Studies on competition for N between arrowleaf clover and gulf ryegrass by using ¹⁵N depleted ammonium nitrate

Abstract

Arrowleaf clover (Trifolium vesiculosum Savi) and ryegrass (Lolium multiflorum Lam.) are commonly grown together but the fertilizer N needs of these plants when grown in mixture are not well understood. Two greenhouse and two field experiments were undertaken to investigate the competitive ability of Yuchi arrowleaf clover and gulf ryegrass for 99% ^15N depleted ammonium nitrate applied to the soil. The objectives of the two greenhouse experiments were to determine the competition for N at different planting densities. Nitrogen was applied to clover and ryegrass grown in pure and mixed stands during early growth. In both experiments, clover and ryegrass in pure stand accumulated similar quantities of fertilizer N. However, ryegrass in mixed stand accumulated several times as much fertilizer N as its clover component. Differences in root mass between the plants in pure stand could not be used to explain the differences in the plants ability to utilize the fertilizer N. These experiments indicated that gulf ryegrass in mixture with arrowleaf clover is much more competitive for soil N than the clover but in pure stand grass and clover have a similar ability to accumulate the soil N. Two field experiments were conducted in 1981-82 and 1982-83 to determine the seasonal competition for N between arrowleaf clover and ryegrass. Nitrogen was broadcast over the field in February, April, and in both February and April. In both experiments, when N was applied in February or both February and April, ryegrass in pure or mixed stand recovered more fertilizer N than the corresponding clover in pure or mixed stand. The April application of N resulted in the clover in pure or mixed stand recovering as much or more fertilizer N as the corresponding ryegrass in pure or mixed stand. Differences in the seasonal fertilizer N recoveries in the plants could not be explained by the time of maximal dry matter production. Both experiments indicated that the competition and ability to accumulate fertilizer N is influenced by the timing of the N application.