Estimating N2-fixation in the field using 15N-labelled fertilizer: Some problems and solutions

Abstract

The 15N-labelled fertilizer dilution technique provides a method of obtaining estimates of biological N2-fixation in the field over the growing season. Field estimates of fixation obtained using peas, french beans, field beans and clover depended on the non-fixing control used. Differences in the N uptake patterns of the legume and control combinations, together with a decrease in the enrichment of plant available soil N with time, were major factors causing this dependency. A simple model of plant N accumulation at decreasing soil enrichment is presented, which explains these errors and allows a more rational choice of non-fixing control. The use of gypsum pelleted 15N fertilizer, or any other treatment which leads to a more stable soil enrichment, reduces errors caused by mismatched N uptake patterns in the two crops.