Climate change impacts on forest biogeochemistry: linking microbial structure and function to ecosystem-level carbon and nitrogen fluxes
The Michigan Nitrogen Deposition Gradient Study
For my Ph.D. work, I'm working in the Michigan Nitrogen Deposition Gradient Study, a long-term ecosystem study that was established in 1987 to examine the effects of climate and atmospheric nitrogen (N) deposition on ecosystem processes. Spanning a 500-km distance, the study area in Michigan, USA, consists of four edaphically and floristically matched sugar maple (Acer saccharum) dominated northern hardwood forest stands on sandy spodosol soil (Typic Haplorthods of the Kalkaska series). The stands span a natural climatic gradient that encompasses the full latitudinal range of northern hardwood forests in eastern North America and the Upper Great Lakes Region, allowing us to generalize our findings across this widespread and important ecosystem in North America.
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Image taken at one of the plots at Site C in Mesick, MI (2022). ​
At each site (n=4), there are six 30-m by 30-m plots, three of which received experimental N additions in the form of 30 kg NO3—N ha-1 yr-1 in 6 equal applications during the growing season beginning in 1994 and three of which receive ambient atmospheric N deposition. The experimental N addition treatment was applied annually over a 24-year period and was terminated following the final application in the summer of 2017.​
For additional information on the Michigan Nitrogen Deposition Gradient Study, visit our website.
Chapter 1: Assessing how nitrogen deposition affects the trajectory of organic matter decomposition and alters relationships with saprotrophic microbes.
Soil organic matter (SOM) is the largest terrestrial organic carbon pool, and potential climate-mediated feedbacks involving SOM decomposition could exacerbate or mitigate climate change. SOM consists of different chemical constituents that exist across a spectrum of chemical complexity from simple to complex substrates, and the relative abundance of these potential substrates for microbial metabolism can have consequences for the fate of soil C stored as mineral SOM under climate change. However, current understanding of the mechanistic controls on SOM chemistry and mineralization, and how these may be affected by climate change, is still incomplete.
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​By using high resolution SOM biochemical (Pyrolysis Gas Chromatography Mass-Spectrometry (py-GCMS)) and soil microbiome datasets, the primary objectives of this study are to assess:
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The trajectory of organic matter decomposition and stabilization from undecayed plant root litter to mineral SOM
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How these processes are altered by anthropogenic N deposition
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Conceptual diagram depicting the hypothesis that organic compounds that constitute the biochemical profile at each decomposition stage will differ, exhibiting compositional overlap to varying degrees as decomposition progresses and amounts in SOM stabilization.
Chapter 2: What is going to happen to the terrestrial carbon sink that was supported by high rates of anthropogenic nitrogen deposition when nitrogen deposition is reduced?
Anthropogenic nitrogen (N) deposition in northern Hemisphere temperate forests has increased forest soil carbon (C) storage, thereby slowing atmospheric CO2 accumulation and subsequent climate warming. At the Michigan Nitrogen Deposition Gradient Study, experimental N deposition, i.e., the N deposition treatment, substantially increased soil organic matter (SOM) accumulation in the organic horizon (i.e., forest floor) (+51%) and the mineral soil (+18%; Zak et al., 2008). This increase in soil C is attributable to mechanistic changes in microbial function, namely the suppression of microbial activity associated with the breakdown SOM. As a result, these forests exhibit reduced rates of leaf litter and SOM decay and therefore result in the accumulation of C in soil, a finding that has been observed across several long-term field experiments. However, since the implementation of emission abatement policies in the 1970’s, atmospheric N deposition has declined globally, and the consequences of this decline are unknown. In this study, we assess the initial recovery (5-years post-termination of high N conditions) of the terrestrial C sink in both the organic and mineral soil horizons.
We show that the increased soil C pool that accumulated under experimental N deposition was not observable 5-years post-termination of the N deposition treatment, and this pattern is, in part, the result of mechanistic changes in microbial activity. This is the first report of forest recovery from long-term historically high N deposition, summarizing >20 years of soil C data both during and post-termination of high N conditions. These findings will be critical in informing Earth System Models of how to parametrize projected forest C stocks in response to reduced anthropogenic N deposition.
Collecting organic horizon samples at Site B in Pellston, MI (2022).
For more information, check out our publication:
Propson, B.E., Zak, D.R., Classen, A., Burton, A.J., & Freedman, Z.B. (2024). Gains in soil carbon storage under
anthropogenic nitrogen deposition are rapidly lost following its cessation. Ecology. https://doi.org/10.1002/ecy.4444
Chapter 3: Measure the expression of microbial genes associated with soil carbon storage to assess underlying genomic mechanism for organic horizon carbon loss during recovery from historically high nitrogen deposition.
The third chapter of my Ph.D. expands upon the findings from my second chapter. Briefly, we determined that the previously documented increases in organic horizon carbon that were observed under the N deposition treatment are no longer present 5-years following the termination of the experimental N inputs and often exhibit significant deficits in carbon content compared to the ambient N deposition treatment. In addition, we observed increases in microbial activities (such as peroxidase extracellular enzyme activity) that may offer a potential explanation for the loss of soil carbon we observed.
The objective of this project is to assess if the reductions in soil C storage and increases in extracellular enzyme activity potential observed 5-years after the termination of high N deposition conditions are associated with increases in the expression of genes encoding enzymes that govern the fate of this C pool. Though still reputable, it is widely acknowledged that measurements of soil enzyme activity potential face criticism for their suitability to be interpreted as accurate metrics of microbial activity and biogeochemical functionality, largely given differences in methodology. These differences create difficulties in attempting to compare results across studies, making it hard to feasibly improve our understanding of the soil C consequences of shifting N availability at larger spatial and temporal scales. Going beyond measurements of enzyme activity potential towards a genomic mechanism mediating this ecosystem-level response to changing N availability will advance our understanding on the genomic controls over OM degradation in a complex, natural soil system.
Collecting organic horizon samples at Site C in Mesick, MI and immediately putting them in liquid N to preserve DNA and RNA to be extracted in the lab (2023).