Mahaney, W. 2007. Soil processes and plant species: Does the reintroduction of native grasses alter soil carbon and nitrogen cycling? Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.
Human activities have altered biodiversity on a global scale, but the ecological implications of shifts in plant species distributions and abundances are poorly understood. While a number of studies have shown that exotic species can dramatically and rapidly alter ecosystem properties (Vitousek and Walker 1989, Evans et al. 2001, Mack and D’Antonio 2003a), little is known about how reintroductions of extirpated species may impact ecosystem properties in restored systems. My dissertation research focuses on how the reintroduction of native prairie C4 grasses into abandoned agricultural fields (old-fields) influences soil carbon © and nitrogen (N) cycling compared to non-native C3 grasses typical of successional communities in southwestern Michigan, USA.
In this dissertation, I explore three main aspects of plant species controls on soil processes: 1) What are the decadal scale impacts of a shift from a C3-dominated to a C4-dominated system on soil properties and processes (Chapter Two), 2) How quickly do the differences in species traits and soil conditions arise (Chapter Three), and 3) Which plant traits are responsible for these differences (Chapter Four)? I addressed these questions in several old-fields at Michigan State University’s W. K. Kellogg Biological Station, using previously established experimental plots of prairie grasses in Chapters Two and Four, and setting up new experimental studies in Chapter Three.
In Chapters Two and Three, I found that C4 species had significantly greater shoot biomass and more recalcitrant tissue compared to the dominant C3 species, and these differences became apparent within two years after the species were established. However, differences between the two groups of species in surface litter and root biomass took longer than two years to develop but were apparent after 11 years. While there was some evidence to suggest that the C4 species had reduced soil inorganic N levels relative to the C3 species after just two years, many of the changes in soil properties took longer than two growing seasons to develop. After 11 years, soils under C4 species had significantly lower inorganic N levels, and slightly lower in situ net N mineralization and nitrification rates when compared to soils under C3 species. I also found limited evidence for increasing soil C pools under C4 species 11 years after reintroduction. Nevertheless, the 13C signal of the C4 species became measurable in the soil within two years.
I examined how litter quality and microclimate affected litter decomposition rates, and found that while Andropogon gerardii (C4 prairie grass) differed from the C3 species in their effect on soil moisture and temperature, these differences did not correspond to differences in decomposition. Instead, species litter quality was more important than microclimate in determining decomposition rates of both C3 and C4 species.
Overall, my results demonstrated that reintroduction of C4 species into old-fields can alter soil processes related to C and N cycling on relatively short timescales. Process rates changed first, with changes in pool sizes of C and N taking longer to become measurable. Improving our understanding of how plant species impact ecosystem properties and what species traits are driving these changes is imperative if we hope to predict the ecosystem-level consequences of changes in species distribution or composition that could occur, and are occurring, as a consequence of changes in agricultural and land use practices, global change, and species introductions.
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