Linking plant community composition and ecosystem dynamics: Interactions of plant traits determine the ecosystem effects of plant species and plant species mixtures

I examined the mechanisms by which plant species and plant species mixtures influence N dynamics, soil phosphorus and decomposition in a California annual grassland. I established plots of eight grassland species in monoculture, and in two-, three- and five-species mixtures. Plant species influenced ecosystem processes through an interaction of different mechanisms, including litter quality and quantity, aboveground live biomass, labile carbon (C) inputs (exudation and tissue turnover), and influences on soil microclimate (Chapter 1). Species affected ecosystem processes through unique combinations of these mechanisms, and the relative importance of these mechanisms changed over time. Litter quality was the dominant mechanism by which plant species influenced early stages of decomposition, but labile C inputs became more important in the later stages of decomposition. Plant species effects on nitrogen (N) cycling were in part dependent on both litter quality and labile C inputs, but plant effects on soil temperature and moisture played a critical role at the time of the growing season when these factors most limited plant growth and microbial activity. Plant composition had only small effects on soil phosphate and microbial phosphorus (P), which were more strongly influenced by other factors such as slope position.; In order to understand the roles plant species play in ecosystems, it is vital to consider how they influence the activity and distribution of other organisms that play large roles in ecosystems. Plant species influenced the timing, type and extent of gopher activity, and in turn, this plant-gopher interaction greatly altered patterns of nitrogen cycling (Chapter 2). Plant species were also associated with distinct functional profiles of the bacterial community (Chapter 3); these profiles were closely related to plant labile C inputs. While these interactions between the plant community and bacterial substrate utilization profiles correlated with ecosystem processes, variations in bacterial community function, independent of plant composition, also correlated with ecosystem processes.; Bacterial C utilization profiles in plant mixtures were not an additive function of the component monocultures, and this may, in part, account for the non-additive effects of plant mixtures on ecosystems. The non-additive effects of plant mixtures were also related to combinations of plant substrates impacting the activity of the microbial community. For example, legumes enhanced decomposition of recalcitrant litter. Another mechanism that accounted for non-additive effects of plant mixtures on ecosystem processes was changes in plant traits when grown in mixture. Shifts in litter quality, labile C inputs, aboveground biomass, and effects on soil temperature all were associated with non-additive effects of plant mixtures on N cycling.; Overall, plant traits can be used to understand many effects of plant communities on ecosystems by providing mechanistic links between ecosystem processes and the interactions between plant traits, plant species, and plants with other organisms.