Estimation of symbiotic N2 fixation by a low-level, large-scale 15N application technique

A modified approach is presented for the 15N isotope dilution method to estimate the proportion of plant nitrogen derived from the atmosphere (Ndfa). This approach utilizes a low-level, large-scale application of 15N enriched fertilizer. In a field study, pea (Pisum sativum L.) was cultivated in two long-term (since 1970) tillage systems (conventional: CT, minimum tillage: MT). Nitrogen (26 g NO3–N ha−1 with 10 at.% 15N) was distributed with a conventional field sprayer to increase the plant available soil 15N level up to the desired range of δ15N>5‰. A formula for determining the desired amounts of 15N application is presented and a model is introduced comparing the apparent Ndfa calculated by field data to the theoretically calculated true values of Ndfa. The model makes it possible to consider three basic factors: (i) the amount of plant available soil nitrogen, (ii) the distribution of 15N enrichment in different soil layers and (iii) the soil N uptake proportions of the legume and the reference plant when the amount of applied 15N fertilizer is the dependent variable. Errors in estimating Ndfa by the 15N dilution method can be quantitatively determined with different soil N uptake patterns of legumes and reference plants at varying 15N enrichment levels. The model shows that results obtained by using the low-level, large-scale 15N enrichment technique produced a slightly smaller margin of error than did the addition of larger amounts of 15N labelled fertilizer when N uptake patterns of the legume and the reference plant differ. The new technique offers distinct advantages, in that it is very practical and makes the enlargement of plots possible at low cost and with marginal effort. The results of the low-level enrichment field study were evaluated using the model as well as the results of an investigation on pea with moderate 15N enrichment by liquid manure (isotope dilution method).