Effects of different composts on soil nitrogen balance and dynamics in a biennial crop succession.

With the one exception of compost derived from food processing industry residues and municipal waste immediately after the first application, differences in N soil concentrations between compost treatments and control were always low, suggesting that, in general, composts released scarce amounts of nitrogen. In addition, no differences were detected among compost types even though they derived from materials of very different origin. The rhythm of nitrogen release from compost appears to be of less importance given the weak rate of compost mineralization. Crop response and N uptake were scarcely affected by compost fertilization, with little difference with respect to the unfertilized treatment. These data are in accordance with the literature, which reports values of available N, in the first year, of less than one fifth of the total N applied with compost. Hence, looking at the short-term, compost does not fulfil the N needs of crops under the pedo-climatic conditions of the experimental environment, so its use as unique source of N for crops is not recommendable. Nevertheless, these indications have to be considered transient, because they are relative to the first years of compost application. Indeed, according to the literature, an increment of both compost mineralization rate and crop recovery efficiency might be expected in the medium-long term. Moreover, the data highlighted the importance of compost in the integration of original N content of soil, with the purpose of maintaining the apparent N balance in equilibrium and avoiding a progressive depletion of the soil N.