Comparação de métodos isotópicos de 15N na determinação da eficiência de recuperação do nitrogênio da adubação verde

The objective of this work was to compare three methods of 15 N isotope application to the soil in order to determine the recovery efficiency of nitrogen derived from green manure by corn ( Zea mays ). The used experimental design was a randomized complete block with six replicates. The treatments consisted of three 15 N isotope methods: indirect method I, isotope dilution with 15 N-labelled synthetic fertilizer applied through a small hole next to each corn plant; indirect method II, isotope dilution with 15 N-labelled synthetic fertilizer spread in the furrow next to the corn planting row; and direct method, application of a legume straw as green manure, treated with 2.23% excess 15 N. The green manure used was jack bean ( Canavalia ensiformis ). Applying synthetic 15 N to the soil (indirect methods I and II) did not interfere in the estimation of the recovery efficiency of N derived from green manure. The recovery efficiency of N from green manure was 17% for the indirect methods, overestimated when compared with that of 7% for the direct method. The direct method is the most adequate to determine the recovery efficiency of N from green manure.