Modelling soil carbon and nitrogen cycles during land use change. A review

Forested soils are being increasingly transformed to agricultural fields in response to growing demands for food crop. This modification of the land use is known to result in deterioration of soil properties, in particular its fertility. To reduce the impact of the human activities and mitigate their effects on the soil, it is important to understand the factors responsible for the modification of soil properties. In this paper we reviewed the principal processes affecting soil quality during land use changes, focusing in particular on the effect of soil moisture dynamics on soil carbon (C) and nitrogen (N) cycles. Both physical and biological processes, including degradation of litter and humus, and soil moisture evolution at the diurnal and seasonal time scales were considered, highlighting the impact of hydroclimatic variability on nutrient turnover along with the consequences of land use changes from forest to agricultural soil and vice-versa. In order to identify to what extent different models are suitable for long-term predictions of soil turnover, and to understand whether some simulators are more suited to specific environmental conditions or ecosystems, we enumerated the principal features of the most popular existing models dealing with C and N turnover. Among these models, we considered in detail a mechanistic compartment-based model. To show the capabilities of the model and to demonstrate how it can be used as a predictive tool to forecast the effects of land use changes on C and N dynamics, four different scenarios were studied, intertwining two different climate conditions (with and without seasonality) with two contrasting soils having physical properties that are representative of forest and agricultural soils. The model incorporates synthetic time series of stochastic precipitation, and therefore soil moisture evolution through time. Our main findings in simulating these scenarios are that (1) forest soils have higher concentrations of C and N than agricultural soils as a result of higher litter decomposition; (2) high frequency changes in water saturations under seasonal climate scenarios are commensurate with C and N concentrations in agricultural soils; and (3) due to their different physical properties, forest soils attenuate the seasonal climate-induced frequency changes in water saturation, with accompanying changes in C and N concentrations. The model was shown to be capable of simulating the long term effects of modified physical properties of agricultural soils, being thus a promising tool to predict future consequences of practices affecting sustainable agriculture, such as tillage (leading to erosion), ploughing, harvesting, irrigation and fertilization, leading to C and N turnover changes and in consequence, in terms of agriculture productio

Analysis of carbon and nitrogen dynamics in riparian soils: Model development

The quality of riparian soils and their ability to buffer contaminant releases to aquifers and streams are connected intimately to moisture content and nutrient dynamics, in particular of carbon (C) and nitrogen (N). A multi-compartment model – named the Riparian Soil Model (RSM) – was developed to help investigate the influence and importance of environmental parameters, climatic factors and management practices on soil ecosystem functioning in riparian areas. The model includes numerous improvements compared to many similar tools, in particular regarding the capability to simulate a wide range of temporal scales, from daily to centuries, along with the ability to predict the concentration and vertical distribution of dissolved organic matter (DOM). The ecological importance of DOM has been highlighted on numerous occasions, and it was found that its concentration controls the amount of soil organic matter (SOM) stored in the soil as well as the respiration rate. The moisture content was computed using a detailed water budget approach, assuming that within each time step all the water above field capacity drains to the layer underneath, until it becomes fully saturated. A mass balance approach was also used for nutrient transport, whereas the biogeochemical reaction network was developed as an extension of an existing C and N turnover model. Temperature changes across the soil profile were simulated using an existing analytical solution of the heat transport equation, assuming periodic temperature changes in the topsoil. To verify the consistency of model predictions and illustrate its capabilities, a synthetic but realistic soil profile in a deciduous forest was simulated. Model parameters were taken from the literature, and model predictions were consistent with experimental observations for a similar scenario. Modelling results stressed the importance of environmental conditions on SOM cycling in soils. The mineral and organic C and N stocks fluctuate at different time scales in response to oscillations in climatic conditions and vegetation inputs/uptake. Low frequency fluctuations with a period larger than 10 y were observed also, which were not connected to any single environmental process

