Inputs of nutrients, such as nitrogen and phosphorus, are essential to agricultural production, and integral to raising productivity. At the same time, a surplus of nutrients in excess of immediate crop needs can be a source of potential environmental damage to surface and ground water (eutrophication), air quality (acidification) and contribute to global warming (greenhouse effect). If soils are farmed and nutrients not replenished, this can lead to declining soil fertility and may impair agricultural sustainability through "soil mining" of nutrients.
Many OECD countries have established goals to reduce nutrient emissions from agriculture. These are closely linked to the need for agriculture to comply with national standards for nitrate and phosphate emissions into aquatic environments. A number of international conventions and agreements also have the objective of limiting and reducing transboundary emissions into the environment, including nutrient emissions from agriculture into surface and ground water, marine waters and the atmosphere.
The OECD soil surface nitrogen balance indicator measures the difference between the nitrogen available to an agricultural system (inputs, mainly from livestock manure and chemical fertilisers) and the uptake of nitrogen by agriculture (outputs, largely crops and forage). A persistent surplus indicates potential environmental pollution, while a persistent deficit indicates potential agricultural sustainability problems. The indicator provides information on the potential loss of nitrogen to the soil, the air, and to surface or groundwater. However, nitrogen loss through the volatilisation of ammonia to the atmosphere from livestock housing and stored manure is excluded from the calculation.
The trend with regard to surpluses in national nitrogen soil surface balances over the last decade is downward or constant for most OECD countries, which suggests that the potential environmental impact from agricultural nitrogen emissions is decreasing or stable. Some countries with a relatively high nitrogen surplus have reported significant reductions, although for a few countries surpluses have risen.
The spatial variation of nitrogen surpluses within a country can be considerable. Regional data suggests that even in countries with a relatively low national nitrogen surplus, nitrate pollution is experienced in some localities, while soil nutrient deficits occur in others.
A second nutrient use indicator, the efficiency of nitrogen use in agriculture, measures the physical nitrogen input/output ratio. This indicator has shown an improvement in nitrogen use efficiency for most countries over the past decade. However, there is considerable variation across countries in the efficiency of using nitrogen in agriculture, and in some cases the efficiency of nitrogen use has deteriorated.