UC Berkeley

Abstract

Climatic and Edaphic Controllers of Soil N Transformations and Microbial Community Composition in Coast Redwood Forests

by

Damon Charles Bradbury

Doctor of Philosophy in Environmental Science, Policy and Management

University of California, Berkeley

Professor Mary K. Firestone, Chair

Coast redwood forests comprise one of the most cherished ecosystems in California, if not the world. There is concern that changes in climate, particularly in summer fog frequency and duration, may negatively impact redwood forests; however, little is known about how climate change will impact soil nitrogen (N) transformations and soil microbial community composition in these forests. These topics deserve further study because soil microbes perform critical ecosystem functions, including decomposition and nutrient transformations, and N is an essential macronutrient that often limits growth in temperate forests. In this dissertation, I examine the influence of climatic and edaphic factors on both soil N transformations and microbial community composition in coast redwood forests, particularly with respect to potential impacts of future changes in climate.

In Chapter 1, I report the results of an initial study conducted to measure rates of gross N mineralization and nitrification in redwood forests, as well as the potential for the dissimilatory reduction of nitrate to ammonium under low redox conditions and at warmer temperatures. Soils were collected in late winter, during the wettest time of year. The two northern redwood forests studied have high gross rates of N mineralization and nitrification as well as ammonium and nitrate consumption and cycle inorganic N tightly during the cool, moist winter. Gross nitrification rates were close to 4 μg N⋅g soil-1⋅day-1 in both sites at field temperature; these relatively high rates indicate that nitrification is not inhibited in redwood forests. Exposing the soils to low redox conditions stimulated DNRA and 15N2O production in these forests, suggesting that both processes can be important in areas of low redox soil, especially at warmer temperatures.

Next, I conducted a reciprocal transplant experiment across the latitudinal gradient of coast redwoods to study the potential for climate change to alter rates of gross N mineralization and nitrification and soil microbial community composition in redwood forests. Soils were reciprocally transplanted among three sites and collected after one and three years. This transplant experiment focused on soil conditions at the end of the summer because this is a time of year when climate can differ dramatically across the north-south gradient of coast redwoods, especially in terms of fog, which is a defining characteristic of coast redwood forests during the otherwise dry summer. Results from analyses of the impact of climate change on soil N transformations and soil microbial community composition are presented in Chapters 2 and 3, respectively.

After one year, rates of gross N mineralization and nitrification, the abundances of ammonia oxidizing bacteria (AOB) and archaea (AOA), and several edaphic characteristics, including soil water availability, were measured (Chapter 2). While rates of gross N mineralization varied among soils of different origins, they did not differ in response to transplanting. In contrast, rates of gross nitrification changed significantly in response to transplantation into a new climate. Gross nitrification was sensitive to water availability, and rates of gross nitrification were very low below a soil water potential of -0.05 MPa. Above this soil water potential, rates varied widely. Rates of gross nitrification were correlated with the abundances of AOA and AOB, which were correlated with water availability. The results suggest that the abundances of AOA and AOB and rates of gross nitrification (and nitrate availability) are likely to be more influenced by changes in summer climate (fog frequency) in coast redwood forests than rates of gross N mineralization (and ammonium availability).

For Chapter 3, the impacts of transplant-induced changes in climate on soil microbial communities were examined. Soil bacterial and fungal communities were examined after one year by terminal restriction fragment length polymorphism (T-RFLP), and bacterial communities were also examined with a high-density 16S rDNA oligonucleotide microarray (G2 PhyloChip) after one and three years. Both fungal and bacterial communities changed in composition after one year, and the patterns in compositional change persisted for bacteria after three years. Soil characteristics interacted with climate to frame the magnitude and character of these changes in community composition; the variability in community composition was correlated with edaphic as well as climatic variables for both bacterial and fungal T-RFLP. Of the over 2,000 bacterial taxa detected on the G2 PhyloChip, 3.2% differed in relative abundance between transplants after one year, while 12.2% differed after 3 years. The bacterial taxa responding to transplant-induced climate change showed strong phylogenetic clustering by net relatedness analyses after both 1 and 3 years. There appear to be taxonomic, and phylogenetic, patterns in the speed of the soil bacterial response to changing climate, and the patterns of community change indicate that the impact of climate change on microbial community composition should be assessed using reasonably long-term (multi-year) experiments.