Carbon and nitrogen dynamics in a soil profile: Model development

In order to meet demands for crops, pasture and firewood, the rate of land use change from forested to agricultural uses has 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 neighbouring areas. To this end, in order to interpret field experiments and elucidate the different mechanisms taking place, numerical tools are beneficial. Microbiological transformations of the soil organic matter, including decomposition and nutrient turnover are controlled to a large extent by soil water content, influenced in turn by climatic and environmental conditions such as precipitation and evapotranspiration. The work presented here is part of the Swiss RECORD project (http://www.cces.ethz.ch/projects/nature/Record), a large collaborative research effort undertaken to monitor the changes in ecosystem functioning in riparian areas undergoing restoration. In this context we have developed a numerical model to simulate carbon and nitrogen transport and turnover in a one-dimensional variably saturated soil profile. The model is based on the zero-dimensional mechanistic batch model of Porporato et al. (Adv. Water Res., 26: 45-58, 2003), but extends its capabilities to simulate (i) the transport of the mobile components towards deeper horizons, and (ii) the vertical evolution of the profile and the subsequent distribution of the organic matter. The soil is divided in four compartments, each representing a different “functional unit”, having different thickness. The three shallower compartments, each variably saturated, correspond to the top soil, the root zone and an intermediate soil layer between the root zone and the aquifer. The deeper compartment represents the unconfined aquifer that receives nutrients infiltrating through the soil profile and always remains water-saturated. Carbon and nitrogen infiltration in the soil profile and their cycling are described by a set of coupled non-linear ordinary differential equations that are numerically integrated. To show the model capabilities in simulating soil nutrients transformations and transport and to illustrate how the model can be used to predict the changes in soil functioning as a result of land use changes, several realistic scenarios, with different soil and vegetation types, were modelled using a stochastically generated precipitation time series

The Finite Volume Point Dilution Method: A tracer technique for monitoring transient Darcy fluxes

Quantification of pollutant mass fluxes is essential for assessing the impact of contaminated sites on their surrounding environment, particularly on adjacent surface water bodies. In this context, it is essential to quantify but also to be able to monitor the variations with time of Darcy fluxes in relation with changes in hydrogeological conditions and groundwater – surface water interactions. The Finite Volume Point Dilution Method (FVPDM) is a new tracer technique that generalizes the single-well point dilution method to the case of finite volumes of tracer fluid and water flush. It is based on an analytical solution derived from a mathematical model proposed recently to accurately model tracer injection into a well. After a brief description of the underlying concepts and mathematical model, an analytical solution is derived for calculating straightforwardly the evolution of concentration of the tracer in the injection well during and after injection operations. It is shown that this new tracer technique is easier to implement in the field than the classical point dilution method while it further allows monitoring changes with time of the magnitude of estimated Darcy fluxes, which is not the case for the former technique. In the scope of the EU FP6 AQUATERRA project, the FVPDM was applied in two experimental sites with contrasted objectives, geological and hydrogeological conditions, and field equipment facilities. In site A, the objective was to estimate contaminant travel times in groundwater to a spring while assessing vertical variations in groundwater fluxes, using a combined FVPDM – classical tracer test, with “non-ideal” experimental conditions. In site B, the purpose was to estimate, in very well controlled experimental conditions, groundwater fluxes flowing out from a contaminated site to a neighbouring river. In both cases, field tracer concentrations monitored in the injection wells were used to fit the calculated modelled concentrations by adjusting the apparent Darcy flux crossing the well screens. Modelling results are very satisfactory and indicate that the methodology is efficient and accurate, with a wide range of potential applications in different environments and experimental conditions, including the monitoring with time of changes in Darcy fluxes

Axysimetrical water infiltration in soil imaged by non-invasive electrical resistivimetry

