Designer compost: facts or fantasy? A case study on compost rich in lignin and low in phosphorus.

Application of compost as a source of stabilized organic matter is one strategy for increasing the organic carbon content of arable land. To meet environmental standards on nutrient leaching, especially the total P content of the composts limits the application rate. A functional compost with high lignin content and low P concentration would be a valuable tool for maintaining soil fertility. We aimed to determine the effect of feedstock composition on P and lignin content in compost. The research is based on three farm compost experiments with three treatments each, and three compost experiments in nature conservation areas. Besides general characteristics, lignin and ergosterol content and total P and N concentrations were measured in the composts. A 12-week incubation experiment was set up to measure N mineralization and P release by the composts when applied to the soil. In general, compost characteristics varied due to differences in feedstock and composting process characteristics. The produced farm composts had higher organic matter contents than the other composts; by feedstock selection farm composts with a total P content lower than 3 g P/kg dry matter were produced. The lignin content for the organic part of the compost was relatively constant for different compost types. The incubation experiments indicated that the effect on P availability in soil was strongly related to changes in soil pH after compost addition. Our results indicate that total P concentration of the feedstock materials should always be assessed in combination with mass reduction of the feedstock mixture during the composting process.