Best Management Practice' (BMP) (7, 46, 52, 151, 209, 222, 242, 245, 271, 272, 309).

Such 'Best Management Practice' (BMP) may include:

- taking light porous soils near coasts (189) out of production,
- an appropriate crop rotation (11),
- conservation tillage and any other measure to control soil erosion (direct drilling, mulching and cultivating with the contour) (13, 38),
- installation of buffer strips (and riparian vegetation) as 'protection zones' around surface waters to avoid nutrient runoff (150, 204, 265),
- applying nitrogen (N) and phosphorus (P) whether in the form of animal manure and slurry, or mineral fertilizers only, in accordance with soil-testing and yield expectation level (48, 335, 381),
- applying manure or fertilizer only at appropriate times, i.e. in line with crop growth,
- incorporating animal manure and slurry at, or immediately after, spreading to avoid runoff and volatilisation,
- creation of favourable conditions for nutrient retention processes in agricultural soils and landscape.

Further general recommendations / possibilities for the reduction of nitrogen (N) and phosphorus (P) losses are given / discussed in the publications: (17, 26, 122, 127, 131, 145, 146, 157, 205, 281, 282, 297).

A very special attempt to reduce the phosphorus (P) load from a lake and a estuary in tropical conditions has been made through the uptake by the Water Hyacinth (Eichhornia crassipes) (349); however, harvesting and composting the plants has been hampered by their high water content.