Project Funding: 2014 - 2017

NRA: 2013 NASA: Carbon Cycle Science   

Funded by DOE

Abstract:
We will assess the influence of topography on multiple belowground processes (soil CO2 efflux, soil C, microbial biomass, root density, root production, and root turnover) and develop a coupled model capable of simulating the water and carbon dynamics in topographically complex terrain. Belowground processes are affected by soil moisture, but current studies seldom connect the impacts of natural variation in soil moisture associated with topography to multiple C cycle processes. Likewise, current Earth system models cannot resolve topographically driven hill-slope soil moisture patterns, and cannot simulate the nonlinear effect of soil moisture on soil processes, especially at high soil moisture. We will fill these gaps by testing six hypotheses: (1) highest soil CO2 efflux will be in locations with the longest duration of optimal soil moisture. (2) Within sites of similar soil moisture there will still be substantial variation in soil CO2 efflux that is controlled by fine-scale spatial variation in root density, litter fall, and associated microbial biomass. (3) At landscape levels, root turnover can be predicted by knowledge of tree species composition alone without incorporating the effects of heterogeneous soil moisture. Alternatively, root turnover is strongly influenced by spatially variable soil moisture in addition to species composition. (4) At landscape levels, root production, root standing crop, and microbial biomass are spatially correlated with aboveground net primary productivity (ANPP), which is controlled by species-specific variation in plant growth rates. In addition, locations of periodic excess water exhibit lower root density, root production, and microbial biomass as a proportion of ANPP than that of dry regions; (5) a coupled model that combines a 1-D Earth system model (CLM) with a 3-D hydrologic model (PHIM) will improve the simulation of belowground C processes at ~10 m resolution in a first-order watershed, relative to a 1-D Earth system model; and (6) the use of spatially distributed root and soil parameters in CLM or in PIHM-CLM will improve the simulation of the spatial heterogeneities of belowground C processes, relative to models that use spatially uniform belowground root and soil parameters. We will test these hypotheses at the Susquehanna/Shale Hills critical zone observatory (SSHOCZO), which has been intensively studied from a hydrologic, geochemical and geophysical perspective. We will measure the vertical root distribution, microbial biomass, soil CO2 efflux and root turnover in relation to topography at the SSHCZO. We will sample 50 macro-sites across the watershed with 4 micro-sites nested within each macro-site (total 200 points). We will add the Penn State Integrated Hydrologic Model (PIHM), which accounts for horizontal groundwater flow, to the Community Land Model Version 4 with terrestrial carbon-nitrogen interactions (CLM4CN), to improve the representation of the land surface and subsurface heterogeneities caused by topography. The proposed high-resolution measurements of soil C processes 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 (PIHM-CLM), 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 microbial processes. 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 PIHM-CLM model, a high-resolution coupled biogeochemical and hydrologic model.