Direct interactions between N fertilizer and organic matter: evidence from trials with 15 N-labelled fertilizer.

This paper focuses on testing the Direct Hypothesis using 15N-labelled fertilizers. This hypothesis formulated by Vanlauwe et al. (2001a) originates from data obtained under controlled laboratory conditions showing interactions between organic matter-C and mineral-N. A series of experiments (lysimeter, nanoplots and microplots) with 15N-labelled fertilizer (urea or ammonium sulfate) and organic matter was carried out to test the Direct Hypothesis at different scales. Details of the trials testing the Direct Hypothesis through the use of 15N-labelled fertilizers and organic matter carried on an Arenosol in Ibadan, southwestern Nigeria are discussed.