Estimating Plant-Available 15N and its Impact on the 15N Dilution Measurement of the Symbiotic Biological Nitrogen Fixation of Red Clover Grown on Different Soils

Abstract

One general problem of the 15N dilution technique for determining symbiotic biological nitrogen fixation (BNF) by legumes is deciding on a suitable reference crop. The reference crop must exhibit a similar temporal and spatial pattern of N uptake and must take up nitrogen from the same soil layers as the N-fixing plant being investigated. To verify these presumptions, mixtures of red clover (Trifolium pratense L.) and ryegrass (Lolium perenne L.) were cultivated on different soils in lysimeters. The organic matter of these lysimeter soils was nearly homogeneously labeled with 15N by long-term 15N fertilization. The 15N abundance of the potentially plant-available nitrogen was investigated via mineralization experiments after Stanford and by hot water extraction. The land-use system on the lysimeter soils had been switched from intensive to ecological farming and no N fertilizer had been administered during the last two years before the clover-ryegrass mixtures were sown. It was found that 80–93% of the plant nitrogen originates from the atmosphere. The impact of different reference crops (Lolium perenne L., Apera spica-venti L., Atriplex patula L.) on the calculated biological N fixation is minor. Taking into account the 15N abundance of the plant-available inorganic nitrogen determined by mineralization after Standford and by hot water extraction, similar results are obtained for biological N fixation.