Nitrogen mineralization and availability in manure composts from Québec biological farms

The real contribution of composts to N availability depends on their characteristics and maturity. A laboratory incubation experiment (140 d) was conducted parallel to a greenhouse study (330 d) in a split-split-plot design, with, respectively, two peat rates (0, 20 gkg −1 soil), five manure composts and four compost rates (0, 250, 500 and 750 gkg −1 soil). Compost N mineralization, orchardgrass (Dactylis Glomerata L.) yield and N uptake were measured. Total amount of mineralized N and yields and N uptake for six cuts of orchardgrass varied significantly with the type of composts and rate. Peat addition temporarily decreased compost N mineralization rate but significantly increased orchardgrass yields and N uptake as compared to peatless treatments. Mineralized N represented <3% of total N, whereas N uptake by orchardgrass represented 13–40% of total N among composts. This low mineralized N value compared to total N and total N uptake was due to a high maturity of the composts studied. This was shown by high humic acid: fulvic acid fraction ratios (3.1 to 4.8) and low nonhumic fraction:humic acid+fulvic acid ratios (0.10 to 0.12), as well as low C/N ratios, high bulk density, high ash content, pH, NO 3 -N and CEC values. Even if peat addition decreased mineralized N basically due to temporary N immobilization, its application significantly increased yields and N uptake probably by improving physical conditions in soil-manure compost mixtures. Peat addition to mature manure composts should be considered as an interesting alternative for horticultural plants sensitive to high NO 3 -N content from mature composts.