Linking Measurable Phosphorus Pools With Simulations of Soil P Dynamics: Results for the Long‐Term Experiment ‘Rostock’

Phosphorus (P) is removed from agroecosystems through harvesting, and sustainable management must include P fertiliza-tion as P availability affects crop performance. However, accurate assessment of plant-­ available P is challenging. In this study,two promising approaches are combined to assess the plant-­ available P of a 22-­ year long-­ term experiment (LTE) near Rostock,Germany. We hypothesize agreement between a modern P test method and process-­ based model estimates of plant-­ availableP. The diffusive gradients in thin films (DGT) technique offers an accurate P test method because it mimics the diffusion anddesorption of soil P in the presence of root uptake. This was applied in a synergetic combination with a state-­ of-­ the-­ art agroeco-system model that was extended with a P cycling module. The simulations and yearly DGT-­ P analyses comprise 4 treatments: noP fertilization, mineral P fertilization with triple-­ superphosphate, organic P fertilization with compost, and mineral plus organicP fertilization. Soils at 0–30 cm depth were sampled in four replicates on a yearly basis between 1999 and 2021. In addition, a Pfractionation was applied for 2015 using the Hedley approach, which made it possible to link non-­ plant-­ available, steady P frac-tions with the respective model pools. The comparison between DGT-­ P determined plant-­ available P up to a depth of 30 cm andthat estimated from the pools of the agroecosystem model AgroC showed agreement with respect to the differences between thetreatments and with respect to the temporal evolution (R2 between 0.65 and 0.7). Less agreement was detected for DGT-­ P and therespective model pools in deeper soil. A closer match over soil depth was found between grouped Hedley P fractions and AgroCmodel pools. Both, model and DGT-­ P analyses indicate that a new plant-­ available P equilibrium will be established under thenew P management after about 12 years for the Rostock site, which points to the resilience of P cycling in agroecosystems. Weconclude that the combined application of DGT-­ P analysis and agroecosystem modeling offers a robust and accurate quantifica-tion of plant-­ available P in the plough layer and can be used to create an agricultural digital twin with respect to soil P availabilityand its impact on crop yield.