Modelling soil P dynamics and P cycling in soils amended with organic waste

The reprioritization of the circular economy in agricultural sector worldwide leads to an increase of the recycled fertilisers production and use. To develop sustainable strategies for soil phosphorus (P) management using recycled P fertilizers, an understanding of their effect on soil P dynamics is required. This study investigates the impact of various dairy processing sludge (DPS) based recycled P fertilisers on soil P build up and availability. The experiments were conducted at incubation, pot, and field scale. A novel isotope pool dilution technique was used to quantify the labile P pool reserves as well as the rate of soil P turnover between the available (soluble) soil P and the labile (rapidly exchangeable) P.

The incubation study was designed using soils with different P fertilisation history and enabled a description of a model of biotic and abiotic soil P cycling in the soils depending on the initial status. The pattern of the soil P build up and turnover between the available and labile P pools were investigated in the soil that received recycled DPS-based fertiliser or mineral fertilisers and the differences in the build-up mechanism was described. Chemically treated DPS-based products demonstrated high potential to replace conventional P fertilisers, however they showed slower rates of available and exchangeable P build up compared to chemical fertilizers, suggesting the need for higher application rates or earlier application times. Thermally treated products exhibited poor agronomic performance as P fertilizers.

The long term changes in soil P dynamics in soil amended with the recycled P fertilisers was further assessed in a long-term pot study where soil was subjected to a P build up and drawdown cycles. The findings revealed that during the period of high P demand, available P levels fell below the baseline values but recovered as crop growth and P demand decreased. The recovery time differed depending on soil P status and DPS amendment.

Finally, a field-scale study was conducted to describe temporal changes in available, exchangeable, and Mehlich3 P pools in P-deficient grassland soil amended with DPS-derived hydrochar and struvite. The results showed that struvite increased available, exchangeable, and Mehlich3 P levels, reaching optimal values for crop production within a month. Hydrochar exhibited slower P release, with P levels remaining low during the dormant period. These findings contribute to the understanding of soil P dynamics and provide insights into the sustainable use of DPS and its derived products as P fertilizers. The results emphasize the need for tailored nutrient management approaches and regulations that account for the slower P turnover rates associated with recycled fertilizers and cautious approach to thermally produced fertilisers application.