Fate of 15N-nitrate applied to a bermudagrass system: assimilation profiles in different seasons

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Pressures to protect water quality and water shortages are leading to increased applications of effluent water on turfgrasses, and there are pressures to disperse effluent throughout the year. These experiments investigated NO3– uptake efficiency by Tifway bermudagrass [Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt Davy] during growth and dormancy cycles, and thus the potential to filter effluent at different times of the year. Turf–soil cores from field plots were placed in controlled environment chambers and fed solutions with 15N-NO3–. Nitrate uptake was, as expected, greatest in summer when plants were growing rapidly. Less than 10% of applied NO3– was recovered from soil after 3 d. The microbial population was elevated, but little 15N was found in soil microbial or organic fractions. The system was inefficient in winter when bermudagrass was dormant; 80 to 90% of the NO3– remained in soil after 16 d. The system was more efficient than expected in spring and fall transition months, with 80 to 90% assimilated within 1 wk. A large portion of applied 15N was held belowground in rhizomes and roots. Competitiveness of the microbial population was greater in transition months than during rapid bermudagrass growth in August when the population itself was larger. Although seasonal differences occurred, bermudagrass roots were consistently more competitive than the microbial population for applied 15N, an observation very different from that with other grass systems.

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