Eutrophication from Agricultural Sources - Pathways for Nutrient Loss to Water with Emphasis on Phosphorus

ERTDI Report 75 (Kiely et al.)

Summary: Synthesis Report for the project: 2000-LS-2.1-M2. Authors: G. Kiely, G.Morgan; R.Moles, P. Byrne; P. Jordan; K. Daly, D. Doody, H. Tunney, I. Kurz, D. Bourke, C. O’Reilly; D. Ryan; N. Holden; E. Jennings, K. Irvine;O. Carton.

Published: 2007

ISBN: 1-84095-245-8

Pages: 10

Filesize: 180KB

Format: pdf

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The main objective of this project was to study phosphorus (P) loss from agricultural land under a range of conditions in Ireland, to quantify the main factors influencing losses and to make recommendations on ways to reduce these losses. This report is a synthesis of the main conclusions and recommendations from the results of the studies. The final reports from the nine individual sub-projects in this project are available from the EPA (www.epa.ie).

Historically Irish soils were low in P and much of the P that is presently in the topsoils on intensive farms was added directly in chemical fertiliser and indirectly in purchased feedstuffs, during the past forty years. This reservoir of P has a considerable influence on P losses from the land. Up to the 1960s, Irish soils had a low average soil-test P level and eutrophication of surface waters due to P loss from agricultural fields was not a matter for concern. Since then there has been a large input of P (about 3 million tonnes) in fertiliser and purchased feed to Irish agricultural land. The surpluses of inputs over outputs have been equivalent to doubling total P in agricultural topsoils, on average. However, the annual use of chemical fertiliser P in Ireland has decreased from about 62,000 tonnes P to about 42,000 tonnes over the last decade due to research advice and consequent changes in farming practice.

Diffuse P loss from agriculture occurs when P is mobilised from the land and transported to water bodies. Fields and farmyards are the main source of P from agriculture, and water from rainfall transports it to water bodies. Fertilising grassland with chemical fertilisers and organic manures can increase P losses by placing high concentrations of potentially mobile P at or near the soil surface, where it can easily be mobilised by water.

The conclusion of experimental investigations at the field-plot scale is that the build-up of soil P is the major factor determining the quantity of P lost from grassland fields to water. Thus, in these field-plot experiments where losses from grazing and cut grass were compared, loss rates were closely related to soil P levels, with relatively small and inconsistent effects associated with the grazing animals. The highest loads and concentrations of P from grassland occurred during autumn via overland flow. Summer losses of P were small as there was little or no overland flow in the summer during the experiment (2001–2003).

Exports of P from nested sub-catchments in three catchments were studied. Phosphorus losses to water from the Dripsey (Co. Cork) and Oona (Co. Tyrone) catchments were consistently high (greater than 2.0 kg/ha/yr total P in 2002) irrespective of position in the catchment, indicating that once P was lost from land, via overland flow or drainage, there was little net attenuation within streams. However, losses from the calcareous Clarianna (Co. Tipperary) catchment were only 10% of those in the Dripsey and Oona catchments and the lower loss rates were not a result of low soil-test P (STP) in the Clarianna catchment. The chemical and hydrological characteristics of soils in each catchment therefore play a major role in determining vulnerability to P losses. In particular the free-draining soils in the Clarianna catchment ensure that surface run-off rarely occurs and the inherent capacity of the soil to absorb P prevents significant losses of P to the river. Laboratory studies indicated that the chemical characteristics of soils can influence land as a source of mobile P. For example, soils that have high amounts of aluminium will hold P more strongly and mineral soils with high organic matter will bind P more loosely.

Dung from grazing animals and soil microbial biomass play an important role in P cycling in Irish grassland soils. The  STP concentrations in soil under dung-pats were shown to increase three- to four-fold compared to areas with no dung-pats. Uneven redistribution of nutrients within grazed grassland, due to livestock (dung-pats, feeding and drinking areas, near gates), may create areas of greater risk of P loss to water.<...>