Fresh and residual plant uptake of recycled and mineral phosphorus fertilizers in a multi-year field experiment

It is unknown whether the heterogeneous composition of phosphorus (P) in recycled fertilizers (i.e., products with a mixture of water soluble and non-water -soluble P forms) influences residual plant availability ( i.e. , from application year to subsequent growing seasons). Consequently, the objective of this study was to monitor how, following a single application, fertilizer solubility and characteristics impacted soil metrics and plant uptake over multiple growing seasons. Outdoor ryegrass plots created in the summer of 2019 in Taastrup, Denmark were fertilized with either a single dose (300 kg ha −1 P) of a commercial meat and bone meal (Øgro™), sewage sludge (sludge), sewage sludge ash (sludge ash), triple superphosphate (TSP) or received no P input. Soil samples and plant biomass cuts (n = 4) were collected between 2019 and 2022. As expected, TSP increased plant-available phosphate (Olsen P) concentrations in 2019 before decreasing in subsequent years. In 2019, the recycled fertilizers only slightly increased Olsen P, except for sludge-amended soils. Over time, Øgro™ maintained a constant soil Olsen P concentration. A higher labile P content (i.e., sequential fractionation water- and bicarbonate-P) increased plant fertilizer recovery (21.3–21.7 % for TSP, sludge and Øgro™) compared to the sludge ash (5 %). Yet, growing season exclusively affected ryegrass yields. Experimental projections expected a plant response based on the proportion of labile to non-labile (i.e., sequential fractionation NaOH- and HCl-P) fertilizer P. As this projection never materialized, the experimental period must be longer than four years to identify pronounced changes in ryegrass yields, plant metrics, and soil P availability. • Starting amendment solubility impacted soluble P concentrations. • Climate over treatment type impacted phosphorus plant metrics. • The impact of recycled amendments on soil residual phosphorus requires long trials.