Impact of agricultural land use and management on available soil phosphorus content in agricultural catchments of Ireland

The build-up of soil phosphorus (P) in agricultural soils exceeding crop requirements can lead to diffuse P losses that could impair surface water quality. Therefore, adequate spatial information is required to develop viable tools and recommendations for sustainable P management at the local scale. Here, we present a database of nearly 8.000 samples, collected over a 12-year period in four meso-scale (~10 km²) agricultural catchments in Ireland. The agricultural area of each catchment is divided into sampling units (up to 2 ha) and soil samples are repeatedly taken from each sampling unit every 4 years. Four soil sampling campaigns were carried out to date. The results were analysed in the context of soil test P values (Morgan’s P) and classified according to the P index system as defined in the Ireland’s Nitrates Action Programme.

Overall, levels of soil test P did not show substantial changes, with the exception of the most recent sampling campaign. However, when the collected data are considered in a spatial context and accompanied with soil data and land use information, they reveal a more complex story. Notable differences in soil P trends are observed at the individual catchments scale and impacted by land use, agricultural management intensity and some soil properties across and within the catchments. Similarly to the overall soil test P trends, the total area under P index 4 soils (above optimal) decreased in the period preceding the most recent sampling campaign. The most notable decreases in P index 4 soils are found in tillage and drystock fields, but also in the catchment dominated by highly stocked dairy farms availing of a nitrate derogation.

Recent increases in soil test P and consequently areas under P index 4 may not be linked to increased organic or mineral P inputs, but rather come as a result of an overall increase in soil pH from increased lime application observed over the most recent period, which had an impact on the extractable Morgan’s P content. On-farm redistribution of fertilizer P inputs to soils with lower P index status has the potential to increase P use efficiency and decrease P loss risk to surface water.