Recycling potential of secondary phosphorus resources as assessed by integrating substance flow analysis and plant-availability.

The plant-availability of phosphorus (P) plays a central role in the ability of secondary P resources to replace mineral fertilizer. This is because secondary P plant-availability varies, often with large fractions of residual P that has no immediate fertilization effect. Therefore, if low quality secondary P fertilizers are applied, they will accumulate in soils that, in the long run, may increase the risk of P runoff and eutrophication. Substance flow analyses (SFA), used to identify potentials for improved P management, have not considered this well-known quality barrier. We, therefore, argue that traditional SFA over-estimates the fertilizer potential of secondary P resources. Using Norway as a case, we present a plant-availability extended SFA methodology that integrates SFA and the concept of relative agronomic efficiency. To account for the plant-available soil P stock and long-term soil interactions, we adjust the Norwegian P fertilization demand based on soil P values. We found that, while the method has uncertainties particularly for long-term estimations, it more realistically estimates secondary P fertilizer potentials and is adaptable to other countries. For Norway, we found the overall secondary P fertilizer potential reduced by 6-55% when considering plant-availability. The most important secondary resource was manure, which had the highest P plant-availability and quantities large enough (10.9kt plant-available P/yr) to meet Norway's entire P fertilization demand (5.8kt plant-available P/yr). However, barriers related to its transportability need to be overcome to efficiently use this resource. Fish sludge was also an important product, with 6.1kt plant-available P/yr but with uncertain plant-availability data. We argue that high quality secondary P resources can theoretically meet Norway's P fertilization demand and, therefore, make Norway mineral P independent. However, it is important that their use is carefully regulated based on plant-availability to eliminate the soil accumulation of both available and residual P.