The transformation and transport of carbon (C), nitrogen (N), and phosphorus (P) in soils are key processes in agro-environmental systems, controlling crop productivity, nutrient use efficiency, and environmental impacts such as groundwater contamination and eutrophication. The agrohydrological model FLOWS provides a physically based framework to simulate the coupled dynamics of water flow, solute transport, and biogeochemical transformations of C–N–P species in soils. A central challenge in applying FLOWS lies in the numerical representation of the strong coupling between hydrological and biogeochemical processes. In the model, soil organic carbon is described through conceptual pools undergoing microbial decomposition, typically represented by first-order kinetics. These processes convert organic matter into dissolved organic carbon (DOC) and inorganic forms such as CO2, with transformation rates strongly controlled by environmental factors including soil moisture, temperature, and substrate availability. Organic carbon dynamics are tightly linked to nitrogen and phosphorus cycling through stoichiometric constraints (C:N:P ratios), which regulate mineralization and immobilization processes. As a result, carbon turnover directly influences nutrient availability, microbial activity, and plant uptake, making it a key driver of agro-environmental system functioning. Nutrient transport is described by advection–dispersion mechanisms combined with nonlinear reaction kinetics, including mineralization, immobilization, nitrification, denitrification, and plant uptake. Additional complexity arises from multiphase transport (e.g., CO2 in gas and liquid phases), sorption processes (e.g., ammonium adsorption), and spatial heterogeneity, including preferential flow paths that enhance nutrient leaching. Overall, the treatment of organic carbon in FLOWS provides a mechanistic link between soil physics and biogeochemistry, playing a crucial role in predicting carbon turnover, nutrient cycling, and environmental impacts in agro-environmental applications. From an agro-environmental perspective, accurately capturing these dynamics is essential for predicting nutrient losses, optimizing fertilizer use, and assessing mitigation strategies. However, this remains a complex task, as in agro-environmental systems these dynamics are strongly influenced by soil physical conditions, which vary in space and time, as well as by management practices such as fertilization and irrigation. The relevance of these processes is demonstrated through a real-field application in which FLOWS was applied in a hydraulically reclaimed coastal area (Arborea, Italy) and coupled with a routing model to simulate water and nutrient fluxes from irrigated fields to a surface drainage network. The model successfully reproduced soil water and solute dynamics and quantified nutrient losses at the field scale, showing that significant amounts of nitrate and phosphorus can rapidly move from the upper soil layers to drainage channels during both irrigation and rainfall events. These losses were driven by mechanisms such as infiltration excess and subsurface lateral flow, highlighting the strong connectivity between agricultural soils and surface waters in shallow groundwater systems. Such applications emphasize the importance of integrated modeling tools for evaluating nutrient export pathways, optimizing irrigation and fertilization practices, and mitigating ecohydrological risks to vulnerable coastal water bodies.
Coupled Transformation and Transport of Organic Carbon and Nutrients in Soils: A Process-Based Representation within the FLOWS Model
Coppola A.
Primo
;Pili F.;Fenu C.;Concas A.
2026-01-01
Abstract
The transformation and transport of carbon (C), nitrogen (N), and phosphorus (P) in soils are key processes in agro-environmental systems, controlling crop productivity, nutrient use efficiency, and environmental impacts such as groundwater contamination and eutrophication. The agrohydrological model FLOWS provides a physically based framework to simulate the coupled dynamics of water flow, solute transport, and biogeochemical transformations of C–N–P species in soils. A central challenge in applying FLOWS lies in the numerical representation of the strong coupling between hydrological and biogeochemical processes. In the model, soil organic carbon is described through conceptual pools undergoing microbial decomposition, typically represented by first-order kinetics. These processes convert organic matter into dissolved organic carbon (DOC) and inorganic forms such as CO2, with transformation rates strongly controlled by environmental factors including soil moisture, temperature, and substrate availability. Organic carbon dynamics are tightly linked to nitrogen and phosphorus cycling through stoichiometric constraints (C:N:P ratios), which regulate mineralization and immobilization processes. As a result, carbon turnover directly influences nutrient availability, microbial activity, and plant uptake, making it a key driver of agro-environmental system functioning. Nutrient transport is described by advection–dispersion mechanisms combined with nonlinear reaction kinetics, including mineralization, immobilization, nitrification, denitrification, and plant uptake. Additional complexity arises from multiphase transport (e.g., CO2 in gas and liquid phases), sorption processes (e.g., ammonium adsorption), and spatial heterogeneity, including preferential flow paths that enhance nutrient leaching. Overall, the treatment of organic carbon in FLOWS provides a mechanistic link between soil physics and biogeochemistry, playing a crucial role in predicting carbon turnover, nutrient cycling, and environmental impacts in agro-environmental applications. From an agro-environmental perspective, accurately capturing these dynamics is essential for predicting nutrient losses, optimizing fertilizer use, and assessing mitigation strategies. However, this remains a complex task, as in agro-environmental systems these dynamics are strongly influenced by soil physical conditions, which vary in space and time, as well as by management practices such as fertilization and irrigation. The relevance of these processes is demonstrated through a real-field application in which FLOWS was applied in a hydraulically reclaimed coastal area (Arborea, Italy) and coupled with a routing model to simulate water and nutrient fluxes from irrigated fields to a surface drainage network. The model successfully reproduced soil water and solute dynamics and quantified nutrient losses at the field scale, showing that significant amounts of nitrate and phosphorus can rapidly move from the upper soil layers to drainage channels during both irrigation and rainfall events. These losses were driven by mechanisms such as infiltration excess and subsurface lateral flow, highlighting the strong connectivity between agricultural soils and surface waters in shallow groundwater systems. Such applications emphasize the importance of integrated modeling tools for evaluating nutrient export pathways, optimizing irrigation and fertilization practices, and mitigating ecohydrological risks to vulnerable coastal water bodies.
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