Axisymetrical infiltration of water in soil has been largely studied since the of tension disc infiltrometers. Procedures have been developed to derive the hydraulic properties of soils from axisymetrical infiltration measurements but rely some simplifying and/or a priori assumptions on the homogeneity of the soil from point of view of its hydraulic properties and its initial water status prior to Such assumptions are difficult to ascertain. We present here an attempt to image in vertical 2D plane the development of the axisymetrical infiltration bulb in soils using Bi-dimensional images of the soil electrical resistivity were obtained at various during the infiltration process by inverting apparent electrical resistivity taken by a 32-electrodes Wenner array with a 10 cm spacing laid across a diameter of the infiltrometer. The inversion was done using the Res2Dinv software. The infiltration experiments used either a CaCl2 or a KBr solution at 40 g/Litre to enhance the soil electrical resistivity contrast, and either 8-cm or 25-cm diameter disks. Most of the infiltration experiments were done at one single water potential (-0.1 kPa) and lasted 3.5 to 5 hours. A multipotential experiment was conducted as classically done to derive hydraulic conductivity values according to Reynolds & ElrickŠs method. At the end of each experiment, the soil was sampled for Cl or Br concentrations on the 2D plane corresponding to the resistivity measurements. Electrical resistivity measurements provided clear images of the infiltration bulb allowed the user to monitor the development of the infiltration bulb through time. The infiltration bulb imaged by resistivimetry at the end of the infiltrations matched well that imaged from the anion concentrations in soil. Some geometrical of the infiltration bulb could be seen both through resistivity and anion measurements and were consistent between both imaging methods. High- geophysical imaging of water infiltration in field soils seems a fruitful approach to development of efficient methods for the hydraulic characterisation of soils

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

River infiltration to a subtropical alluvial aquifer inferred using multiple environmental tracers

Chloride (Cl−), stable isotope ratios of water (δ18O and δ2H), sulfur hexafluoride (SF6), tritium (3H), carbon-14 (14C), noble gases (4He, Ne, and Ar), and hydrometry were used to characterize groundwater-surface water interactions, in particular infiltration rates, for the Lower Namoi River (New South Wales, Australia). The study period (four sampling campaigns between November 2009 and November 2011) represented the end of a decade-long drought followed by several high-flow events. The hydrometry showed that the river was generally losing to the alluvium, except when storm-derived floodwaves in the river channel generated bank recharge—discharge cycles. Using 3H/14C-derived estimates of groundwater mean residence time along the transect, infiltration rates ranged from 0.6 to 5 m yr−1. However, when using the peak transition age (a more realistic estimate of travel time in highly dispersive environments), the range in infiltration rate was larger (4–270 m yr−1). Both river water (highest δ2H, δ18O, SF6, 3H, and 14C) and an older groundwater source (lowest δ2H, δ18O, SF6, 3H, 14C, and highest 4He) were found in the riparian zone. This old groundwater end-member may represent leakage from an underlying confined aquifer (Great Artesian Basin). Environmental tracers may be used to estimate infiltration rates in this riparian environment but the presence of multiple sources of water and a high dispersion induced by frequent variations in the water table complicates their interpretation

Documented spatial data set containing the subdivision of the basins into groundwater systems and subsystems, the selected locations per subsystem and a description of these sites, available data and projected additional measurements and equipment

The establishment of tools for trends analysis in groundwater is essential for the prediction and evaluation of measures taken within context of the Water Framework Directive and the draft Groundwater Directive. This report describes the spatial data sets which will be used for the purpose of detection, aggregation and extrapolation of temporal trends in groundwater quality. Trend analysis methods will be applied and tested at various scales and in various hydrogeological situations. The report contains a description of the studied sub-basins in TREND 2, including information on hydrogeology, land use and pressures, available data and projected additional measurements. Major differences between the sub-basins and the data sets are described to examine consequences for the work on trend detection. One of the challenges for TREND 2 is to define criteria for the application of various statistical and deterministic trend approaches for a range of hydrogeological conditions, spatial scales and types of groundwater monitoring. An overview of these conditions, scales and monitoring types is provided in the present report.FP6 Integrated Project AquaTerra Integrated Modelling of the river-sediment-soil-groundwater system; advanced tools for the management of catchment areas and river basins in the context of global change (Project no. 505428 - GOCE

