SOIL MICROBIAL COMMUNITY CARBON AND NITROGEN DYNAMICS WITH ALTERED PRECIPITATION REGIMES AND SUBSTRATE AVAILABILITY
Tiemann, Lisa K.
University of Kansas
Ecology & Evolutionary Biology
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Understanding the nature and extent of the feedback between soil microorganisms and ecosystem processes is of great concern as we are faced with multiple elements of global environmental change. In this dissertation, I explore how anthropogenically induced environmental changes affect soil microorganisms' resource use, and how, in turn, changes in microbial resource use alters ecosystem processes. These explorations were conducted in grassland systems, which contain 12% of global soil carbon (C) stocks and can serve as large C sources or sinks depending on environmental conditions such as nitrogen (N) availability and precipitation regimes. Nitrogen availability in grasslands can control plant primary productivity as well as rates soil organic matter decomposition and the fate of soil organic C. In grassland systems undergoing N addition through fertilization, resource inputs to soils increase in both quality and quantity. In Chapter 1, I investigate the fate of enhanced biomass inputs due to N addition by determining the direct and indirect effects of N addition on the activity of the soil microbial decomposers. Through measurement of extracellular enzyme activities and isotopic analysis of the microbial biomass relative to substrate sources, I found enhanced mineralization of newly incorporated soil organic C with N addition. This increase in soil C break down was associated indirectly with N addition through increases in plant litter quality and not directly with increased soil N availability. These results suggest that increased biomass input resulting from N addition does not necessarily result in increased soil C accrual. Climate change in the Great Plains region will likely cause increases in drought severity and precipitation event size with little change in annual precipitation totals. Precipitation events, particularly those following periods of drought, can create large flushes of resources for microbial communities, but these same pulses also can cause high levels of physiological stress and disturbance. When faced with increased soil moisture stress and re-wetting disturbance, microorganisms must accumulate and release protective osmolytes. The acquisition and release of protective osmolytes, apparently of sufficient magnitude to influence ecosystem level N and C fluxes, makes understanding the mechanisms behind these fluxes critical for predicting not only microbial community responses to global change, but ecosystem responses as well. In Chapter 2, 3 and 4, I use soils from four locations across the Great Plains precipitation gradient in a combination of laboratory and in situ soil incubations to explore the effects of soil moisture stress on flows of C and N though the microbial biomass. In Chapter 2, I focus on links between soil moisture stress and resource use efficiency by manipulating the frequency and magnitude of soil wetting and drying cycles in laboratory soil incubations. As soil moisture stress was increased with longer drought intervals and larger water pulse events, I saw a decline in C use efficiency and a 360 - 4800% increase in net N mineralization in soils from four sites along the Great Plains precipitation gradient. In Chapter 3, I employed the use of stable isotopes at the end of a similar incubation, to trace the C and N during a soil wetting-drying cycle. In this study I found that increased levels of soil moisture stress shifted microbial preference from N-rich protective osmolytes to N-free osmolytes. I also found that soils from the mesic end of the precipitation gradient were more sensitive to changes in soil moisture stress than soils from the semi-arid end of the gradient and that nitrification appeared to be less sensitive than denitrification, leading to increased soil nitrate concentrations and a decoupling in the N cycle. Finally, in Chapter 4, I reciprocally transferred soils between four study sites along the precipitation gradient and allowed them to incubate in situ for 1.5 and 2.5 y. After collection I assessed nitrification and denitrification potentials and the abundance of functional genes associated with these processes. I compared effects of both the initial community composition and the change in environment on the process rates. I found that as soil moisture stress increased across the precipitation gradient, nitrification potential decreased and nitrification functional gene abundance increased. Depending on soil origin, denitrifiers were either sensitive, resistant or functionally redundant after 1.5 y of altered precipitation regimes. In contrast, after 2.5 y denitrifiers in soils of all origins exhibited declines in process rates and functional gene abundance with increased soil moisture stress. Overall, I found that microbial communities are sensitive to environmental change, and as these communities shift in structure and function C and N cycling in these grasslands is altered. In particular, the perturbations explored in this dissertation, N addition and climate change, may induce increased rates of C release and N loss from these grassland soils.
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