Elsevier

Geoderma

Volume 151, Issues 3–4, 15 July 2009, Pages 179-190
Geoderma

Preferential flow and slow convective chloride transport through the soil of a forested landscape (Fougères, France)

https://doi.org/10.1016/j.geoderma.2009.04.002Get rights and content

Abstract

This study aims to assess the water flow and non-reactive solute transfers occurring in a glossic acidic soil under a beech forest in Brittany (Fougères, France). The specific objectives were to study the water and chloride transfers in this soil, to understand the spatial and temporal variability of these transfers and to produce a data set in this forest site for future modelling. For this, we carried out a field tracer experiment and sprinkled chloride enriched solution over two areas of soil (2 × 66 m2) in March, 2006. Subsequently, we monitored the composition of the soil solutions collected by zero tension plate lysimeters and ceramic cup lysimeters installed at depths between 0 and 240 cm, over a period of 18 months.

We prove that preferential flow through rapid-mobile porosity and slow transfers by convective flux though slow-mobile porosity coexist in the soils of the experimental plot, and that the time scales brought into play ranged from a few days to a yearly scale. The transfer velocities ranged between 2.38 mm day 1 for the slowest convective flux and 600 mm day 1 for the fastest preferential flows. We also prove that the rapid-mobile porosity represents only a small proportion of the soil volume (the mean of all depths, except 10 cm, was about 11%) but the quantity of solute transferred, which by-passes a large part of the rooting zone, may be important (around 17% of the tracer mass applied). The rapid transfer is mainly governed by the soil moisture combined with precipitation intensities and the slow transfer mainly by the cumulative percolation flux. Both transfers are also characterised by wide spatial and temporal variability. The wide transfer variability may be explained by the hydrodynamic dispersion related to the heterogeneity of the slow and rapid porosities, combined with the impact of the 2006 growing season, which slowed down the tracer displacement. Lastly, the experiment proves that the zero tension plate lysimeters mainly collect rapid drainage water, as preferential flows, while the ceramic cup lysimeters mainly collect slow-mobile water mixed with rapid drainage water.

Introduction

Assessing potential effects of modifications or disturbance on forest ecosystems, such as climate change or modifications of silvicultural practices, requires the correct modelling of physical, chemical and biological processes. Most of these processes are controlled by the soil water. The flow pathways and the velocities at which water is transferred through the soils also influence the biogeochemical processes that affect solute bioavailability for trees, thus controlling soil fertility and forest productivity. In the context of climate change, with a possible increase in growing season duration (Schär et al., 2004), modelling water flow is essential to compute precise input–output nutrient budgets (Ranger and Turpault, 1999), which are valuable diagnostic tools to determine the sustainability of forest ecosystems. In the nutrient budget approach, nutrient fluxes in soil and nutrient leaching are often obtained by coupling the water fluxes from a deterministic flow model to nutrient concentrations in soil water. Nutrient leaching, which is one of the terms of the budget, is particularly sensitive to water flow.

The transfer of water in soils is usually split into slow water movement through the soil matrix by slow convective flux, resulting from a piston-like flow (Jardine et al., 1990, McDonnell, 1990, Waddington et al., 1993, Rawlins et al., 1997, Sidle et al., 2000), and rapid or preferential flow via specific pathways which bypass a large proportion of the soil (Jardine et al., 1989, Kung, 1990, Tsuboyama et al., 1994, Köhne et al., 2009). As recalled by Deurer et al. (2003), the causes of preferential flow are related to hydraulic characteristics such as the occurrence of macropores (Beven and Germann, 1982, Jarvis, 2007), dual or multimodal pore-size distributions (Gerke and van Genuchten, 1993, Zurmühl and Durner, 1996), discontinuities of texture (Kung, 1990), the occurrence of biopores such as root channels (Bramley et al., 2003, Parsons et al., 2004) or the occurrence of glossic structures (tongues) as shown in Diab et al. (1988).

Forest soils require particular attention (Sidle et al., 2001, Uchida et al., 2001) mainly because their structure is rarely disturbed, hence the presence of well developed aggregates and pore systems, which are known to influence flow variability (Jarvis, 2007). Most agricultural soils are disturbed annually in the upper decimetres due to tilling operations, while the soil structure of forest soils and particularly the macropores and biopores (Beven and Germann, 1982, Hagedorn and Bundt, 2002) may last for decades or more, and may be continuous from the surface to the deepest soil horizons. For these reasons, the heterogeneity of forest soil structure is thought to have considerable effects on water flow.

