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The effects of drought on plant and soil microbial communities and functioning during tallgrass prairie restoration
Eckhoff, Kathryn
Eckhoff, Kathryn
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Abstract
Once widespread, over 90% of the historic North American tallgrass prairie has been lost, primarily due to its conversion to row-crop agriculture. The conversion of prairie to monoculture crops reduces diversity, displaces many native prairie species, and diminishes ecosystem functions. In order to restore this diversity and functioning, agricultural practices (i.e., tilling) are ceased and native seed is added to the site. Despite similarity in the approach to restore prairie, restoration outcomes vary immensely. Climate has been suggested underlie variable restoration outcomes, particularly climate during the establishment of a restoration when seeds are emerging. Yet, there are few experiments that examine at the long-term effects of initial year conditions, or that utilize rainfall manipulations to elucidate the effects of drought on restoration trajectories. In this dissertation, I assessed how initial year climate alters the restoration outcomes of plant and soil microbial communities. All of my research was conducted within a long-term experiment, the Sequentially Restored Prairie (SRP), at Konza Prairie Biological Station in Manhattan, KS. This experiment consists of prairie restored every other year in an agricultural field using identical restoration methods beginning in 2010. The first objective of my dissertation research was to evaluate the short-term and long-term effects of establishment year on the plant communities of tallgrass prairie restoration (Chapter 2). I utilized the first four restorations in SRP (SEQ I-IV), established from 2010 to 2016 under a wide range of climate to test whether climate-driven variation in communities among establishment years persisted on short-term (five years) and long-term (decadal) time scales. Over the short-term, compositional trajectories of four assembled communities showed significant differences in initial community composition. Communities changed dynamically over time in a similar trajectory due to a flush of annual volunteers followed by the establishment of sown perennial species, but all communities remained distinct from each other in year five. Precipitation in June and July of the establishment year explained coarse measurements of plant composition (i.e., species richness and grass/forb cover), with wet establishment years resulting in ‘grass years’ and dry establishment years resulting in ’forb years.’ Additionally, initial differences in community composition, species richness, and grass/forb cover in the first two communities established under average precipitation and drought conditions persisted for 9 and 11 years, with low interannual variability in composition over the long term, indicating persistently different states on a decadal time scale. Thus, year effects resulting from stochastic variation in climate have lasting effects on community assembly outcomes. While establishment year may have a large effect on the outcome of a restoration, there are very few studies that directly test the effects of drought on developing prairie. Therefore, the second objective of my dissertation research was to elucidate the effect of drought on plant community development and productivity in restored tallgrass prairie by installing rainfall reduction (66% reduction May-October) paired with ambient rainfall control plots for the first three years of restoration (Chapter 3). I measured plant community composition and species richness as well as aboveground net primary productivity (ANPP) over the first three years of restoration (SEQ V; 2018-2020). Illustrating the necessity of studying drought, a natural drought occurred during the growing season in 2018, the first year of the restoration and reduced precipitation experiment. In order to examine the generalizable effects of drought on developing prairie, I also included plant composition and ANPP from SEQ I (established in 2010 under average precipitation) and SEQ II (established in 2012 under drought). In the rainfall manipulation experiment, age of restoration affected plant composition, richness, and ANPP, but there was no effect of reduced precipitation. I attributed no effect of drought treatment to the severe natural drought in the establishment year that caused a drought signature in both the reduced and ambient precipitation treatment. Comparison of similar-aged communities restored in different years showed all communities changed with age and were separated by establishment year corresponding with variation in precipitation. While species richness in prairie established in drought was initially low, richness increased to the level of prairie established in average conditions. Comparing prairies restored under contrasting climate revealed that drought in the establishment year can have large effects on short-term restoration outcomes, whereas experimentally elucidating direct effects of drought is complicated by drought years, made more frequent due to climate change, which can obscure the effect of imposed drought treatments. My third objective aimed to reveal the short-term response of soil microbial community composition and enzyme activity to prairie restoration, and the extent to which multi-year manipulated drought from the onset of restoration affects recovery (Chapter 4). Soil microbial communities and enzyme activity were quantified for the first three years of restoration under ambient and reduced precipitation (66% reduction May-October). Due to the occurrence of a natural growing season drought at the onset of the restoration, coinciding with the rainfall reduction experiment, the objectives of this study were expanded to model the short-term recovery rate of the active soil microbial biomass and community components using co-located prairies restored with the same methods. Potential extracellular enzyme activity (EA) increased with restoration age for all enzymes analyzed, implicating a return in soil properties degraded during agriculture, i.e., nutrient cycling, soil organic matter. While carbon-acquiring EA increased with restoration age, the activity of these enzymes also increased under reduced precipitation in the third year of restoration. I attributed this to an increase in carbon inputs from previous year litter in reduced precipitation, suggesting drought conditions could alter carbon cycling in restored prairie. Unlike EA, there was no effect of drought or restoration on phospholipid fatty acid (PLFA) biomarker profiles. Examining soil microbial biomass PLFA profiles across the chronosequence of identically restored prairie demonstrated that PLFA biomass of total, fungal, and arbuscular mycorrhizal fungi, as well as the fungi:bacteria ratio, increased over 9 years. Restorations established in drought years were not lower than those established in wet or average precipitation, indicating that drought does not cause a delay in the recovery of active microbial biomass. While drought may not affect soil microbial communities, reduced precipitation can alter carbon inputs and carbon cycling in restorations established in a multi-year drought, which could have long-term implications for soil carbon cycling and sequestration.
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2022-12-31
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University of Kansas
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This item contains archived web content.
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Keywords
Ecology, Conservation biology, Community assembly, Restoration Ecology, Tallgrass prairie, Year effects