Contrasting phosphorus build‑up and drawdown dynamics in soils receiving dairy processing sludge and mineral fertilisers

Sustainable utilisation of waste from the food industry is required to transition to a circular economy. The dairy industry relies on high phosphorus (P) inputs and produces large quantities of P-rich dairy processing sludge (DPS). Recycling DPS into P fertilisers provides an opportunity to decrease the reliance on chemical P fertilisers. However, current soil nutrient management planning (NMP) is based on chemical P and does not account for recycled alternatives. A pot trial using a novel isotope pool dilution technique was used to describe build-up and drawdown cycles of P in soils fertilised with DPS. Changes in available, exchangeable, and Mehlich3 P (M3-P) pools were recorded over 36 weeks of grass growth. Results demonstrated that in the period of high P demand (12 weeks), these P pools were depleted. As crop growth and demand decreased, available P recovered through mobilisation of P from exchangeable P and M3-P reserves. DPS allowed available P to recover and build up to agronomic target levels after 24 weeks. Using DPS, build-up of available and exchangeable P was slower but P use efficiency was higher at stages of slow growth. Dairy waste created a more stable P pool which could be utilised by crops over a growing season indicating that NMP needs to account for this in the decision support for growers. Isotope studies revealed that extractive agronomic tests do not capture drawdown in P reserves.