Towards better utilisation of soil phosphorus in managed grassland systems

Managed grassland are an important agricultural land use and their productivity will need to increase over the coming decades to help meet the rising global food demand. Meanwhile, phosphorus (P) inputs into grasslands may decrease as a result of higher fertiliser prices or stronger agri-environmental regulations. Better utilisation of soil P pools and higher use efficiency of P inputs may help increase yields with lower inputs. The main objective of this thesis is to investigate mechanisms and driving factors that determine P availability in grassland soils. In a review of the existing literature on P fertilisation of grasslands, a large variation in the response of grass to P fertilisation was documented. Application rate and soil P status were major factors determining the success of fertilisation, but the presence of legumes, the soil pH, and organic matter content appeared to impact the results as well. This thesis also shows the potential role of grass species selection and management of soil fauna for increasing P use efficiency, through a series of greenhouse and lab experiments. Deep-rooting grass species with long roots were better able to resist P deficiency, and grew better than other species both when P fertilisation was applied or withheld. Additionally, various earthworm species were shown to greatly increase available P concentrations in their casts, compared to the bulk soil. These local P ‘hotspots’ could be utilised by grass and led to yield increases under P-limited growing conditions. When chemical composition of the earthworm casts was further explored, a higher pH in the casts was found to have a minor effect on P availability, and it was hypothesised that competition for adsorption sites between dissolved organic matter and orthophosphate could contribute to the higher P availability. This thesis provides a variety of avenues to increase P use efficiency in managed grasslands, which should be explored further in field and farms studies to assess their true potential.