PhD Student: Chase Kasmerchak
Within the experimental aspen stands at Arlington Agricultural Research Station, there are distinct differences in litter chemistry between genotypes, specifically with condensed tannin (CT) and phenolic glycosides. Differences in litter chemistry affect decomposition rates and ultimately, the relative proportions of carbon that is readily available for microbial decomposition and carbon that is bound and protected within soil aggregates. The plots at our research site include populations of high and low tree density plots. Low tree density plots are meant to resemble the modern silviculture management practice of intermediate thinning, while high density plots are meant to resemble unthinned forests. To better understand how differences in litter chemistry effect soil microbial activity, I will conduct a laboratory incubation using soil from the high and low tree density environments and tree litter from two genotypes – one with high CT and one with low CT contents. This experiment will allow us to measure microbial activity directly through rates of CO2 and N2O production. I will also look at the carbon content within aggregate fractions and readily accessible carbon from field soils, along with in-situ formation of aggregates in the lab using artificial soil, tree litter, and microbial inoculum from the field soils. By linking these data to rates of mineralization and nitrification, and the composition and function of soil microbial communities, we will better understand how forest management strategies effect nutrient cycling, element transformations, and longer-term carbon storage in forests with varying degrees of genetic diversity.
Tropical land-use change effects on soil microbial communities and soil carbon storage capacity
PhD Student: Emily J. Diaz Vallejo
Emily’s first PhD chapter was published in Biotropica. Abstract: Modifications to vegetation and soil due to changes in land use have the potential to alter the soil microbiome, with consequences for carbon and nutrient cycling. Despite the important function of soil microorganisms, little is known about their response to land-use change, especially in tropical regions where current rates of land conversion are greatest. The aim of this meta-analysis was to examine how land-use change influences soil microbial properties in tropical ecosystems and to identify current trends and knowledge gaps in the literature. We identified 83 published paired studies that reported data on microbial biomass, abundance, composition, and enzyme activity under representative land-use changes in the tropics. We calculated response ratios for studies that compared the following: reference forests to (a) agriculture, (b) pastures, (c) plantations, and (d) secondary forests. Soil microbial biomass decreased with forest conversion to agriculture and plantations. Microbial biomass response to land-use change depended on rainfall classes, although this was the only microbial variable which had sufficient data to test for a rainfall effect. Microbial abundance and enzyme activity showed variable results depending on the type of forest conversion. Microbial diversity and richness did not show any pattern with forest conversion or recovery to secondary forests. Published studies were not representative of the range of biophysical conditions observed in the tropics. Sites in moist regions in the American tropics were overrepresented. To better predict how land-use change affects the soil microbiome and its contribution to nutrient cycling, research should reflect observed environmental variation in the tropics.
Vulnerability of carbon in buried soils to climate change and landscape disturbance
This project will test the potential for deep buried soil organic matter to become a carbon source in response to changes in climate or land use that affect the connectivity of buried soils to the atmosphere. The research aims to understand (1) how soil burial contributes to the persistence of carbon in the form of soil organic matter and (2) whether exposure to surface conditions can trigger the decomposition of ancient carbon. The proposed study site is located in the U.S. Great Plains, where climate-driven loess deposition during the late Pleistocene and Holocene resulted in sequences of buried soils in thick loess deposits. The vulnerability of ancient organic matter to changing environmental conditions will be measured in two ways. First, changes in organic matter age, composition and bioavailability will be quantified along eroding and depositional field toposequences, where the paleosol exists at varying degrees of isolation from the modern landscape surface. Second, laboratory manipulations will measure the effects of carbon substrates, nitrogen availability, and microbial composition on ancient organic matter decomposition and mobilization in gaseous and dissolved forms. This study combines a geomorphic approach drawing from paleoclimatic reconstructions with advanced geochemical, spectroscopic and metagenomic techniques to generate new knowledge on environmental controls on carbon biogeochemistry. Funded by NSF Geobiology and Low Temperature Geochemistry. Collaborators: Dr. Joe Mason, Dr. Asmeret Asefaw Berhe and Dr. Marie-Anne de Graaff.
