| A growing supply of composted industrial, municipal and agricultural wastes are available to agriculture as a source of crop nutrients and organic matter. Perennial forages may be ideally suited for fertilization with slow N release amendments such as composts. However, the impact of composts on soil organic matter (SOM) components and their decomposition rates in soil is much less understood than for manures. The study objectives were to (i) assess the response of perennial forages, grown with or without a legume component and on contrasting soils, to different composts and (ii) to characterize the dynamics of SOM as affected by forage crop, fertility treatment and soil type through SOM fractionation and stable isotope techniques. Compost treatments were applied in 1998--99, and supplied up to 10 Mg C ha-1 and 900 kg N ha-1. Soil coring at seeding (1998) and in 2000 and 2001 allowed changes in SOM, LF, HF, SOM size separates, and microbial biomass to be quantified.; The highest quality compost (C:N 9.8; 4% N) produced timothy yields (and crop C soil returns) matching that from fertilizer or liquid manure treatments. Sewage sludge and dairy manure compost yields failed to differ from the unfertilized control. The legume (Trifolium pratense L.) in the mixed forage acted as an "N-buffer" producing forage yields and quality (% protein) closer in range for all treatments than in the grass crop.; Gains in SOM were influenced largely by amendment and not crop or soil type. Composts increased soil organic C (up to 66%) and N, reduced soil bulk density, and increased soil moisture content. Gains in SOC with inorganic fertilizer (0.9 Mg C ha-1 y-1) were not detectable until 2001. One year after application, composts alone produced significant gains in the quantity, and modified the quality, of all densiometric and size fractions, and composts and manure alone increased soil microbial biomass carbon.; Differences in 12C/13C composition between the sewage sludge compost and soil allowed detection of compost C in SOM fractions. The 13C enriched nature of the mature compost enabled characterization of the distribution and dynamics of this compost in all SOM components. |
