How can shifts in species composition influence ecosystem processes? Impacts of reestablishing C4 grasses into old-field communities on C and N cycling

Mahaney, W., K.A. Smemo, and K.L. Gross

Presented at the ASM at Estes Park (2006-09-01 to 2012-09-23 )

Humans are altering biodiversity and species distributions around the world, but the ecological implications of these changes are poorly understood. A mechanistic understanding of how plant species impact soil processes is needed to improve our ability to predict how future shifts in species composition and abundance may affect ecosystem function. Agriculture in southwestern Michigan has reduced soil carbon © and nitrogen (N) levels dramatically and, following abandonment, old-fields that were once dominated by C4 grasses typically contain mainly non-native C3 species. We examined whether the reintroduction of C4 grasses into old-fields has altered soil C and N cycling, over what timescale these impacts are measurable, and whether specific plant traits can predict those impacts. C4 species typically have higher biomass and more recalcitrant tissue (both above- and below-ground) than C3 grasses, and these differences in litter quality and quantity are expected to affect C and N cycling. To examine species’ decadal scale impacts, we measured plant and soil properties under monocultures of a C4 species (Andropogon gerardii) established in 1994 in two old-fields and compared this to samples from adjacent C3 grass-dominated plots. Andropogon had greater aboveground biomass (6-10 times), belowground biomass (1.3-2 times), and tissue C:N (2 times) than the C3 dominated plots. Soils under Andropogon had significantly less inorganic N than under the C3 species (5.46  0.66 vs. 6.97  0.39 mg N/g soil). There were also some subtle differences in soil total C, and we are exploring this in greater detail using stable isotope analysis to determine if species differences are affecting total, labile and recalcitrant soil C pools. We are also investigating if these soil differences are related to species’ N uptake/utilization or plant-soil feedbacks mediated through the microbial community. These preliminary findings have implications for our ability to predict how species invasions, extinctions, or shifts in relative abundance may alter soil processes, as well as how C4 grass reintroduction may be used to restore these processes.

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