Three keys to unlock legacy phosphorus for sustainable crop production : models, budgets, and expert elicitation

Davies, Jennifer and Haygarth, Philip and Davies, Jessica and Blackwell, Martin and Janes-Bassett, Victoria (2025) Three keys to unlock legacy phosphorus for sustainable crop production : models, budgets, and expert elicitation. PhD thesis, Lancaster University.

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Abstract

The term “legacy phosphorus (P)” is used to refer to the accumulated P in soils and wider catchments resulting from past anthropogenic inputs and its subsequent impact in the environment. To explore how the potential of soil legacy P can be “unlocked” for sustainable crop production this thesis used three “keys”: process-based models, substance flow analysis (SFA) models and expert elicitation. Using the process-based model N14CP, the study demonstrates that legacy P has the potential sustain crop yields for 50-83 years and 155-217 years on a cereal and permanent grassland plot, respectively with no applications of P fertiliser. However, addressing key "blind spots" in our empirical understanding of P cycling may help explain the sustained yields and soil P stocks in some long-term, no-input systems that N14CP cannot capture. Utilising an SFA model, P balances and internal flows were estimated for an organic mixed farming estate in the UK. This work showed that SFA models can help farmers to track P flows and identify inefficiencies, especially in systems reliant on manure recycling. However, their effectiveness depends on accurate data and application over multiple cropping seasons. Finally, this thesis identifies significant gaps between scientific perspectives on legacy P and farmers' practices. It highlights that many farmers view legacy P as inaccessible to crops and that socio-economic barriers greatly influence P management practices on farms. The development of tools and strategies that align with farmers' needs, supported by farmer engagement could help to address this. As data availability and processing capacity continue to grow, so does the opportunity for integrated P research to address the gaps in our understanding of P cycling in the environment. However, legacy P challenges will persist unless there is widespread adoption of strategies that prevent further P accumulation and effectively manage existing legacy P stocks.