Phosphorus and organic soils under grassland production: Assessment of phosphorus losses for a more sustainable agriculture
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Date
2019-06-19Author
Gonzalez, Jose
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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.