Improving plant availability of recycled phosphorus with organic acids

Increasing the efficiency of phosphorus (P) use through nutrient circularity is essential for securing P sufficiency and reducing pollution. The release of P in recycled materials can be enhanced with organic acids produced from cellulose. In this study, fertilisers based on sewage sludge (SS) and poultry manure (BR) were treated with increasing levels (0%, 8%, and 20%) of hydroxy carboxylic and carboxylic acid mixture to determine the effect of acidification on the availability of P and other selected elements. According to sequential extractions, acidification increased the share of labile P in both products. In SS, which was dominated by refractory metal phosphates, a higher acid dose was required for the effect, and a smaller share of total P was transformed than in the BR, which was already rich in easily accessible P. Besides P, acidification increased the mobility of several elements but induced no harmful element accumulation in ryegrass leaves. The increased P lability recorded by fractionation in SS was reflected in ryegrass P uptake, cumulative yield, apparent P recovery, and mineral-P equivalence. In BR exhibiting superiority to mineral P, the effect of acidification on plant growth and P uptake was negative. Acidification treatment offers an option to enhance the plant availability of circular P, but the treatment needs to be tailored according to product characteristics. Optimising the application rate and acid composition might allow increasing effectiveness while avoiding negative growth effects.