ESTIMATING PLANT-AVAILABLE N-15 AND ITS IMPACT ON THE N-15 DILUTION MEASUREMENT OF THE SYMBIOTIC BIOLOGICAL NITROGEN-FIXATION OF RED-CLOVERGROWN ON DIFFERENT SOILS

One general problem of the N-15 dilution technique for determining sym biotic biological nitrogen fixation (BNF) by legumes is deciding on a suitable reference crop. The reference crop must exhibit a similar tem poral and spatial pattern of N uptake and must take up nitrogen from t he same soil layers as the N-fixing plant being investigated. To verif y these presumptions, mixtures of red clover (Trifolium pratense L.) a nd ryegrass (Lolium perenne L.) were cultivated on different soils in lysimeters. The organic matter of these lysimeter soils was nearly hom ogeneously labeled with N-15 by long-term N-15 fertilization. The N-15 abundance of the potentially plant-available nitrogen was investigate d via mineralization experiments after Stanford and by hot water extra ction. The land-use system on the lysimeter soils had been switched fr om intensive to ecological farming and no N fertilizer had been admini stered during the last two years before the clover-ryegrass mixtures w ere sown. It was found that 80-93% of the plant nitrogen originates fr om the atmosphere. The impact of different reference crops (Lolium per enne L., Apera spica-venti L., Atriplex patula L.) on the calculated b iological N fixation is minor. Taking into account the N-15 abundance of the plant-available inorganic nitrogen determined by mineralization after Standford and by hot water extraction, similar results are obta ined for biological N fixation.