Modelling carbon and nitrogen turnover in variably saturated soils

Natural ecosystems provide services such as ameliorating the impacts of deleterious human activities on both surface and groundwater. For example, several studies have shown that a healthy riparian ecosystem can reduce the nutrient loading of agricultural wastewater, thus protecting the receiving surface water body. As a result, in order to develop better protection strategies and/or restore natural conditions, there is a growing interest in understanding ecosystem functioning, including feedbacks and nonlinearities. Biogeochemical transformations in soils are heavily inﬂuenced by microbial decomposition of soil organic matter. Carbon and nutrient cycles are in turn strongly sensitive to environmental conditions, and primarily to soil moisture and temperature. These two physical variables affect the reaction rates of almost all soil biogeochemical transformations, including microbial and fungal activity, nutrient uptake and release from plants, etc. Soil water saturation and temperature are not constants, but vary both in space and time, thus further complicating the picture. In order to interpret ﬁeld experiments and elucidate the different mechanisms taking place, numerical tools are beneﬁcial. In this work we developed a 3D numerical reactive-transport model as an aid in the investigation the complex physical, chemical and biological interactions occurring in soils. The new code couples the USGS models (MODFLOW 2000–VSF, MT3DMS and PHREEQC) using an operator-splitting algorithm, and is a further development an existing reactive/density-dependent ﬂow model PHWAT. The model was tested using simplified test cases. Following verification, a process-based biogeochemical reaction network describing the turnover of carbon and nitrogen in soils was implemented. Using this tool, we investigated the coupled effect of moisture content and temperature ﬂuctuations on nitrogen and organic matter cycling in the riparian zone, in order to help understand the relative sensitivity of biological transformations to these processes.

David Andrew Barry, Jordi Batlle-Aguilar, Alessandro Brovelli

Natural ecosystems provide services such as ameliorating the impacts of deleterious human activities on both surface and groundwater. For example, several studies have shown that a healthy riparian ecosystem can reduce the nutrient loading of agricultural wastewater, thus protecting the receiving surface water body. As a result, in order to develop better protection strategies and/or restore natural conditions, there is a growing interest in understanding ecosystem functioning, including feedbacks and nonlinearities. Biogeochemical transformations in soils are heavily inﬂuenced by microbial decomposition of soil organic matter. Carbon and nutrient cycles are in turn strongly sensitive to environmental conditions, and primarily to soil moisture and temperature. These two physical variables affect the reaction rates of almost all soil biogeochemical transformations, including microbial and fungal activity, nutrient uptake and release from plants, etc. Soil water saturation and temperature are not constants, but vary both in space and time, thus further complicating the picture. In order to interpret ﬁeld experiments and elucidate the different mechanisms taking place, numerical tools are beneficial. In this work we developed a 3D numerical reactive-transport model as an aid in the investigation the complex physical, chemical and biological interactions occurring in soils. The new code couples the USGS models (MODFLOW 2000–VSF, MT3DMS and PHREEQC) using an operator-splitting algorithm, and is a further development an existing reactive/density-dependent ﬂow model PHWAT. The model was tested using simplified test cases. Following verification, a process-based biogeochemical reaction network describing the turnover of carbon and nitrogen in soils was implemented. Using this tool, we investigated the coupled effect of moisture content and temperature ﬂuctuations on nitrogen and organic matter cycling in the riparian zone, in order to help understand the relative sensitivity of biological transformations to these processes.

2009

Modelling of mechanisms affecting nitrogen and carbon cycles in soils subject to land use change

