Extracellular enzyme activity, greenhouse gas emissions and soil microbial communities responses to drought and defoliation in northern temperate grasslands
Creators: Rajper, Ahsan
Publisher: name: University of Alberta Library
Year: 2023

<p>Grassland ecosystems cover more than 40% of the global land area and provide many ecological goods and services, therefore, it is important to sustain these terrestrial ecosystems. One of the ecological services they provide is their potential to act as a carbon (C) sink. However, northern temperate grasslands, which are arid/semi-arid in nature, are specifically vulnerable to climate change (e.g. drought) and natural as well as anthropogenic disturbances such as overgrazing.  This study was conducted, at seven different locations across a climate gradient, to test the effects of drought (45% rainfall reduction using rainout shelters) and defoliation on different grassland soil components to understand how these disturbances affect C and nutrient cycling. Defoliation was applied by clipping experimental plots either once or twice a year at two variable heights (3 and 7 cm). A combination of five defoliation treatments was applied including control (no-defoliation). In the first study, I tested the effect of drought and defoliation on soil extracellular enzyme activity. Five enzymes were selected based on their role in C (β-glucosidase, β-cellobiosidase and β-xylosidase), N (N-acetyl-β-glucosaminidase) and P (Acid phosphatase) cycling. I found that the activity of all enzymes decreased with drought, except that β-xylosidase increased with drought conditions, suggesting a shift in biogeochemical processes of these soils under future drought. Furthermore, β-glucosidase activity was reduced under intermediate defoliation. In the second study, I tested the effects of drought and defoliation on greenhouse gas (CO2, N2O and CH4) emissions over two growing seasons (April to September in 2017 and 2018). I found that intermediate defoliation (i.e., defoliation once a year) reduced CO2 emissions as compared to heavy defoliation (i.e. defoliation twice a year). No treatment effects were observed on N2O and CH4 fluxes. Furthermore, I found that defoliation frequency, not timing, was the driving factor for CO2 emissions. Soil temperature and extracellular enzyme activity were the best predictors for greenhouse gas emission rates. Findings suggest that annual single-event defoliation could reduce CO2 emissions in future drought conditions in northern temperate grasslands. In the third study, I tested the effects of drought and defoliation on soil microbial communities in two contrasting grasslands (Kinsella vs Mattheis). I found that drought affected beta diversity at Kinsella (wetter site) where there was a significant effect of drought on beta diversity between as well as within groups (ambient vs drought). Soil bacterial communities were affected by drought; however, soil fungi showed resistant and/or even favored drought conditions. At Kinsella, xylanolytic bacteria (involved in breakdown of xylan) were increased under drought conditions. Overall, the findings of this thesis imply that drought effects were more consistent on enzymes, greenhouse gas emissions and soil microbial communities, while defoliation effects were limited and dependent upon drought treatment and local climate. Results from this thesis suggest that single annual defoliation (specifically in early season) have the potential to reduce CO2 emissions and increase C stocks in northern temperate grassland soils under future drought conditions.</p>