Carbon and nitrogen dynamics in a soil profile: Model insights and application to a restored Swiss riparian area

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. During restoration, environmental conditions (such as vegetation type and water availability) are manipulated to create ecological conditions suitable for the successful establishment of a target composition of species. Often, ecological restoration induces changes to adjacent ecosystems. This is the case of riparian ecosystems, and their restoration to their original undisturbed situation is likely to cause changes in nutrient cycles. For example, following the restoration of a riparian zone, microbial communities adapted to one set of environmental conditions have to acclimatize to another, and the subsequent changes in the composition of the biomass populations might induce changes in soil organic matter mineralization and soil respiration rates. Since the biogeochemical cycles are tightly interconnected, these changes can trigger nutrient storing or release, therefore inducing changes in nutrient cycles of adjacent ecosystems. Overall, the effects of the restoration activities on the hydrologic regime, soil properties and vegetation are still largely unknown and poorly understood. Within the RECORD project (http://www.cces.ethz.ch/projects/nature/Record), a large collaborative research effort undertaken to monitor and understand the changes in ecosystem functioning in riparian areas undergoing restoration, a numerical model has been developed to simulate the vertical transport of the mobile C and N components in a soil profile (model development discussed in the companion submitted abstract Batlle-Aguilar et al.). In the model, microbial decomposition of the soil organic matter drives biogeochemical transformations of C and N, while the activity of the soil biota is primarily controlled by the soil moisture content. The temporal evolution of the soil properties measured at one location of the RECORD experimental site, in a mixed forest dominated by ash and maple characteristic for the transition from riparian to upland forest, was used to validate the model and to gain insights into the key factors controlling the nutrient turnover. The site is located next to the Thur River where a revitalization project involving removal of levees has been implemented to create more natural conditions in the riparian zone. Soil water content and temperature at several depths were monitored continuously between October 2008 and October 2009. In October 2008, January 2009 and in biweekly frequency between April and October 2009, topsoil and soil solution at several depths were sampled. The soil solution samples were analysed for major carbon and nitrogen species, and the soil samples for denitrification enzyme activity, potential nitrification and related properties. In addition, soil respiration and N2O emissions were measured at each sampling event. Preliminary modelling results are shown, together with a discussion of the most influential parameters and processes controlling C and N turnover in riparian soils

Using hydraulic head, chloride and electrical conductivity data to distinguish between mountain-front and mountain-block recharge to basin aquifers

This work is distributed under the Creative Commons Attribution 4.0 License.Numerous basin aquifers in arid and semi-arid regions of the world derive a significant portion of their recharge from adjacent mountains. Such recharge can effectively occur through either stream infiltration in the mountain-front zone (mountain-front recharge, MFR) or subsurface flow from the mountain (mountain-block recharge, MBR). While a thorough understanding of recharge mechanisms is critical for conceptualizing and managing groundwater systems, distinguishing between MFR and MBR is difficult. We present an approach that uses hydraulic head, chloride and electrical conductivity (EC) data to distinguish between MFR and MBR. These variables are inexpensive to measure, and may be readily available from hydrogeological databases in many cases. Hydraulic heads can provide information on groundwater flow directions and stream–aquifer interactions, while chloride concentrations and EC values can be used to distinguish between different water sources if these have a distinct signature. Such information can provide evidence for the occurrence or absence of MFR and MBR. This approach is tested through application to the Adelaide Plains basin, South Australia. The recharge mechanisms of this basin have long been debated, in part due to difficulties in understanding the hydraulic role of faults. Both hydraulic head and chloride (equivalently, EC) data consistently suggest that streams are gaining in the adjacent Mount Lofty Ranges and losing when entering the basin. Moreover, the data indicate that not only the Quaternary aquifers but also the deeper Tertiary aquifers are recharged through MFR and not MBR. It is expected that this finding will have a significant impact on the management of water resources in the region. This study demonstrates the relevance of using hydraulic head, chloride and EC data to distinguish between MFR and MBR