The effect of defoliation on growth and N 2 fixation by Medicago sp. Grown alone or with ryegrass

Abstract

The effect of repeated defoliation on N 2 fixation by Medicago littoralis (medic) grown alone or with Lotium multiflorum (ryegrass) has been studied using an 15N dilution technique. The 15N label had been incorporated into the organic fraction of the soil. The 15N enrichments of clippings of the non-fixing reference plant (ryegrass) grown alone were relatively constant, suggesting that the enrichment of the soil mineral N taken up by the plants was also constant. Thus the major source of error of the 15N dilution technique was removed. In pots, repeated defoliation drastically reduced N 2 fixation by the medic plants, whether grown alone or in competition with ryegrass. By contrast, in field microplots, water, rather than clipping, appeared to control yield and hence N 2 fixation. There was a direct linear relationship between medic yields and amounts of fixed N, in both pot and field experiments. Enrichments of the N of non-fixing ryegrass tops and roots differed. These differences were observed in the absence of legume but also depended upon the extent of competition from medic. In most cases, the results suggested little transfer of fixed N from medic to ryegrass, even after continued defoliation. These variations in ryegrass N enrichments may be due to discrimination against 15N during the distribution of 14N and 15N around the plant, and they produce some minor differences in the calculated amounts of N fixed by the medic. As an example, the amounts of fixed N in the medic were 71.1, 70.6, 69.8 mg pot −1, using the 15N atom% enrichment of N of the ryegrass tops, whole plant, and roots, respectively.