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NUTRIENTS Sources, Pathways and Amounts of Nutrient Losses(149) 2. Sources of nitrogen (N) nutrient losses from agriculture 3. Sources of phosphorus (P) losses / fluxes from agriculture 4. Pathways / transport of lost nutrients to estuaries 5. Amounts of nutrients lost from agriculture
Both the nutrients nitrogen (N) and phosphorus (P) are entering seas and oceans directly from the coasts, through rivers and streams discharging to them and - in the case of nitrogen - also as atmospheric wet or dry deposition. In addition to losses caused through agricultural activities there are unavoidable natural losses of nutrients, mainly of nitrogen (N) by de-nitrification, volatilisation and leaching, but also of phosphorus (P) mainly through erosion (29,327, 374). However, other human activities (29) are also contributing considerably to losses of nitrogen (N) (18) and phosphorus (P) to the environment (230), i.e. losses of nutrients are not only originating from agricultural activities, but also from human activities such as urbanisation (34, 66, 91, 139, 140, 152, 240, 255, 263, 367, 375), industries (31, 32, 135, 137, 205, 264, 325, 342, 361) and traffic (f.i. losses of nitrogen oxides by combustion in cars or electricity generations) (139, 229, 282). In the case of nitrogen (N) attempts have been made to establish global balances (19, 20, 236, 296). Because sources of losses from urbanization and industries in most cases can be well detected and located, pollution caused by them is called 'point pollution'. In contrast nutrient losses from agriculture are generally defined as 'non-point or diffuse pollution' (2, 121, 163, 166, 168, 208, 327), though there are also specific cases of urban 'non-point' pollution (226). An exception are fish breeding systems by aquaculture (64, 96, 112, 113, 136, 191, 345; specifically phosphorus: 363, 373) and specific concentrated animal breeding or feed lots, dairy or pig units (133) which might be considered as agricultural 'point pollution' sources.
In agriculture pollution, i.e. losses or fluxes of nitrogen (N) (231) and phosphorus (P), start on the agricultural farm, on the cultivated fields, on the grazed (209, 210, 246, 290) or cut grassland (46), and in the forests (125), originating from animal excreta, application of animal manure, slurry (296), mineral fertilizers or N-fixation (230). Depending on the given conditions (soil type, temperature, humidity, bacterial activity) and N-dynamics (244, 318) nitrogen (N) losses or fluxes are in the form of nitrate (NO3), ammonia (NH3/NH4), nitrous oxide (N2O) and NOX. Nitrate (NO3), which is very mobile in the soil solution, when not taken up by the growing plants, may be leached to the ground water or enter a subsurface flow. Through this it may enter streams and rivers (24, 56, 197, 215, 217, 218, 219, 222, 234, 239, 242, 246, 247, 250, 254, 268, 279, 287, 305, 308, 315, 318). The concentration of nitrate-N in the ground water is therefore also influenced by the travel time within the flow-path (117, 118, 243). Nitrogen (N) is also lost through volatilisation/evaporation of ammonia (NH3), which relatively quickly reacts to ammonium (NH4). Such losses are mainly originating from animal husbandry, either directly from the animal excreta themselves, or through losses in collecting, storing and applying animal manure or slurry, or inappropriate disposal practices. Agriculture (mainly husbandry) constitutes approximately 50% of the total nitrogen-N-deposition and 25% of the total acidifying effect of atmospheric deposition (4). Further nitrogen (N) losses occur in form of nitrous oxide (N2O) (20, 236, 298) and NOX resulting from de-nitrification or nitrification, nitrogen (N)-reactions in the soil and plant emissions (NOX) (302, 306). In comparison to common agriculture high losses of nitrogen (N) may originate from intensive horticulture and fruit-growing (e.g. strawberries, citrus) especially when grown on sandy, permeable soils (12, 285). However, such losses are generally restricted to limited areas or regions. 3.