David Andrew Barry, Jordi Batlle-Aguilar, Alessandro Brovelli

In order to meet demands for crops, pasture and firewood, the rate of land use change from forested to agricultural uses steadily increased over several decades, resulting in an increased release of nutrients towards groundwater and surface water bodies. In parallel, the continuous degradation of natural ecosystems such as riparian zones, contribute to diminish their capacity to provide ecological services, i.e. reduce the impacts of deleterious human activities on both surface and groundwater. Land use changes and restoration practices are known to affect both soil nutrient dynamics and their transport to neighbouring areas. Biogeochemical transformations in soils are heavily influenced by microbial decomposition of soil organic matter. Carbon and nutrient cycles are in turn strongly sensitive to environmental conditions, primarily to soil moisture. This physical variable affects the reaction rate of almost all soil biogeochemical transformations, including microbial and fungal activity, nutrient uptake and release from plants, etc. Soil water saturation is not constant neither in time nor in space, thus further complicating the picture. In order to interpret field experiments and elucidate the different mechanisms taking place, numerical tools are beneficial. The impact of hydroclimatic variability (soil moisture in particular) on soil carbon and nitrogen cycles was highlighted along with the consequences of land use changes from forest to pasture and/or agriculture. To this end, a mechanistic model based on the compartment model of Porporato et al. (2003) was developed. The predictive capabilities of the model to forecast the effects of land use changes over carbon and nitrogen dynamics were shown, modelling four different scenarios, intertwining two different climate conditions (with and without seasonality) with two contrasting soils having physical properties that are representative of forest and agricultural soils. Synthetic time series of precipitation, and therefore soil moisture evolution in time, were generated using a stochastic approach. The model was subject to isothermal conditions and other considerations such as average values of carbon and nitrogen concentrations over the rooting depth. As well an average microbial organic decomposition rate was considered. Simulation results demonstrated higher concentrations of carbon and nitrogen in forests soils as a result of higher litter decomposition than in agricultural soils (Figure 1). High frequency changes in water saturation under seasonal climate scenarios were shown to be commensurate with carbon and nitrogen concentrations in agricultural soils. Forest soils have different properties to agricultural soils, leading to an attenuation of the seasonal climate-induced frequency changes in water saturation, with accompanying changes in carbon and nitrogen concentrations. The model was shown to be capable of simulating the long term effects of modified physical properties of agricultural soils as a consequence of tillage, ploughing and harvesting. It is, thus, a promising tool to predict carbon and nitrogen turnover changes due to changes in land use. These results encourage further model improvements aimed at accounting for the complexity of the soil system and its processes for better understanding soil nutrients turnover.

2009

Carbon, oxygen and nitrogen dynamics in a soil profile: Model development and application

David Andrew Barry, Jordi Batlle-Aguilar, Alessandro Brovelli

In order to meet demands for crops, pasture and firewood, the rate of land use change from forested to agricultural uses steadily increased over several decades, resulting in an increased release of nutrients towards groundwater and surface water bodies. In parallel, the degradation of riparian zones has diminished their capacity to provide critical ecosystem functions, such as the ability to control and buffer nutrient cycles. In recent years, however, the key environmental importance of natural, healthy ecosystems has been progressively recognized and restoration of degraded lands towards their former natural state has become an area of active research worldwide. Land use changes and restoration practices are known to affect both soil nutrient dynamics and their transport to neighboring areas. To this end, in order to interpret field experiments and elucidate the different mechanisms taking place, numerical tools are beneficial. Microbial decomposition is the main driving force on biogeochemical transformations of soil organic matter and soil nutrients. The activity of the soil biota is primarily controlled by water availability and by the pore-solution oxygen concentrations, which ultimately depend to a large extent on meteorological conditions, e.g., precipitation. In this work a model is presented that simulates carbon and nitrogen turnover and transport in a 1D profile under variably-saturated conditions. The model is based on the mechanistic batch model of Porporato et al. (Adv. Water Res., 26: 45-58, 2003), but extends its capabilities to simulate the vertical transport of the mobile components. Furthermore, oxygen dynamics are included such that the pore-water concentration is dependent on microbial degradation rates and soil moisture level. The model was applied to simulate the effect of land use change from forested to agricultural soils on pedo-fauna activity and nutrient distribution and abundance across the vertical profile. External forcing, i.e., precipitation time series, were generated stochastically. Modeling results showed that soil tillage practices, which modify soil structure and thus soil aeration, were responsible for higher decomposition and mineralization rates in agricultural soils, thus for the lower soil carbon and nitrogen concentrations. Furthermore, higher plant uptake rates and leaching of agricultural soils contributed also to lower nitrate concentrations in such soils.

