How to Evaluate Nitrogen Release Process from Compost.

It is important to synchronize the nitrogen (N) supply process in soil or cultivation media to N uptake of crop, both for high N use efficiency and less environmental impact. Thus, in planting system which only compost fertilizer is used as N resource, evaluation of the N release process is the primary need to assess the effectiveness of the fertilizer. Up to now, the most accurate method to evaluate N release is 15N labelling of compost fertilizer. However, the 15N labelling must be done during the composting process, which is impossible to those already sold in the market. We change thinking to label soil active N pools, which are the main sources of mineral N from soil during compost incubation in soil for a short period (such as 60 days). In this way, the mineral N during the incubation of the mixture of soil and compost can be differentiated and the N release pattern of compost can be evaluated. However, to do this, the most difficult is to evenly label 15N the different active N pools, such as microbial biomass N, salt extractable N, and slow decomposable N, because there is significant 15N discrimination between N pools. Then, we tested a series of ways to evenly label 15N to the different active N pools, in which the ¦Ä15N of soil mineral N, microbial biomass N, salt extractable N, and slow decomposable N were measured. We want to the variation of ¦Ä15N between these pools to be less than 1¡ë, which can be ideal in the evaluation of N release from compost. During the method development, the quantitative relationship between these N pools was also clarified, helping to understand the mineralization- immobilization turnover process. Keywords: nitrogen release, compost, 15N label, evaluation