CONTINUOUS DEFOLIATION OF PERENNIAL RYEGRASS (LOLIUM-PERENNE) AND WHITE CLOVER (TRIFOLIUM-REPENS) AND ASSOCIATED CHANGES IN THE COMPOSITIONAND ACTIVITY OF THE MICROBIAL-POPULATION OF AN UPLAND GRASSLAND SOIL

A microcosm study was conducted to investigate the effect of continuou s plant defoliation on the composition and activity of microbial popul ations in the rhizosphere of perennial ryegrass (Lolium perenne) and w hite clover (Trifolium repens). Continuous defoliation of ryegrass and clover resulted in significant (P<0.01) increases in soil microbial b iomass, although whilst increases were measured from day 2 in soil sow n with clover significant increases were only seen from day 21 in soil sown with ryegrass. These increases were paralleled, from day 10 onwa rds, by increases in the numbers of culturable bacteria. Numbers of Ps eudomonas spp. also increased in the later stages of the study. No inf luence on culturable fungal populations was detected. Whilst shifts in the composition of the microbial populations were measured in respons e to defoliation there was little effect on microbial activity. No cha nges in either dehydrogenase activity or microbial respiration in the rhizosphere of ryegrass or clover were measured in response to defolia tion, but both dehydrogenase activity and microbial respiration were g reater in ryegrass than clover when values over the whole study were c ombined. Continuous defoliation resulted in significant (P<0.001) redu ctions in the root dry weight of ryegrass and clover, of the order 19% and 16%, respectively.