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- All Subjects: Photodegradation
- Creators: Hall, Sharon
Description
The nutrient dynamics of degradation have been studied almost exclusively in mesic (not arid or semi-arid) ecosystems. In arid ecosystems, we do know that photodegradation can cause significant mass loss and that lignin plays a dual role in the processes of degradation: it slows biodegradation due to its rigid chemical structure but can speed up photodegradation via the carbon mineralization process. This experiment attempts to assess the nutrient dynamics of nitrogen (N) and phosphorus (P) that occur while overall mass is being lost via photodegradation. We took Ambrosia deltoidea litter from 5 sites within the Phoenix city core and 5 sites downwind of the city of Phoenix. Half of this litter was N and P enriched from a previous experiment and half was control. We split the litter into UV opaque and UV transparent litter bags that had holes punched in them to allow microbial interaction. These bags were picked up at sampling periods of 10, 20, 30, and 40 weeks. All samples were then tested for mass loss, lignin content, carbon (C) content, N content, and P content. We found that downwind samples lost more mass than the core. There was little effect over time on N content and little disparity in P trends between the samples. P behaved as expected with an initial rise due to microbial interaction and then a decline as the microbes released P. Lignin concentration rose in a similar fashion at both core and downwind sites confirming that lignin remains in litter through the process of photodegradation. One interesting result was an logarithmic-like decrease in C:N ratio and C:P ratio for the downwind samples but a fairly constant ratio in the core samples. It is clear that these decreasing ratios result not from increased N or P, but instead from rapidly decreasing C. Overall, we conclude that neither N nor P is affected significantly by photodegradation at either site. N deposition appears to slow mass loss, but speed up N release, at least in the early stages of decomposition.
ContributorsChristman, Maximilian Peter (Author) / Ball, Becky A. (Thesis director) / Hall, Sharon (Committee member) / Barrett, The Honors College (Contributor) / School of Sustainability (Contributor) / School of Mathematical and Statistical Sciences (Contributor)
Created2013-12
Description
Decay of plant litter represents an enormous pathway for carbon (C) into the atmosphere but our understanding of the mechanisms driving this process is particularly limited in drylands. While microbes are a dominant driver of litter decay in most ecosystems, their significance in drylands is not well understood and abiotic drivers such as photodegradation are commonly perceived to be more important. I assessed the significance of microbes to the decay of plant litter in the Sonoran Desert. I found that the variation in decay among 16 leaf litter types was correlated with microbial respiration rates (i.e. CO2 emission) from litter, and rates were strongly correlated with water-vapor sorption rates of litter. Water-vapor sorption during high-humidity periods activates microbes and subsequent respiration appears to be a significant decay mechanism. I also found that exposure to sunlight accelerated litter decay (i.e. photodegradation) and enhanced subsequent respiration rates of litter. The abundance of bacteria (but not fungi) on the surface of litter exposed to sunlight was strongly correlated with respiration rates, as well as litter decay, implying that exposure to sunlight facilitated activity of surface bacteria which were responsible for faster decay. I also assessed the response of respiration to temperature and moisture content (MC) of litter, as well as the relationship between relative humidity and MC. There was a peak in respiration rates between 35-40oC, and, unexpectedly, rates increased from 55 to 70oC with the highest peak at 70oC, suggesting the presence of thermophilic microbes or heat-tolerant enzymes. Respiration rates increased exponentially with MC, and MC was strongly correlated with relative humidity. I used these relationships, along with litter microclimate and C loss data to estimate the contribution of this pathway to litter C loss over 34 months. Respiration was responsible for 24% of the total C lost from litter – this represents a substantial pathway for C loss, over twice as large as the combination of thermal and photochemical abiotic emission. My findings elucidate two mechanisms that explain why microbial drivers were more significant than commonly assumed: activation of microbes via water-vapor sorption and high respiration rates at high temperatures.
ContributorsTomes, Alexander (Author) / Day, Thomas (Thesis advisor) / Garcia-Pichel, Ferran (Committee member) / Ball, Becky (Committee member) / Hall, Sharon (Committee member) / Roberson, Robert (Committee member) / Arizona State University (Publisher)
Created2020