CAJ | 학술논문

Planting grass and legume mixtures on improved grasslands has the potential advantage of realizing both higher yields and lower environmental pol ution by optimizing the balance between applied N fertilizer and the natural process of legume biological nitrogen ifxation. However, the optimal level of N fertilization for grass-legume mixtures, to obtain the highest yield, quality, and contribution of N2 ifxation, varies with species. A greenhouse pot experiment was conducted to study the temporal dynamics of N2 ifxation of alfalfa (Medicago sativa L.) grown alone and in mixture with smooth bromegrass (Bromus inermis Leyss.) in response to the addition of fertilizer N. Three levels of N (0, 75, and 150 kg ha–1) were examined using 15N-labeled urea to evaluate N2 ifxation via the 15N isotope dilution method. Treatments were designated N0 (0.001 g per pot), N75 (1.07 g per pot) and N150 (2.14 g per pot). Alfalfa grown alone did not beneift from the addition of fertilizer N;dry matter was not signiifcantly increased. In contrast, dry weight and N content of smooth bromegrass grown alone was increased signiifcantly by N application. When grown as a mixture, smooth bromegrass biomass was increased signiifcantly by N application, resulted in a decrease in alfalfa biomass. In addition, individual alfalfa plant dry weight (shoots+roots) was signiifcantly lower in the mixture than when grown alone at al N levels. Smooth bromegrass shoot and root dry weight were signiifcantly higher when grown with alfalfa than when grown alone, regardless of N application level. When grown alone, alfalfa’s N2 ifxation was reduced with N fertilization (R2=0.9376, P=0.0057). When grown in a mixture with smooth bromegrass, with 75 kg ha–1 of N fertilizer, the percentage of atmospheric N2 ifxation contribution to total N in alfalfa (%Ndfa) had a maximum of 84.07 and 83.05%in the 2nd and 3rd harvests, respectively. Total 3-harvest%Ndfa was higher when alfalfa was grown in a mixture than when grown alone (shoots:|t|=3.39, P=0.0096;root:|t|=3.57, P=0.0073). We believe this was due to smooth bromegrass being better able to absorb available soil N (due to its ifbrous root system), resulting in lower soil N availability and al owing alfalfa to develop an effective N2 ifxing symbiosis prior to the 1st harvest. Once soil N levels were depleted, alfalfa was able to ifx N2, resulting in the majority of its tissue N being derived from biological nitrogen ifxation (BNF) in the 2nd and 3rd harvests. When grown in a mixture, with added N, alfalfa established an effective symbiosis earlier than when grown alone;in monoculture BNF did not contribute a signiifcant portion of plant N in the N75 and N150 treatments, whereas in the mixture, BNF contributed 17.90 and 16.28%for these treatments respectively. Alfalfa has a higher BNF efifciency when grown in a mixture, initiating BNF earlier, and having higher N2 ifxation due to less inhibition by soil-available N. For the greatest N-use-efifciency and sustainable production, grass-legume mixtures are recommended for improving grasslands, using a moderate amount of N fertilizer (75 kg N ha–1) to provide optimum beneifts.