Towards an understanding of the dynamics of compost N in the soil-plant-atmosphere system using 15N tracer

The principal aim of the present review is to synthesize and evaluate published information on the N fertilizer value of composts, and their effect on the utilization of conventional N fertilizers by crops. We have examined the literature where the dynamics of N in the soil-plant-atmosphere continuum are traced using composts that were either artificially enriched in the N-15 stable isotope (in units of atom % N-15 excess) or had a natural N-15 abundance (delta N-15 in units of aEuro degrees or per mil) due to isotope discrimination processes that occur during composting. The methods used to produce artificially-enriched composts and to test uniformity of labelling are reviewed. Limited data show that composts are generally inferior sources of N for crops compared with their raw materials due to a lower N mineralization capacity. Immobilization of fertilizer N increases in compost-amended soils and may reduce recovery by a crop, but fertilizer N losses are reduced overall. However, co-application of compost and urea should be avoided due to the risk of increased NH3 volatilization due to the action of compost-derived urease. High annual rates of compost application can exacerbate environmental problems including nitrate contamination of groundwater. Efforts are required to improve the N fertilizer value of composts by minimizing NH3 volatilization losses during composting. More attention should also be given to the use of the natural N-15 abundance of compost as a tracer.