Seasonal changes of N2-fixation by Trifolium repens in an upland Mediterranean grassland
2004
Tsialtas, J.T. | Kassioumi, M. | Veresoglou, D.S.
Six successive 15N treatments were applied during the growing season of 1997 to an upland Mediterranean grassland in order to measure the N2-fixation (%N(dfa), the percentage of nitrogen derived from atmosphere) by Trifolium repens. Nitrogen-15 (as (15NH4)2SO4, 12.5% enriched) was injected at two soil depths (5 and 15 cm), in three randomly selected 1 mx1 m squares for each treatment. %N(dfa) was determined by 15N-enriched dilution method using the six most current non-legume species (Poa pratensis L., Lolium perenne L., Festuca valida (Uechtr.) Penzes, T. repens L., Taraxacum officinale Weber ex Wigg, Plantago lanceolata L., Achillea millefolium L.) as reference plants. %N(dfa) and the absolute amount of fixed N (Symbiotic N2 Fixed, SNF) were influenced by the reference species, the depth of 15N application and the time of above-ground biomass harvest. Estimations of %N(dfa) and SNF were higher when 15N was applied at 15 cm depth. T. officinale as a reference species gave the highest estimations for %N(dfa) and SNF by T. repens. Estimations for the two parameters were lower when L. perenne was the reference plant. In general, dicots gave higher and independent from 15N application depth values of %N(dfa) and SNF compared to grasses. Dicots seemed to be more suitable as reference species. An increase of %N(dfa) with the progress of the growing season was attributed to the exhaustion of soil N with the progress of the growing season since a negative relation was found between the two parameters. %N(dfa) was negatively but not significantly related to soil water content at the two soil depths (5 and 15 cm). SNF was mainly determined by the biomass production of T. repens and not by %Ndfa during the growing season and for this reason seasonal patterns of SNF were completely different compared to those of %N(dfa).
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