The dynamics of root carbon have been studied on a limited number of plants under a limited range of environmental conditions. This is particularly true of temperate pasture grasses. It was proposed to investigate rhizosphere carbon flow within Lolium perenne, to determine environmental factors affecting root exudation and root respiration. A number of microcosms were designed in which ryegrass could be pulse labelled with 14C-CO2, and the fate of the label determined within a plant-soil system. A wide range of factors were found to affect the distribution of carbon within the plant and soil under laboratory conditions. These factors include plant stage of development, micro-organisms, temperature, soil pH, soil water stress and soil anaerobism. A field experiment was carried out to estimate rhizosphere carbon flow under 'natural' conditions and to determine if the laboratory studies could be related to field conditions. Results showed that rhizosphere carbon flow varied greatly depending on environmental conditions. It was proposed that the major loss of newly photoassimilated carbon was dominated by root respiration rather than by root exudation. Inoculation with micro-organisms affected carbon loss from plant roots as exudates. The pattern of carbon distribution within the plant and rhizosphere varied greatly depending on the inoculant.

Heather plants were established, by Dr. John Miles, in gaps created in existing birch stands in 1979 at Craggan (NJ190322) and birch trees planted in heather moorland during 1977-1980 at Delnalyne (NJ189175). This study attempts to identify the processes leading to differential soil development beneath the two contrasting vegetation types over the period 1985-88 i.e. c 6 years after establishment. Field work involved the chemical analysis of soil solution and estimates of microbial biomass and basal respiration rates under birch and heather at both sites. Laboratory-based experiments were conducted to investigate the trends observed in the field. Major changes in soil processes were attributed to rapid growth and therefore nutrient uptake by the transplanted species. However, some of the changes due to birch trees, which were observed during studies of birch chronosequences by Miles &'38 Young (1980) and Miles (1981) can be attributed to high nutrient inputs in throughfall and litter under birch, which affect abiotic processes and stimulate microbial activity and nutrient transformation rates. Root inputs also influenced biological processes.