Renewable P sources: P use efficiency of digestate, processed animal manure, compost, biochar and struvite

In the attempt to close nutrient cycles, organic fertilizers and soil improvers are getting interest as renewable P sources for crops. However, both the P availability of these compounds for crops and the underlying mechanisms are not fully understood. In this study composts (n = 8), biochars (n = 5), animal manure and processed animal manure (n = 13), digestates and processed digestates (n = 15) and blends of digestates with compost/animal manure/mineral fertilizers (n = 15) were analyzed for chemical composition, organic matter stability and P use efficiency (PUE). Biodegradability (=holocellulose/lignin ratio) proved to be a good indicator for organic matter stability and can successfully replace time-consuming incubation experiments in standard analyses of organic fertilizers. The PUE of digestates, struvites, animal manure products and blends of digestate with compost/animal manure/mineral fertilizers was determined by the NH₄⁺-N, Mg and Fe content of the organic fertilizers. The PUE can be predicted by PUE = 61.34 + 8.59*NH₄⁺-N/P + 42.25*Mg/P - 8.09*Fe/P (R² = 0.71). As increasing amounts of NH₄⁺-N and Mg stimulate the formation of soluble struvite crystals, increasing PUE is explained by an increasing amount of P as struvite. The PUE of biochars and composts was determined by the Ca/P and Al content of the organic fertilizers. Here, PUE can be predicted by PUE = 88.87-1.07*Ca/P + 6.08*Al/P (R² = 0.93). As increasing amounts of Ca stimulate the formation of highly stable apatite crystals, increasing PUE is explained by an increasing amount of P in the form of apatite.