NITROGEN MINERALIZATION AND AVAILABILITY IN MANURE COMPOSTS FROM QUEBEC BIOLOGICAL FARMS

The real contribution of composts to N availability depends on their c haracteristics and maturity. A laboratory incubation experiment (140 d ) was conducted parallel to a greenhouse study (330 d) in a split-spli t-plot design, with, respectively, two peat rates (0, 20 g kg(-1) soil ), five manure composts and four compost rates (0, 250, 500 and 750 g kg(-1) soil). Compost N mineralization, orchardgrass (Dactylis Glomera ta L.) yield and N uptake were measured. Total amount of mineralized N and yields and N uptake for six cuts of orchardgrass varied significa ntly with the type of composts and rate. Feat addition temporarily dec reased compost N mineralization rate but significantly increased orcha rdgrass yields and N uptake as compared to peatless treatments. Minera lized N represented <3% of total N, whereas N uptake by orchardgrass r epresented 13-40% of total N among composts. This low mineralized N va lue 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 ac id fraction ratios (3.1 to 4.8) and low non-humic fraction:humic acidfulvic acid ratios (0.10 to 0.12), as well as low C/N ratios, high bul k density, high ash content, pH, NO3-N and CEC values. Even if peat ad dition decreased mineralized N basically due to temporary N immobiliza tion, its application significantly increased yields and N uptake prob ably by improving physical conditions in soil-manure compost mixtures. Feat addition to mature manure composts should be considered as an in teresting alternative for horticultural plants sensitive to high NO3-N content from mature composts.