2009

Carbon, oxygen and nitrogen dynamics in a soil profile: Model development and application

David Andrew Barry, Jordi Batlle-Aguilar, Alessandro Brovelli

In order to meet demands for crops, pasture and firewood, the rate of land use change from forested to agricultural uses steadily increased over several decades, resulting in an increased release of nutrients towards groundwater and surface water bodies. In parallel, the degradation of riparian zones has diminished their capacity to provide critical ecosystem functions, such as the ability to control and buffer nutrient cycles. In recent years, however, the key environmental importance of natural, healthy ecosystems has been progressively recognized and restoration of degraded lands towards their former natural state has become an area of active research worldwide. Land use changes and restoration practices are known to affect both soil nutrient dynamics and their transport to neighboring areas. To this end, in order to interpret field experiments and elucidate the different mechanisms taking place, numerical tools are beneficial. Microbial decomposition is the main driving force on biogeochemical transformations of soil organic matter and soil nutrients. The activity of the soil biota is primarily controlled by water availability and by the pore-solution oxygen concentrations, which ultimately depend to a large extent on meteorological conditions, e.g., precipitation. In this work a model is presented that simulates carbon and nitrogen turnover and transport in a 1D profile under variably-saturated conditions. The model is based on the mechanistic batch model of Porporato et al. (Adv. Water Res., 26: 45-58, 2003), but extends its capabilities to simulate the vertical transport of the mobile components. Furthermore, oxygen dynamics are included such that the pore-water concentration is dependent on microbial degradation rates and soil moisture level. The model was applied to simulate the effect of land use change from forested to agricultural soils on pedo-fauna activity and nutrient distribution and abundance across the vertical profile. External forcing, i.e., precipitation time series, were generated stochastically. Modeling results showed that soil tillage practices, which modify soil structure and thus soil aeration, were responsible for higher decomposition and mineralization rates in agricultural soils, thus for the lower soil carbon and nitrogen concentrations. Furthermore, higher plant uptake rates and leaching of agricultural soils contributed also to lower nitrate concentrations in such soils.

2009

Modelling of mechanisms affecting nitrogen and carbon cycles in soils subject to land use change

David Andrew Barry, Jordi Batlle-Aguilar, Alessandro Brovelli

2009

Modelling carbon and nitrogen turnover in variably saturated soils

David Andrew Barry, Jordi Batlle-Aguilar, Alessandro Brovelli

Natural ecosystems provide services such as ameliorating the impacts of deleterious human activities on both surface and groundwater. For example, several studies have shown that a healthy riparian ecosystem can reduce the nutrient loading of agricultural wastewater, thus protecting the receiving surface water body. As a result, in order to develop better protection strategies and/or restore natural conditions, there is a growing interest in understanding ecosystem functioning, including feedbacks and nonlinearities. Biogeochemical transformations in soils are heavily inﬂuenced by microbial decomposition of soil organic matter. Carbon and nutrient cycles are in turn strongly sensitive to environmental conditions, and primarily to soil moisture and temperature. These two physical variables affect the reaction rates of almost all soil biogeochemical transformations, including microbial and fungal activity, nutrient uptake and release from plants, etc. Soil water saturation and temperature are not constants, but vary both in space and time, thus further complicating the picture. In order to interpret ﬁeld experiments and elucidate the different mechanisms taking place, numerical tools are beneficial. In this work we developed a 3D numerical reactive-transport model as an aid in the investigation the complex physical, chemical and biological interactions occurring in soils. The new code couples the USGS models (MODFLOW 2000–VSF, MT3DMS and PHREEQC) using an operator-splitting algorithm, and is a further development an existing reactive/density-dependent ﬂow model PHWAT. The model was tested using simplified test cases. Following verification, a process-based biogeochemical reaction network describing the turnover of carbon and nitrogen in soils was implemented. Using this tool, we investigated the coupled effect of moisture content and temperature ﬂuctuations on nitrogen and organic matter cycling in the riparian zone, in order to help understand the relative sensitivity of biological transformations to these processes.

2009