Estimation of the contribution of biological N-2 fixation to two Phaseolus vulgaris genotypes using simulation of plant nitrogen uptake from N-15-labelled soil

A technique for the application of the N-15 isotope dilution technique for the quantification of plant associated biological nitrogen fixation (BNF) was tested and applied to quantify the BNF contribution to two genotypes of Phaseolus vulgaris. The technique makes use of sequential measurements of the N-15 enrichment of soil mineral N, and the uptake of labelled N by the ''N-2-fixing'' plant, to simulate its uptake of soil N (the ''soil to plant simulation'' technique). The test was made with two non-N-2-fixing crops (non-nodulating beans and wheat) and two bean genotypes (PR 923450 and Puebla 152), at two levels of N fertilizer addition (10 and 40 kg N ha(-1)), to compare the actual N uptake with that simulated from the soil and crop N-15 data. The simulation of the soil N uptake by the non-nod bean crop using this ''soil to plant simulation'' technique underestimated by 20 to 30% the true N uptake, suggesting that the mineral N extracted from soil samples taken from the 0-15cm layer had a higher N-15 enrichment than that N sampled by the roots of this crop. In the case of the wheat crop the simulation resulted in a much greater underestimation of actual N uptake. In general the results using this technique suggested that BNF inputs to the bean cultivars was higher than would be expected from the nodulation and acetylene reduction data, except for the early PR beans in the 40 kg N ha(-1) treatment. In this case the total N and simulated soil N accumulation were well matched suggesting no BNF inputs. An allied technique (the ''plant to plant simulation technique'') was proposed where the N-15 enrichment of soil mineral N was simulated from the data for total N and labelled N accumulation taken from sequential harvests of either of the non-N-2-fixing control crops. This was then utilized in combination with the labelled N uptake data of the other crop to simulate its soil N uptake. However, the results using either technique indicated that the wheat and non-nod or nodulating beans exploited pools of N in the soil with completely different N-15 enrichments probably due to differences in exploitation of the soil N with depth.