The influence of plant species on soil processes in a Tasmanian grassland

Many ecological processes are mediated by plant-soil interactions and feedbacks, thus the examination of interactions between the plant community and soil processes is crucial to further understanding how ecosystems function. Environmental change will influence how terrestrial ecosystem work but since plant communities are also likely to change in composition, it is possible that changes in plant community composition will have impacts on ecosystem processes larger than the impacts of the environmental changes themselves. Thus, this study investigated the effect of plant species on soil processes in order to understand the extent by which global change might affect soil processes via shifts in plant species composition. Using a native temperate grassland community co-dominated by a C\(_4\) grass, Themeda triandra, and a C\(_3\) grass, Austrodanthonia caespitosa, with a C\(_3\) grass, Austrostipa mollis, as a sub-dominant species, this study examined the effect of plant species on soil microbial community composition, litter decomposition and nitrogen (N) transformation processes, as well as how plant species influences the impact of elevated carbon dioxide (CO\(_2\)) and warming on litter decomposition. The co-occurring grass species differed in their associated microbial community composition examined by a molecular fingerprinting technique. The two dominant species, Themeda triandra and Austrodanthonia caespitosa, were more similar to each other in their bacterial and arbuscular mycorrhizal community composition than either was to the sub-dominant species, Austrostipa mollis, but not in their fungal community composition. Plant species not only affected microbial community composition but also microbial community function. Using a \(^{15}\)N isotope tracing technique, coupled with quantitative molecular techniques and soil incubation assays, it was found that the co-occurring plant species differed substantially in N transformation rates as well as the abundance and activity of their associated microbial groups (ammonia-oxidising bacteria, ammonia-oxidising archaea and fungi) that are involved in N mineralisation and nitrification processes. Further examination also revealed that autotrophic nitrification dominates nitrate production in this grassland, however, there was some indication that nitrification by fungi may also contribute substantially to nitrate production. Litter decomposition was influenced by both physical and biochemical quality, as decomposition rates increased with decreasing litter particle sizes and were strongly correlated with litter quality measured by litter N content and C:N ratio. The effect of plant species on litter decomposition was therefore largely driven by differences in litter quality. One of the predicted impacts of global change is its effect on litter quality of individual species, with potentially significant ramifications for ecosystem nutrient cycling. Therefore, the effect of global change on litter decomposition was examined using a reciprocal incubation experiment in order to assess the relative importance of changes in litter quality and soil microbial community function in litter decomposition rates under simulated global change. The study utilised plant and soil materials from a long running free-air-CO\(_2\)-enrichment (FACE) facility established in the same grassland community, in which the community has been exposed to elevated CO\(_2\) (550 ppm) and warming (2.0 °C) treatments since 2002. It was found that litter decomposition, assessed by C mineralization rates, was more strongly influenced by global change-induced alterations in soil community function than litter quality. Further, soil microbial communities exposed to both experimental warming and elevated CO\(_2\) concentrations had a substantially increased ability to decompose added plant litter, regardless of that litter’s source. Despite this, the consistent difference between C\(_3\) and C\(_4\) litter decomposition means that a shift in the relative abundance of C\(_3\) and C\(_4\) species is also likely to alter decomposition processes. Therefore, these co-occurring grass species exert a very strong influence on both the soil microbial community and soil N and C cycling. Hence, any changes in the relative dominance of these species are likely to lead to relatively large and important alterations of nutrient cycling. Such fine-scale differences among largely similar and co-occurring species have not been demonstrated before. It is therefore likely that such specific relationships also exist in many other systems, and that even slight changes in plant community composition, for whatever reasons, will lead to alterations of ecosystem function.