Rhizosphere carbon flow and the associated soil microbial community of Lolium perenne : the influence of defoliation and non-structural plant carbon
Grassland productivity is commonly limited by availability of nutrients, and is dependent on cycling through the microbial biomass. Although the importance of the soil microbial community is recognised, the role of rhizosphere C-flow in mediating microbial activity and nutrient cycling is unclear. This thesis aimed to adopt an integrated approach to investigate factors regulating rhizosphere C-flow in grazed ecosystems, and determine associated impacts on the soil microbial community. Lolium perenne cultivars contrasting in accumulation of non-structural carbon were used as models to study impacts of defoliation on root exudation, and consequences for the microbial community. Experiments in axenic culture found that defoliation induced a large, transient increase in exudation, but this was inconsistent and not cultivar-specific. Utilisation of a lux-marked biosensor for C-flow, demonstrated that increased exudation was a rapid response to defoliation (<180 min), and that the rate of subsequent recovery was cultivar-specific. The involvement of root re-uptake processes was investigated through the use of a plasmalemma-transport inhibitor and 14C-tracers. However, these studies were inconclusive and the physiological basis for defoliation-induced exudation remains unresolved. In soil, the high NSC accumulating cultivar (AberDove) was more productive than the lower NSC accumulating cultivar (S23). In plants carbon partitioning was altered to re-establish photosynthetic capacity to a greater extent in AberDove, compared with S23. However, impacts of defoliation on the soil microbial community were greatest associated with S23, both in the short term and over the growing season. Impacts on the microbial community were apparent in physiological diversity (SCsups) and structure (PLFA), and in rates turnover determined in a 14C pulse-chase experiment. The cultivars can be considered to have contrasting strategies for recovery following defoliation. In AberDove, NSC is utilised to re-establish photosynthetic capacity, whereas in S23, altered C-flow from roots impacts on microbial community structure, turnover and nutrient transformations.