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Topographical Controls on Belowground Carbon

Forest on toe slope in Shale Hills CZO. How does topographical position affect carbon fluxes belowground?

Forest on toe slope in Shale Hills CZO. How does topographical position affect carbon fluxes belowground?

July 1, 2014 – June 30, 2017. U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, Terrestrial Ecosystem Sciences Program

David Eissenstat, Jason Kaye, Yuning Shi, Ken Davis, Chris Duffy, Henry Lin, Kusum Naithani and Luke McCormack

Project Summary

Belowground processes are strongly affected by soil moisture. Current studies are generally limited to sites with relatively uniform topography, and seldom connect the impacts of natural variation in soil moisture associated with topography to multiple C-cycle processes. Current Earth system models cannot resolve topographically driven hill-slope scale soil moisture patterns, and cannot simulate the nonlinear effect of soil moisture on soil respiration, especially under high soil moisture conditions.  In this project we will assess the influence of topography on multiple belowground processes (soil CO2 flux, soil C, root density, root production, and root turnover) and develop a coupled modeling system capable of simulating the water and carbon dynamics of this complex system.

The study site is the Susquehanna/Shale Hills critical zone observatory (SSHCZO), which has been intensively studied from a hydrologic, geochemical and geophysical perspective. We will measure the vertical root distribution, soil respiration and root turnover in relation to topography at the SSHCZO to address the hypotheses. We will sample 50 macro-sites across the watershed with 4 micro-sites nested within each macro-site (total 200 points). We will also add a spatially-distributed land surface hydrologic model, Flux-PIHM (Flux Penn State Integrated Hydrologic Model), which accounts for horizontal groundwater flow, to the Biome-BGC, which is the current carbon and nitrogen biogeochemistry model in the latest version of the Community Land Model (CLM4), to improve the representation of the land surface and subsurface heterogeneities caused by topography. Numerical experiments using the coupled model (referred to as Flux-PIHM-BBGC) will be performed to examine if the coupled model with lateral flows performs better than a one dimension biogeochemistry model. The proposed high-resolution measurements of soil respiration, soil C, root density distribution, and root turnover at the SSHCZO will provide important spatially distributed a priori parameter values and boundary conditions for modeling, and provide an unprecedented chance to comprehensively evaluate the coupled model fidelity (Flux-PIHM-BBGC), improve our modeling skills at high resolution and low-order watersheds, and investigate the impacts of landscape variation on belowground C processes.

This study will determine the effects of topographic and hydrologic variation on root and soil respiration as well as their subsequent contributions to ecosystem net primary production (NPP).  The study will also link aboveground drivers, such as tree species composition, litter fall and aboveground tree growth to belowground processes such as soil respiration, root standing crop, root production and root lifespan.  It is expected that the key drivers of variation in belowground processes will be identified by this study, which will enable more efficient characterizations of C processes in sites that lack the wealth of data available in the SSHCZO. One of the primary products of the study will be the coupled Flux-PIHM-BBGC model, a high-resolution coupled biogeochemical land surface and hydrologic model. These crucial data and model will be publically available for future work investigating both short- and long-term processes of the Earth system.