Competition between subterranean clover and rygrass for uptake of N-15-labeled fertilizer

Grasses and legumes are often grown together for improving quality of forag e and for better yield when soil N availability is limiting. One compatible mixture is Trifolium subterranium L., subterranean clover and Lolium multi florum Lam, ryegrass. Experiments were conducted with plants grown in a gla sshouse and plant growth chambers to determine the competitive ability of t hese plants for fertilizer N. Fertilizer N was enriched with N-15 to measur e the contribution of dinitrogen fixation and fertilizer N to the growth of clover. In pure stands, with increased fertilizer N, the legume took up si milar quantities of mineral N as the grass to make up for the deficit due t o less dinitrogen fixation but in mixed stands the grass by far outcompeted the legume. The growth of clover suffered due to lack of N both from less dinitrogen fixation and the inability to compete with the grass for mineral N. Increasing levels of fertilizer N reduced dinitrogen fixation by the cl over. When growing with the clover the grass did not receive N from the clo ver. A laboratory experiment using N-15 label on pure stands of the two spe cies indicated that the grass had an inherent capability of absorbing almos t twice the amount of mineral N as the legume under the same conditions eve n when root weight and volume was not larger for the grass. The results of this research provide insight into the often observed phenomenon that growt h of clover is reduced when grown with grass in proportion to the amount of mineral N provided.