image: Regional coupled C-N-H2O cycle processes and associated driving mechanisms
The regional C-N-H2O coupling cycle is a strong coupling process that occurs between physicochemical process and biogeochemical processes in terrestrial and aquatic ecosystems through hydrological flux, and it is closely related and intercoupled biogeochemical processes.
The primary focus of C, N, and H2O cycles from a point scale to a slope scale is on C, N, and H2O exchange flux, biophysical soil-vegetation-atmosphere processes, hydrological C and N processes (surface flow, interflow and seepage flow) and morphological transformation slope processes.
At a watershed scale, the C, N and H2O cycle is the basic unit and the basic subsystem of regional coupled C-N-H2O cycling research, we were attentive to the dynamic mechanisms of C and N exchange and their biogeochemical processes, resulting in the riverine-watershed transport of these elements.
The regional coupled C-N-H2O cycle is a continuum which is composed of atmosphere, terrestrial, freshwater, estuarine and marine systems, defined as the flow of material and energy from several river basin subsystems and the physical connection between water, gas and aerosol flux, emphasis on the interaction of elements at a regional scale(Figure 1).
The regional C-N-H2O coupling cycle involves regional C-N-H2O transport and transformation processes, regional biogeochemical C-N-H2O processes, C and N interactions at the terrestrial-atmosphere interface, and interactions of C and N exchange at the terrestrial-freshwater interface (Figure 2).
The process of carbon transport and transformation includes the transportation of various forms of carbon from the upstream to the downstream, and the transformation between particulate, dissolved and gaseous states.
The loss of N is a continuous dynamic process that occurs in watersheds under four interactional types: the soil erosion process, the rainfall-runoff process, the surface solute dissolution process and the solute infiltration process.
Biogeochemical processes include the temporary storage of vegetation biomass(algae growth and mortality, deposition and decomposition), C, N and P exchanges between sediment and water, organic compound mineralization, adsorption/desorption of mineralized P particulates and denitrification processes, which can all contribute to the persistence of sediment, C and nutrients in lakes and reservoirs.
C and N interactions at the terrestrial-atmosphere interface: Changes of regional N and S deposition have an impact on the C, N emission and export of watersheds, rivers to estuaries and marine environments. Additionally, the influence of N deposition on terrestrial C sources and sinks differ.
Interactions of C and N exchange at the terrestrial-freshwater interface: Elements input from land to freshwater ecosystem have an impact on terrestrial and aquatic ecosystem under the combined action of coupled cycle. Nutrient cycles from river networks to seas further increase differences between the reactivity, loss process and flux of nutrient elements on the terrestrial-water interface, while these cycles also impact interface and transport factors associated with the terrestrial-water C cycle, which constitute the theoretical principles behind the region-scale C-N-H2O coupling cycle (Figure 2).
Climate change, such as CO2 increase and N input, impacts the regional C-N-H2O coupling cycle by driving nutrient cycling, primary productivity, microbial activity and abiotic processes.
With the increase of greenhouse gas CO2 concentration, the Nr deposition caused by artificial atmospheric N emission changes the production of watershed system and produces environmental effects (water acidification, eutrophication, water pollution, etc.), improves the CO2 absorption and N2O emission of water body, affects the N cycle at the land-water interface, and accelerates the C-N coupling cycle in the aquatic ecosystem (Figure 3).
As we move into the future, studying the C-N-H2O coupling cycles of regional ecosystems will increasingly emphasize the human-environment relationship as being the core research, coupled with soil, hydrology, atmosphere and other factors, accentuating natural geographical processes under the influence of anthropogenic activities.
In the future, regional research methods will emphasize the integration of "sky-ground-air integration" and we must further strengthen our understanding of the impact of atmospheric N deposition on regional C-N-H2O coupling cycles.