Comparison of three methods for assessing mineral nitrogen released by soil under maize

Field experiments were carried out in 1990 and 1991 on seven sites to compare estimates of inorganic N released by soil under a maize crop by three methods : (i) N removed by the crop without N fertilizer, (ii) non labelled N removed by maize given a 'IN dressing and (iii) N released during the crop season by bare soil in iron tubes driven into the soil. Field estimates of N supply were characterized by a heterogeneity due to the variability of both soil inorganic N and N uptake by maize. Soil supplies estimated by crop uptake methods were slightly higher than direct measurements in the tubes. It was assumed that mineral N losses occurred by diffusion from inside to outside the tubes. The inaccuracy of the measurements of soil N supply in the field is one of the main obstacles to providing N fertilizer forecasts for crops and to studies of the relationships between soil N behaviour in the laboratory and in field trials.