Soil Hydrology and Reactive Transport of Carbon and Nitrogen in a Multi-scale Landscape - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 108

8 Soil Hydrology and Reactive Transport
of Carbon and Nitrogen in a Multi-scale
Landscape
Christopher Duffy* and Nikolaos Nikolaidis
Abstract
This chapter examines the role of soil in water filtration, its impact on carbon and nitrogen in
biogeochemical transformations and its relation to the larger landscape, where soil functions are key
to clean water, essential to human sustenance. Soil composition and chemical weathering are essential
factors in soil structure and formation, which affect the hydrologic properties of soil and chemical
transport significantly. The role of clays on aggregation, carbon (C) sequestration, pH, etc., carbon/
nitrogen/phosphorus (C/N/P) cycles and plant growth (plant exudates) on reactive transport are exam-
ined. Examples of water filtration and solute transformation of a functioning soil (producing clean
water) and of failure to transform (producing toxicity and contamination) are presented. Special focus
is given on the parameterization of hydrologic and reactive transport models that cover a range of
scales from soil profile, to hill slopes and the catchment. A variety of modelling strategies presently
exist for biogeochemical modelling, and typically each focuses on a particular scale, with scale-
appropriate processes, mechanisms and states. They range from bottom-up approaches, where plot-
scale studies use intensive monitoring and detailed local modelling of process-level biogeochemical
cycles for C and N, to regional- and continental-scale approaches to simulating the C-N dynamics in
atmospheric models that, by necessity, may neglect the details of the processes understanding obtained
in the plot-scale research. Ecosystem approaches extend the plot-scale models for C-N and water to
landscape scales maintaining systematic processes, but may not include detailed geospatial struc-
ture and coupled hydrodynamic processes of the larger catchment and river basin. A strategy for
merging scales and concepts intrinsic to plot-, landscape- and catchment-scale carbon-based biogeo-
chemical research is proposed. The approach will describe existing process models for each scale of
research, including hydrologic impacts. We propose a strategy for an integrated hydrodynamic ap-
proach to numerical modelling that can resolve the geospatial characteristics of water, carbon and
nitrogen cycles over entire catchments, including first- and second-order streams, which link plot
and hill-slope studies within this framework. The approach integrates plot-to-catchment scales for
mesoscale model application for scales that range from 100 m to 10 5 km 2 . The approach has important
implications for ongoing soils research at Critical Zone Observatories, which are advanced field re-
search facilities, to scale up their science understanding to larger domains and will improve the pro-
spect of carbon-nitrogen management greatly.