Environmental controls on the abundance, diversity, growth, and activity of ammonia-oxidizing microorganisms in temperate forest soils
The goal of my dissertation research was to investigate the structure and function of ammonia-oxidizing microbial communities in temperate forest soils. Accomplishing this goal required a hybrid approach: I used modern molecular biology techniques alongside soil biogeochemical measurements and framed my research using ecological theory largely developed in plant systems. All of my field work was done at Coweeta Hydrologic Laboratory, a Forest Service Station and Long Term Ecological Research Site near Otto, NC. Watershed-level land use manipulations have been performed at Coweeta since the 1930s, including clear-cutting, fertilizing, liming, burning, grazing by cattle, and replanting entire watersheds in white pine. While these treatments were originally imposed to assess the effects of land use on water yield, they have resulted in changes in soil characteristics as well. Working at Coweeta has therefore allowed me to sample ammonia-oxidizer communities across a gradient of soil variables, such as pH and nitrogen (N) availability, within the geographically-constrained area of the Coweeta Basin.
First, I used amplicon-based pyrosequencing to independently assess the diversity of ammonia-oxidizing archaea (AOA) and bacteria (AOB) at several sites within Coweeta. I found that AOA and AOB diversity were a function of both resource availability (i.e. N availability) and environmental harshness (i.e. soil pH) in line with general ecological theory developed for plant systems by Tilman and Grime, respectively. Next, I tested whether AOA and AOB were substrate or nutrient limited in this system by adding either N or a nutrient solution containing both potassium and phosphorus to soil incubations and assessing the growth response of AOA and AOB using quantitative polymerase chain reaction (qPCR). I found strong evidence for substrate limitation by AOB and a marginally-significant positive effect of nutrient addition on growth of AOA. Another intriguing finding from this study was that both AOA and AOB grew during unamended soil incubations. Unamended (buried-bag) incubations have been used to estimate in situ rates of nitrification for over 50 years. By measuring the growth of AOA and AOB alongside nitrification during buried-bag incubations, I discovered that AOA are the dominant ammonia-oxidizers in temperate forest soils. However, I found that AOA are much less efficient at using the energy from ammonia oxidation to create biomass than AOB in the forest soils I sampled.
Overall, I found that temperate forest soils contain low abundances of AOA and AOB, with relatively low diversity in both groups. This is especially true for the diversity of AOA, where a single taxon dominated the community at every site. Soil pH and N availability seem to be major selective forces for forest soil ammonia oxidizers, though other nutrients such as potassium and phosphorus may regulate the activity of AOA as well. AOA are most-likely the dominant ammonia oxidizers in temperate forest systems, though this may change with increased disturbance. In a broader sense, I found that ecological theory developed for plant communities was applicable to chemoautotrophic microbes despite the large differences in life history between these groups of organisms.