Effect of phosphate supply and competition from grasses on growth and nitrogen fixation of Medicago trunculata

Abstract

Medic plants and ryegrass were grown in small pots in a glasshouse in monoculture (six plants per pot) or in mixture (three plants of each species) at eight rates of phosphate application. After 10 weeks, the rate of nitrogen fixation was estimated by the acetylene reduction (AR) technique on the intact plants in soil, after which the plants were harvested, both shoots and roots. The yield of medic, in both monoculture and mixture, increased with increasing rate of phosphate application to an optimum at a level of 160 ppm phosphorus and then decreased again at higher phosphate levels. AR activity was similarly dependent on phosphate application and was optimal at the same phosphate level as was yield. AR activity was extremely low in the ryegrass monocultures, the yield of which was optimized at only 10 ppm phosphorus. The community x phosphate interaction on AR activity per plant of medic was not statistically significant, there being considerable variability in this measurement; the suppression of nitrogen fixation by competition from ryegrass near its own phosphate optimum was not therefore established in this study, but remains a possibility. Ryegrass plants benefited individually from growing in mixed culture with the legume, producing as much shoot dry matter from three plants in mixture as from six in monoculture. The advantage in mixture was due either to reduced competition offered by medic or to transfer of fixed nitrogen from legume to grass. Higher concentrations and contents of nitrogen per plant in mixture provided circumstantial evidence for the latter. However, on a pot basis, nitrogen content of ryegrass was similar in pure culture and in mixture. The relevance of the data to the results of a survey of farmers' fields is discussed.