Evaluation of phosphorus supply capacities in Irish grassland soils

Expansion of the Irish agri-food sector is expected to enhance the pressure to intensify grassland systems, with increases in stocking rates and extension of the grazing season. Phosphorus (P) is a key nutrient to support herbage yields and farm exports. Improving P use efficiency in grassland soils is imperative in light of the increasing fertilizers costs and sustainable intensification. This thesis investigated the capacity of temperate grassland soils to sustain P cycling and supply P for plant uptake. The objectives of this thesis are to a) characterize soil P fractions and investigate their relationship with ancillary soil properties and agronomic status; b) to assess the distribution of particulate and dissolved P in the labile P fractions and importance of soil constituents in P mobilization and retention processes; c) evaluate the role of grassland management in soil biological P cycling and d) assess the effects of soil P status and soil drainage class in the grassland soil microbial communities.

The majority of soil P was in unavailable forms with a substantial contribution of organic P (Po) forms for soil P reserves. Soil Al, Fe and Ca influenced specific P fractions and P the retention and solubilisation processes. Soil Ca concentrations were associated with P retention of labile dissolved inorganic and organic P in non-calcareous grasslands and confounded the efficacy of Morgan’s-P agronomic P test to predict plant available-P. Water extractable P contained substantial amounts of organic and particulate forms whereas chemical reagents (NaHCO3) and agronomic tests targeted physico-chemically bound P. Soil microbial communities were notably impacted by alterations in soil P availability and soil drainage class. Poorly drained soils with low P availability presented higher proportions of Acidobacteria and symbiotic fungi, whereas Zygomycota and Ascomycota showed higher abundance towards higher P availability. Key microbial groups with known capabilities for the use of soil P reserves (e.g symbiotic fungi and genera Mortierella and Fusarium) were identified in high and low soil P status grasslands. This thesis results provided an in depth analysis of soil P cycling in grassland soils, across varies soil properties and management systems. My findings highlight agronomic and environmental aspects to be address in order to increase soil P use efficiency, to refine agronomic testing and to improve plant-P supply capacity throughout key soil microbial groups.