Since it is always difficult to predict the spatial variability of the soil structure and thus to predict precisely what will occur on a specific forest site, the collection of information from field tracer experiments, such as the flow velocity or the porosity involved in transfer, is a crucial step for the precise calibration and setting-up of deterministic flow or transport models (Ptak et al., 2004). This study aims to assess the water flow and non-reactive solute transfers occurring in a glossic acidic soil under a beech forest in Brittany. The specific objectives were to study the water and chloride transfers in this soil, to understand the spatial and temporal variability of these transfers and to produce a data set from this forest site for future modelling.

Section snippets

Materials and methods

We carried out a field tracer experiment in an established experimental site located in Fougères forest, western France. For this, we sprinkled chloride enriched solution over two areas of soil (2 × 66 m2) in March, 2006 and monitored the composition of the soil solution collected by zero tension and tension lysimeters over a period of 18 months.

Hydrology

The cumulative precipitation was 876.4 mm for the hydrological year 2005–2006 and 1195.2 mm for 2006–2007 (Fig. 2a). The cumulative Penman evapotranspiration was 477.3 mm and 440.4 mm respectively for the above years.

In 2005 and 2006, the daily volumetric soil moisture values (Fig. 2b) followed a classic scheme with a period of stability before the growing season, a major decrease during the growing season (beginning around 1st June) and an increase from the beginning of autumn to reach soil

Discussion

In order to explain the chloride transfer through the soil, soil porosity needed to be divided into two components: i) the rapid-mobile porosity, responsible for the rapid transfer of water through distinct pathways travelling across the soil matrix and by-passing most of its volume and ii) the slow-mobile porosity, in which most of the solutes are transferred slowly by convective flux through the soil matrix.

The mixing phenomenon which may occur between old water and new water brought in from

Conclusion

The qualitative approach proves that preferential flows through rapid-mobile porosity and slow transfers by convective flux through slow-mobile porosity coexist in the soils of the experimental plot, and the time scales brought into play ranged from a few days to a yearly scale. The rapid transfer is mainly governed by the soil moisture combined with precipitation intensities and the slow transfer is mainly due to the cumulative percolation flux. Moreover, the rapid-mobile porosity involves

Acknowledgements

We would like to thank all the technicians without whom this project would not have been possible, in particular C. Antoine, S. Didier and L. Gelhaye from INRA Nancy.

This work was financed by the EFPA department (INRA), the GIP ECOFOR and by the Office National des Forêts in the context of one of the Environmental Research sites on “Lowland beech” part of F-ore-T network.

The English was corrected by Aldyth Nys.

References (44)

  • MaciejewskiS.

    Numerical and experimental study of solute transport in unsaturated soils

    Journal of Contaminant Hydrology

    (1993)
  • PtakT. et al.

    Tracer tests for the investigation of heterogeneous porous media and stochastic modelling of flow and transport – a review of some recent developments

    Journal of Hydrology

    (2004)
  • WaddingtonJ.M. et al.

    Runoff mechanisms in a forested groundwater discharge wetland

    Journal of Hydrology

    (1993)
  • AllenA. et al.

    Impact of afforestation on ground water resources and quality

    Hydrogeology Journal

    (2001)
  • Baize, D. and Girard, M.C., 1998. A Sound Reference Base for Soils: The "Référentiel Pédologique". Translation by...
  • BarbeeG.C. et al.

    Comparison between suction and free-drainage soil solution samplers

    Soil Science

    (1986)
  • BevenK. et al.

    Macropores and water flows in soil

    Water Resources Research

    (1982)
  • BonneM. et al.

    Etude hydrochimique saisonnière dans trois sols acides: composition des eaux gravitaires et des solutions extraites à pF 4.4

    Science du Sol

    (1982)
  • BramleyH. et al.

    Floodwater infiltration through root channels on a sodic clay floodplain and the influence on a local tree species Eucalyptus largiflorens

    Plant and Soil

    (2003)
  • EdwardsW.M. et al.

    Rainfall intensity affects transport of water and chemicals through macropores in no-till soil

    Soil Science Society of America Journal

    (1992)
  • GerkeH.H. et al.

    A dual-porosity model for simulating preferential movement of water and solutes in structured porous media

    Water Resources Research

    (1993)
  • HainesB.L. et al.

    Soil solution nutrient concentrations sampled with tension and zero-tension lysimeters: report of discrepancies

    Soil Science Society of America Journal

    (1982)
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