Nitrogen cycling during tropical forest succession
Former Student: Sanober Mirza, Holstrom Environmental Scholar
This has been a collaborative project among multiple graduate and undergradaute students in the lab, including Emily Diaz Vallejo, Elliot Vaughan, Sanober Mirza and Olivia Lopez. The objective of this project has been to quantify the variation in different nitrogen (N) cycling processes with forest succession. Nitrogen plays two important roles in Earth’s climate. As a plant nutrient, the availability of N affects plant growth and the uptake of carbon (C) from the atmosphere into plant biomass. The accumulation of C in long-lived biomass and in soils contributes to reducing the amount of CO2 in the atmosphere. Secondly, excess N can lead to the production of N2O, which is a more potent greenhouse than CO2. Land-use change, specifically deforestation and reforestation, can affect N availability for plant growth and N2O production. Long-term agricultural use can deplete nitrogen sources, even in tropical soils where N is not expected to limit productivity. Through the evaluation of nitrogen-fixing tree species, N in litterfall, soil N, and N mineralization, we will gain a comprehensive picture of tropical N-cycle variation. Soils have been sampled from a well-replicated chronosequence in Puerto Rico with pastures, 29-year-old forests, 69-year-old forests, and primary forest. We sampled soils before and after Hurricane Maria, which made landfall very close to our field sites, to understand disturbance effects on soil N cycling processes and their recovery. The results from this study can be applied to forest management and land-use decisions, with implications for the global climate.
ADVANCE Partnership: From the Classroom to the Field: Improving the Workplace in the Geosciences
The geosciences is one of the least diverse disciplines in STEM. One reason for this is the hostile climate experienced by many women and people of color in the geosciences due to sexual and racial harassment. This collaborative project with the Earth Science Women’s Network, the Association for Women Geoscientists and the American Geophysical Union aims to improve work climate conditions and increase gender equity in the geosciences by developing bystander intervention workshops for department heads, chairs, and faculty to appropriately respond to harassment, bullying and discrimination. Goals include harassment awareness and prevention training in the teaching of ethical conduct in research guidelines; developing and incorporating geoscience-relevant scenarios in training and teaching materials, including field research and educational settings; incorporating intersectionality to recognize disproportionate impact of those who may face sexual, racial and gender harassment, and collaborating with professional society partners for national dissemination, implementation, and sustainability. Funded by NSF ADVANCE Partnership program. Team members include: Billy Williams, American Geophysical Union; Blair Schneider, Association for Women Geoscientists; Allison Mattheis, California State University, Los Angeles; and leaders from the Earth Science Women’s Network: Rebecca Barnes, Colorado College; Asmeret Asefaw Berhe, University of California, Merced; and Meredith Hastings, Brown University. Learn more about this project here.
Humanities Education for Anti-Racism Literacy in the Sciences and Medicine (HEAL)
The project seeks to provide models and examples of transformative higher education by drawing on humanities research to advance anti-racist practices and pedagogies in science, technology, engineering, mathematics, and medicine (STEMM). Beginning with an acknowledgement of pervasive systemic racism, our aim is to center the educational experiences of Black, Native, Indigenous and other students of color to build more accurate narratives about histories of racism in the sciences and medicine, allowing us to better understand persistent underrepresentation and to develop educational tools for building a more equitable university and society. Team members include: Cheryl Bauer-Armstrong (Native Education); Christy Clark-Pujara (Higher Education); Elizabeth Hennessy (Coordinator and Higher Education); R. Justin Hougham (Environmental Education & Equity); Erika Marín-Spiotta (STEM Higher Education); Maxine McKinney de Royston (Black Madison Voices); Todd Michelson-Ambelang (Libraries); Troy Reeves (Oral History Program); Robin Rider (Libraries and Archives); Monica M. White (Community Engagement); and Cleo Woelfle-Erskine (Native Education). Funded by The Andrew W. Mellon Foundation’s Just Futures Initiative.
Marin-Spiotta BiogeoLab 