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Soil Microbial Responses to Different Precipitation Regimes Across a Southwestern United States Elevation Gradient

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Soil organic carbon (SOC) is a critical component of the global carbon (C) cycle, accounting for more C than the biotic and atmospheric pools combined. Microbes play an important role in soil C cycling, with abiotic conditions such as soil

Soil organic carbon (SOC) is a critical component of the global carbon (C) cycle, accounting for more C than the biotic and atmospheric pools combined. Microbes play an important role in soil C cycling, with abiotic conditions such as soil moisture and temperature governing microbial activity and subsequent soil C processes. Predictions for future climate include warmer temperatures and altered precipitation regimes, suggesting impacts on future soil C cycling. However, it is uncertain how soil microbial communities and subsequent soil organic carbon pools will respond to these changes, particularly in dryland ecosystems. A knowledge gap exists in soil microbial community responses to short- versus long-term precipitation alteration in dryland systems. Assessing soil C cycle processes and microbial community responses under current and altered precipitation patterns will aid in understanding how C pools and cycling might be altered by climate change. This study investigates how soil microbial communities are influenced by established climate regimes and extreme changes in short-term precipitation patterns across a 1000 m elevation gradient in northern Arizona, where precipitation increases with elevation. Precipitation was manipulated (50% addition and 50% exclusion of ambient rainfall) for two summer rainy seasons at five sites across the elevation gradient. In situ and ex situ soil CO2 flux, microbial biomass C, extracellular enzyme activity, and SOC were measured in precipitation treatments in all sites. Soil CO2 flux, microbial biomass C, extracellular enzyme activity, and SOC were highest at the three highest elevation sites compared to the two lowest elevation sites. Within sites, precipitation treatments did not change microbial biomass C, extracellular enzyme activity, and SOC. Soil CO2 flux was greater under precipitation addition treatments than exclusion treatments at both the highest elevation site and second lowest elevation site. Ex situ respiration differed among the precipitation treatments only at the lowest elevation site, where respiration was enhanced in the precipitation addition plots. These results suggest soil C cycling will respond to long-term changes in precipitation, but pools and fluxes of carbon will likely show site-specific sensitivities to short-term precipitation patterns that are also expected with climate change.

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2019

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Sources and Decomposition of Dissolved Organic Matter in Desert Streams

Description

Dissolved organic matter (DOM) is an important part of aquatic foodwebs because it contains carbon, nitrogen, and other elements required by heterotrophic organisms. It has many sources that determine its molecular composition, nutrient content, and biological lability and in turn,

Dissolved organic matter (DOM) is an important part of aquatic foodwebs because it contains carbon, nitrogen, and other elements required by heterotrophic organisms. It has many sources that determine its molecular composition, nutrient content, and biological lability and in turn, influence whether it is retained and processed in the stream reach or exported downstream. I examined the composition of DOM from vascular wetland plants, filamentous algae, and riparian tree leaf litter in Sonoran Desert streams and its decomposition by stream microbes. I used a combination of field observations, in-situ experiments, and a manipulative laboratory incubation to test (1) how dominant primary producers influence DOM chemical composition and ecosystem metabolism at the reach scale and (2) how DOM composition and nitrogen (N) content control microbial decomposition and stream uptake of DOM. I found that differences in streamwater DOM composition between two distinct reaches of Sycamore Creek did not affect in-situ stream respiration and gross primary production rates. Stream sediment microbial respiration rates did not differ significantly when incubated in the laboratory with DOM from wetland plants, algae, and leaf litter, thus all sources were similarly labile. However, whole-stream uptake of DOM increased from leaf to algal to wetland plant leachate. Desert streams have the potential to process DOM from leaf, wetland, and algal sources, though algal and wetland DOM, due to their more labile composition, can be more readily retained and mineralized.

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Date Created
2018

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Drivers of Spatiotemporal Variability of Ecosystem Metabolism in Aridland Streams

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Stream metabolism is a critical indicator of ecosystem health and connects stream ecology to global change. Hence, understanding the controls of metabolism is essential because streams integrate land use and could be net sources or sinks of carbon dioxide (and

Stream metabolism is a critical indicator of ecosystem health and connects stream ecology to global change. Hence, understanding the controls of metabolism is essential because streams integrate land use and could be net sources or sinks of carbon dioxide (and methane) to the atmosphere. Eleven aridland streams in the southwestern US (Arizona) across a hydroclimatic and size (watershed area) gradient were surveyed, and gross primary production (GPP) and ecosystem respiration (ER) were modeled and averaged seasonally over a period of 2-4 years. The seasonal averaged GPP went as low as 0.001 g O2m-2d-1 (Ramsey Creek in 1st quarter of 2017) and as high as 14.6 g O2m-2d-1 (Santa Cruz River in 2nd quarter of 2017), whereas that of ER ranged from 0.003 (Ramsey Creek in 1st quarter of 2017) to 20.3 g O2m-2d-1 (Santa Cruz River in 2nd quarter of in 2017). The coefficient of variation (CV) of these GPP estimates within site ranged from 42% (Upper Verde River) to 157% (Wet Beaver Creek), with an average CV of GPP 91%, whereas the CV of ER ranged from 32% (Upper Verde River) to 247% (Ramsey Creek), with an average CV of ER 85%. Among 4 main categories of hypothetical predictors (hydrology, nutrient concentration, local environment, and size) on CV and point measurement of stream metabolism, the following conclusion was made: hydrologic variation only predicted the ER and CV of ER but not the GPP or CV of GPP; light and its CV controlled GPP and its CV, respectively, whereas temperature was one of the controlling factors for ER; CV of nutrient concentration was one of the drivers of CV of GPP, nitrate concentration was correlated with point measurement of GPP and ER while soluble reactive phosphorus (SRP) concentration was only relevant to GPP; watershed area was correlated with CV of GPP, while depth mattered to both GPP and ER. My work will enhance our understanding of streams at multiple temporal and spatial scales and ultimately will benefit river management practice.

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2022