N and P nutrient leaching from long-term slurry application on permanent grassland on clay loam soil

Over one-third of rivers and a quarter of lakes in Ireland are failing to meet their environmental quality standards for nutrients. This is impacting Ireland’s goal to improve water quality and achieve its EU Water Framework Directive targets. Nitrogen (N) and phosphorus (P) leached from different forms of fertilizer application may contribute to groundwater and surface water contamination causing numerous environmental issues in the agroecosystem and health problems in human beings. There is a lack of knowledge on the magnitude of the effects of the nutrient management strategies and their long-term implementation on N and P nutrient leaching from the soil to the groundwater and surface waters in Ireland. The accurate optimization of the nutrient management strategies requires monitoring of soil water concentrations under the range of management strategies and quantification of the N and P leaching from the soil profile.

The objective is to study the N and P leaching in organic and inorganic forms from a long-term application of cattle and pig slurry and mineral NPK fertilizer over the drainage seasons 2021/22 in Hillsborough (54°28′ 0′′ N, 6°6′ 0′′ W), Northern Ireland, UK. The long-term experiment in Hillsborough was set up in 1970 to measure the effects of frequent applications of organic and inorganic nutrients on plant productivity and soil biogeochemistry. It was set up in a split-plot design with nutrient management as a main factor and grass-species biodiversity (rye-grass vs multi-species sward treatment) as a subplot factor replicated three times. The multispecies sward was reseeded (05/08) after ploughing (31/07) in spring 1969. Leaching was measured in six nutrient management (control, synthetic fertilizer, pig and cattle slurry with high and low rates) and both biodiversity treatments. Soil solution from an 80 cm dept was extracted using suction cups every ten days over a 5-month drainage period. N and P leaching were quantified from nutrient concentrations of various species multiplied by an effective rainfall obtained through Schulte’s Soil Moisture Deficit hybrid model. The total N, total oxidized N, ammonium, total P, and dissolved reactive P have been analyzed, the N and P losses have been quantified for the above species. Nutrient concentrations and losses were analyzed as repeated measures with a linear mixed effects model in R.

Most of the concentrations of various nutrient species were affected by an interactive effect between nutrient management and biodiversity level. The influence of permanent grassland renovation and reseeding is also evident. However, the concentrations are low, below the water quality thresholds defined by water quality legislation and national background levels in groundwater or surface waters despite the high rates of continuous nutrient applications for over 50 years. This is likely associated with the high natural attenuation capacity of the soil driven by clay-loam texture. The data will be further used in the optimization of the nutrient management strategies using an ecosystem process-based model DayCent. We believe that the results of this study will have direct implications for agri-environmental policies in the Rep. of Ireland and in the UK.