Sources of phosphorus (P) losses / fluxes from agriculture (364,
365, 369, 370). In the past in the industrialized countries the main source of phosphorus pollution has been the use of phosphorus (P)-containing detergents (115) and the lack of, or the use of, unpurifying sewage plants (160). With the introduction of phosphorus (P)-free detergents and the installation of phosphorus (P)-precipitating sewage plants, sewage plants with a primary and secondary treatment (81, 159), the absolute amount of phosphorus (P) lost to waters has decreased significantly. This has, on the other hand, led to a steep proportional increase of phosphorus (P)-losses from agriculture (237). In the soil phosphorus (P) is - in contrast to the rather mobile nitrate (NO3) - quite immobile (63). Hence, under normal conditions the topsoil (ploughing layer) will be the richest in phosphorus (P). Erosion (327, 374) can therefore result in great particulate losses of phosphorus (P) to surface waters (197, 359), rivers and streams (56, 68, 86, 121, 150, 175, 210, 211, 378) (run-off), especially just after manure or fertilizer application and when phosphorus (P) in form of animal manure or mineral fertilizer is spread on the surface of permanent grassland or no-till-systems. It may then be dissolved in rain or surface water and be washed into water systems (48, 210, 377, 379). Such transport of phosphorus is taking place in various solubility forms (348). However, P-loadings based on regional figures from cultivated soils to surface waters have been overestimated (184). In spite of its immobility leaching of phosphorus (P) may become a problem only under special soil conditions and on soils highly saturated with phosphorus (P) (331). This may be the case where extensive amounts of animal manure or slurry from feed lots have been applied on a surface which is proportionally too small. However, the knowledge in P-dynamics in fresh water and marine environments is still incomplete and the mechanisms controlling the loss of phosphorus (P) to drainage water (and its further transport) are imperfectly understood (362). 4. Pathways / transport of lost nutrients (15, 18, 124) to estuaries (23) Lost nitrogen (N) in form of
nitrate may be transported with the ground water or through a subsurface
flow to rivers and streams and further on to the estuaries. A very specific case of nutrient transportation by winds is the dust deposition in the North Atlantic Ocean from North African sources (62, 241, 257). However, this dust generally having mean concentrations of calcium (Ca) gives a neutralizing effect (278), counteracting acidification. Both nutrients, phosphorus (P), and to a lesser extent nitrogen (N), may also be washed into water systems through run-off (48, 68, 150, 210, 211, 228, 260, 292) or by erosion adsorbed to soil particles, transported by rivers discharging to the sea (22) or ocean (266) or by floods (208) and deposited / buried there as sediment (30, 44, 76, 80, 95, 98, 123, 139, 350, 354, 357, 365, 371), which later may be released under different redox conditions (76, 118, 206, 283, 320, 365). This pathway is also the main transport system for phosphorus (P) - losses from agriculture. Phosphorus (P) transport through leaching and ground water or subsurface flow is limited, due to the unsaturated P-fixing capacity of the subsoil to very specific cases only and mainly occurs through artificial drainage systems. When nitrogen (N) in form of nitrate (NO3) is transported by rivers (276) or streams to the estuaries it will be denitrified to N2 (converted under anaerobic conditions by bacteria from NO3 - NO2 - NO - N2O - N2) (230, 232, 279, 295, 298, 302). This process is influenced by the stream channel size (216). Figures showing to what extent the nitrogen (N)-load of rivers in the form of nitrate is reduced by de-nitrification during transport vary greatly. It is assumed, however, that most of the nitrate transported with rivers will not reach the estuaries or continental shelves due to the de-nitrification process (20, 30, 62, 79, 123, 203, 236, 270, 307). Further reduction of nitrogen (N) due to the same process occurs in over-nutrified estuaries or continental shelves, where it will either be further denitrified (302, 306, 307) or buried within the sediments. 5.
Amounts of nutrients lost from agriculture Figures concerning the extent to which agriculture and other human activities are contributing to nutrient (nitrogen (N) - and phosphorus (P)) pollution vary very greatly. They largely depend on the intensity of agriculture, particularly on the size and concentration of cattle feed lots at the coasts and within the catchment basins of rivers discharging to the sea or ocean, as well as on the intensity of urbanization, industrialization and traffic. It is assumed that up to 90% of diffuse or non-point losses of nitrogen (N) - and up to 85% of phosphorus (P) (364) - is originating from agriculture, particularly in regions such as Western Europe, USA, Australia and New Zealand. There are however, great variations, because there are limited precise figures based on exact measurements (100, 180, 198, 216, 328). Also, measurements made during different seasons of a year (32, 183, 201, 211, 213, 219, 228, 276, 310, 329, 334), identify the influence of seasons on nutrient loads and nutrient dynamics (104, 131, 144, 162, 233, 256). Hence, figures of various publications are estimates. Exact figures, estimates, projections as well as nutrient ratios (ratio between nitrogen - N - and phosphorus - P - and f.i. carbon - C -) are presented in the following publications: a)
losses / flux and/or transported amount of nitrogen and phosphorus:
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