There are two types of pollution sources:
• Point source – this included industrial discharges; sewage treatment continuous and storm discharges; and accidental spillage from farm slurry stores; and
• Diffuse sources – these as the name suggest cannot be attributed to a point or incident. Unlike point source pollution it cannot be seen and may affect both water and air.
Diffuse pollution can occur from range of activities including agriculture, forestry, mining, construction and urban actions. It is due to the day to day activities over a larger area. Diffuse pollutants from agriculture includes, silt from soil erosion, nutrient loss from fertilisers and manure and pesticides.
Nutrients may be in the form of nitrates or phosphates. Over 70% of Nitrates and 40% of Phosphates in English waters come from agriculture practices.
As illustrated earlier nitrate levels in water, especially groundwater, is on the increase. No one farmer is responsible for this as it is a cumulative affect. It is for this reason there are a number of initiative to address diffuse pollution. WAgriCo is one such initiative.
But how does the release of nitrates occur from the application of fertilisers and manures – this is explained by the ‘Nitrogen Cycle’.
The Nitrogen Cycle in soils is a very dynamic process but has great practical implications on modern agriculture and protection of nitrate pollution of watercourses and is illustrated in the diagram below.
Most plants can absorb nitrogen in both the nitrate and ammonium forms. However, ammonium-N is rapidly nitrified to nitrate-N which is the main form absorbed by agricultural crops.
Soil organic matter is the main ‘pool’ of nitrogen in the soil but in organic form is not available for plant uptake. Organic matter may be of recent origin (organic manures, crop debris) or much older. However, in order for the organic matter to decompose it is necessary to have a population of living bacteria and other organisms in the soil, these are called the Microbial biomass. Leguminous plants have symbiotic micro-organisms (e.g. rhizobia) associated with their root systems. These micro organisms form part of the microbial biomass and ‘fix’ nitrogen gas from the atmosphere. This is termed Nitrogen fixation and is the only way that organisms can attain nitrogen directly from the atmosphere.
Key processes in the Nitrogen Cycle are:
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