Fertilizer effects of increasing loads of composts and biosolids in urban greening

Abstract The fertilizer effect of increasing loads (4, 8 and 12 kg dm m −2 ) of four waste-based products (biosolid amended with Ca(OH) 2 , biosolid without lime amendment, biosolid compost and biowaste compost) was compared to mineral fertilization (10–30 kg N m −2 ) in a 2-year pot experiment with perennial ryegrass ( Lolium perenne , cv. Napoleon). Addition of 4 kg dm m −2 of biosolids to the soil resulted in a fast establishment of grass cover and moderate grass growth the first and second year. The highest load of biosolids resulted in growth significantly higher than mineral fertilization with 30 g nitrogen m −2 the first season. Addition of biowaste compost had an equivalent effect on plant growth during establishment and first growing season as 10–20 g nitrogen m −2 for all application levels, whereas biosolid compost had no effect on plant growth. Plant-available nitrogen (PAN) varied between 16 and 27% of N added by the biosolids in the first year depending on biosolid type and load. PAN was lower for the biosolid amended with lime (Ca(OH) 2 ) than for the biosolid with no lime addition. For biowaste compost first year PAN was 6–11%, whereas the biosolid compost had no nitrogen fertilizer effect. The second year PAN was lower, varying between 0.8 and 6.8% for biosolids and biowaste compost. When establishing a grass cover, the biosolids should be used in moderate loads to avoid too intensive plant growth. As the biowaste compost has a lower nitrogen fertilizer effect, it can be used in higher loads than the biosolids. The biosolid compost should be regarded as a soil amendment with no nitrogen fertilizer effect.