Phosphorus and organic soils under grassland production: Assessment of phosphorus losses for a more sustainable agriculture

Abstract

The intensification of agriculture in Ireland aims to increase the exports of dairy and beef products considerably in the coming years, which inevitably implies the expansion of grassland systems to areas that were previously uncultivated. This reclamation of new land is generally made on peats and other related soils rich in organic matter (OM) that typically are located in upland areas, within catchments defined as High Status Waterbodies (HSW) in the European Water Framework Directive (WFD). Previous Irish studies at laboratory scale have shown organic soils to have low phosphorus (P) sorption ability compared to mineral soils. However, the optimum fertiliser P management for these soils is as yet unknown. This is especially important when P fertilisers are applied to these soils to improve grass productivity under land reclamation and expansion. This thesis derived an improved management strategy that accounts for rates, timing and frequency of P additions to minimise P losses to waterbodies from this soil type. Soil samples with contrasting OM content from different HSW catchments across the country were collected and subjected to a series of agronomic, soil chemistry, rainfall and leaching experiments. Compared to mineral soils, organic soils exhibited an immediate yield response to P applications due to the absence of a build-up phase resulting in a higher proportion of added P available for plant. The limited ability of these soils to adsorb added P into the soil matrix was further evidenced by the diminished ability of organic soils to accumulate added P when soil P fractions were examined following P additions. The low pH of some of the organic soils immobilised freshly applied P, rendering it unavailable for plant uptake. There was a small turnover in recalcitrant soil P pools in organic soils in the short term, and the ability to supply P from non-labile to labile pools was limited. Single fertiliser applications exhibited significantly higher P loads in surface runoff compared to split applications, although the decay rate and time at which P loads in overland flow returned to basal concentrations were similar across the different P treatments. Leachate P losses were very small and did not follow a clear pattern from the different P rates and timing regime treatments. However, nitrification was the main process dominating nitrogen (N) losses in the leached water in organic soils. This study established the management criteria to minimise losses from organic soils and demonstrated the risk of P transfer from these soils to water due to an inability to build-up their P reserves, suggesting that great care should be taken when they are bought into agricultural production. Optimum management strategies were proposed to mitigate potential P losses, such as liming prior any P fertilisation program or the adoption of a “little and often” approach when P fertilisers are to be applied to newly reclaimed areas under this soil type. Sustainability of these systems will require careful use of P in terms of rate, timing and frequency to protect the quality of waters in catchments with organic soils.