Plant Available Nitrogen
Across diverse ecosystems worldwide, rhizospheres (composed of plant roots and their associated microbiome) are known to extend beyond soil horizons into weathered bedrock. These deep rhizospheres drive water and carbon cycling meters below the base of soil, supporting forest function. Little is known, however, about how these deep rhizospheres drive nutrient cycling at depth. In her dissertation research, Kelsey combines stable isotope and other biogeochemical techniques to explore how nitrogen (N), the most limiting nutrient to plant growth in terrestrial ecosystems, is cycled throughout deep rooting profiles in the critical zone, asking: (1) how do the chemical forms and concentrations of biologically available N change year round; (2) what is the source and fate of this N; and (3) to what extent do dominant tree species utilize N stored within weathered bedrock?
Kelsey and her colleagues have sampled for plant available nitrogen throughout the entire profile of the unsaturated zone at Rivendell using the VMS for more than two years, including sampling of : dissolved organic N (DON), ammonium (NH4+) and nitrate (NO3-). She has also conducted multi-year monitoring of NH4+ and NO3- standing stock, net mineralization in soil and bulk carbon and nitrogen stable isotopes in soil to generate a baseline of N and C content at our site. Together, these data form the basis for assessing biogeochemical N dynamics throughout the entire unsaturated (i.e. vadose) zone at Rivendell. Kelsey also routinely samples the dominant canopy species for foliar N and C content and stable isotope analysis.
Analyses indicate that the majority of N found in the rock moisture zone is DON, and it is found at ecologically significant concentrations. Total N increases with depth and is dynamic across seasons. The covariation of TN, NH4+, NO3-, and C dynamics also suggest N cycling by plants and/or microbes in the weathered bedrock rhizosphere at Rivendell. Additional analyses are underway to better understand the fate of N in the rock moisture zone, and additional gas sampling and N isotope analysis of dissolved N compounds sampled from the VMS will be performed to take a closer look at nitrogen cycling.
Additionally, to investigate plant uptake of N from rock moisture held in weathered bedrock, in summer 2020 Kelsey conducted an experiment testing N uptake capacity of Douglas fir fine roots growing in soil vs weathered rock. Fine roots from Douglas fir in the soil and saprolite were collected and incubated in 15N-labelled NH4+, NO3- and organic N solutions. Differences in d15N values between the control and 15N treatment roots will be used to calculate N uptake per gram of root. Results from this experiment will inform future research into differential gene expression in roots growing in soil versus weathered bedrock.
Systems biology of the soil microbiome (“Microbes Persist”)
The “Microbes Persist” Soil Microbiome project is a DOE-funded Science Focus Area (SFA) led by Lawrence Livermore National Lab Senior Staff Scientist Jennifer Pett-Ridge.
Microorganisms play key roles in soil carbon turnover and stabilization of persistent organic matter via their metabolic activities, biochemistry, and extracellular products. Microbial residues are the primary ingredients in soil organic matter (SOM), a carbon pool that is critical to Earth’s soil health. The Soil SFA is investigating how microbial cellular-chemistry, functional potential, and ecophysiology fundamentally shape soil carbon persistence. Project members are characterizing this via stable isotope probing (SIP) of genome-resolved metagenomes from soils at three California grassland sites located in Hopland, CA (from Hopland Research and Extension Center) the Angelo Reserve, and Sedgwick Reserve. The SFA focuses on soil moisture as a ‘master controller’ of microbial activity and mortality, since altered precipitation regimes are predicted across the temperate U.S.The ultimate goal of the project is to determine how microbial soil ecophysiology, population dynamics, and microbe-mineral-organic matter interactions regulate the persistence of microbial residues under changing moisture regimes.
Direct and indirect responses of meadow species/systems to altered rainfall in California grassland
In a large field experiment that has been running continuously since 2001, Blake Suttle and his students and collaborators have studied direct and indirect effects of altered rainfall regimes in meadow food webs and ecosystems. Blake rigged a clever underground sprinkler system to alter the intensity or duration of the annual rainfall in large, replicated experimental plots. Every year, twelve 70-m2 plots of open grassland are subjected to rainfall changes predicted by the Hadley 2000 climate model for the California North Coast (doubled precipitation during the winter (January-March)). Twelve more plots receive rainfall as predicted under the leading Canadian climate model (double the amount of rain falling later in spring (April-June)). A third group of twelve control plots receive ambient rain. Changes in seasonal water availability had pronounced effects on individual grass, forb, and arthropod species, but as precipitation regimes were sustained across years, feedbacks and species interactions overrode individual physiological responses, and produced patterns opposite to what would have been predicted from the physiological ecology of perennial (native) versus annual (exotic) grasses. Conditions that sharply increased production and diversity through the first 2 years reversed, and caused
simplification of the food web and deep reductions in consumer abundance after 5 years. Changes in these natural grassland communities suggest a prominent role for species interactions in ecosystem response to climate change.
Long-term research on simulated climate change in meadow ecosystems
A 2005 Berkeley Ph.D., Meredith Thomsen has continued her research at the Angelo as a Professor in the Biology Department at the University of Wisconsin-La Crosse. Since 2006, she has made annual trips with students and UW-L colleague Tim Gerber to help sample the rainfall addition project started by Blake Suttle in 2001. Student participants are a mix of Biology majors and Biology Education majors. All students gain first-hand research experience, and, with Tim’s guidance, the Biology Education majors prepare lesson plans based on the research which align with national science standards. Research experience for future teachers broadens the societal impact of our work, as those students become teachers and share the experience in their K-12 classrooms.