Ozone and nitrogen controls on carbon allocation within plants and soil

Abstract

This thesis focuses on the impact of elevated ozone (O3) and/or nitrogen (N) on semi-natural vegetation, with an emphasis on C-partitioning within and between plant and soil. The project reports several studies allied to the exploration of the impacts of elevated O3 and N employing short-term studies in laboratory-based controlled-environment chambers and solardomes plus long-term studies at free-air O3 fumigation sites in the Swiss Alps and at Keenley Fell, Northumberland, UK. A solardome study indicated that both the grass Dactylis glomerata, and the forb Ranunculus acris exhibited increased senescence, and reduced C-allocation below-ground, when exposed to elevated [O3]. Furthermore, N exacerbated the O3-induced reduction in the root biomass of D. glomerata. This finding led to a mechanistic exploration of C-partitioning in response to short-term (three week) exposure of D. glomerata to a combination of elevated O3 and N inputs in self-built fumigation chambers. Plants were pulse-labelled with 14C, and the fate of the recent photosynthate then traced in nine plant and soil C-pools. The study revealed a reduction in below-ground respiration (incorporating root and soil microbial respiration) in high N treated plants, and a significant antagonistic interaction between O3 and N effects on soil microbial biomass. To relate the findings to below-ground responses in an intact ecosystem, impacts of long-term O3 and N exposure on soil microbial community diversity and C metabolism were investigated in a sub-alpine grassland. Terminal Restriction Fragment Length Polymorphism (T-RFLP) analysis and Community Level Physiological Profiling (CLPP) using 14C labelled root exudate substrates and leaf litter, revealed no effects of O3 and N on the soil bacterial diversity, and limited impacts on C substrate turnover. Moreover, in a long-term study on a traditional UK haymeadow, three years of elevated O3 and N inputs did not result in significant changes in above-ground biomass of any plant functional group. However, a significant O3 x N interaction on below-ground biomass of the sward was observed with reduced root biomass in high [O3] plots. The variation in cover of individual plant species was not explained by either O3 or N when analysed by redundancy analysis (RDA). Overall, this study suggests that N deposition subtly modifies vegetation responses to O3 stress and highlights the potentially significant role played by rising levels of N deposition and O3 as drivers of changes in carbon allocation in the natural environment. Key words: Ozone; nitrogen; carbon allocation; grassland; microbial diversity