Feedstock Mixture Composition as Key Factor for C/P Ratio and Phosphorus Availability in Composts: Role of Biodegradation Potential, Biochar Amendment and Calcium Content.

Agricultural soils in northwest Europe as well as in parts of the USA have excessive P levels. To meet European environmental standards on nutrient leaching in soils with a high P load, the total P content of manures and composts limits their application rate. The aim of the present study was to determine the effect of feedstock composition on C/P ratio and P availability in mature compost. A functional compost with a high C/P ratio and a high organic matter content is a valuable tool for maintaining soil fertility. We produced composts with C/P ratios of 60-180 and high organic matter contents, i.e., 43-72% on dry matter basis. P concentration in the compost increased and C/P ratio decreased due to mass reduction during the composting process, which was determined by the biochemical composition, i.e., the biodegradation potential of the feedstock mixture. The feedstock selection also had indirect effects on P availability due to its effect on compost characteristics such as pH, which strongly reduced the readily available P concentrations at $$\text{pH}_{\text{H}_{2} {\text{O}}}$$ > 8.5. Compost pH was found to be an important indicator for readily available P concentrations in the studied composts with $$\text{pH}_{\text{H}_{2} {\text{O}}}$$ between 7.1 and 9.4, and was positively related to total Ca content in the compost, indicating an interaction between Ca and P. In contrast, P availability was not related to the organic matter content of the composts. Mixing biochar (pyrolyzed organic matter) into the compost further reduced the readily available P in two types of compost by 10-64%, even after addition of a small amount of additional mineral P. This was observed for two types of biochar with different chemical composition. The reduction was higher for the biochar with the highest Ca, Fe and Al concentrations, indicating the role of these elements for P binding in the biochar-blended compost. Graphical Abstract: [ABSTRACT FROM AUTHOR]