Quantifying and modelling the impact of land consolidation and field borders on soil redistribution in agricultural landscapes (1954–2009)
Introduction
During the last decades, soil erosion rates in cultivated areas of Western Europe have intensified and have become a problematic issue for farmers and rural communities. When it is triggered by heavy storms, soil erosion and the associated muddy floods can have disastrous and costly consequences (e.g. Evrard et al., 2007a, Pimentel et al., 1995). Additional concerns about soil erosion are related to its subsequent negative impacts, such as water pollution, decline in biodiversity and crop yields or reduction of soil water storage capacity or organic carbon sequestration (e.g. Andraski and Lowery, 1992, Berger et al., 2006, Boardman and Poesen, 2006, Papiernick et al., 2009). Because soil is a non-renewable resource at human timescales, soil protection is crucial. Quantification of erosion and deposition rates and the identification of their driving processes and their spatial variability therefore constitute a prerequisite to develop and implement soil protection strategies.
Traditionally, soil redistribution processes observed on croplands in western Europe were mainly attributed to water erosion processes. In that case, transport intensity is controlled by topographical settings such as slope, drainage area and planform curvature (Chaplot and Le Bissonnais, 2003, Foster, 1986, Poesen, 1984). However, research has shown that tillage operations result in significant soil redistribution in intensively farmed cropland (e.g. Govers et al., 1994, Lindstrom et al., 1992). Tillage erosion results from the net downslope translocation of soil, controlled by slope gradient change, during farming operations (Boardman et al., 1994, Lindstrom et al., 1990, Govers et al., 1994, Montgomery et al., 1999, Van Muysen et al., 1999). As shown by Govers et al. (1994), water and tillage-induced erosion depend on different topographical parameters resulting in a specific spatial signature in the landscape. Tillage erosion is the most intense on landscape positions where water erosion is minimal (i.e. on convexities and in the proximity of upslope field boundaries), whereas areas of tillage deposition often coincide with areas of maximal water erosion as hollows (Govers et al., 1994, Govers et al., 1996, Van Oost et al., 2000).
The hydrological and sedimentological connectivity across cultivated hillslopes is to a large extent controlled by the presence of field borders and associated linear elements, such as hedges, roads, furrows and grass strips, which induce landscape fragmentation (Follain et al., 2006, Szilassi et al., 2006, Van Oost et al., 2000). Vegetated borders (e.g., grass strips, hedges and grassed waterways) can reinfiltrate surface runoff and trap sediment transported by water (Caubel et al., 2003, Evrard et al., 2008, Van Dijk et al., 1996). In contrast, concave anthropogenic features (e.g., furrows) provide preferential drainage pathways, thereby increasing hydrological and sedimentological connectivity across the landscape. Linear landscape elements with a compacted surface (e.g. roads and land tracks) have a limited infiltration capacity and then enhance runoff and hydrological connectivity (Forman and Alexander, 1998, Wemple et al., 1996). In tilled fields, all types of field borders act as lines of zero-flux (Dabney et al., 1999, Van Oost et al., 2000). Consequently, tillage-induced deposition and erosion preferentially occur upslope and downslope of field borders that are oriented parallel to contour lines.
Field borders can therefore act as barriers to water and sediment fluxes generated by both water and tillage erosion (Dabney et al., 1999, De Alba, 2003, Govers et al., 1999, Knapen et al., 2008, Van Dijk et al., 2005). Interaction between erosion and deposition processes at the vicinity of field borders leads to the development of anthropogenic linear landforms of several metres width (e.g., ridges-and-furrows, headlands, and lynchets). These features are common in the agricultural landscapes of Western Europe (Callot, 1980, Hooke, 1988, Zadora-Rio, 1991). These landforms are not conserved after the removal of the field border but will instead keep evolving, and may finally lead to the formation of undulations (Chartin et al., 2011, Houben, 2008). Assessing the effect of field borders and their potential removal is then essential to understand past, present and future spatial patterns of soil redistribution and soil properties in the current global change context.
Agricultural policy and mechanisation led to the massive removal of field borders through the implementation of numerous land consolidation schemes between 1960 and 1990 in Western Europe (Baudry and Burel, 1984, Vitikainen, 2004), and is still ongoing in some regions. The use of 137Cs-technique can therefore offer a solution to evaluate soil redistribution over these last decades (e.g., Ritchie and McHenry, 1990, Rogowski and Tamura, 1965, Walling and Quine, 1992).
This study aims to quantify and improve our understanding of the effects of land consolidation and field borders on mid-term soil erosion and on agricultural landscape evolution. In order to achieve this objective, we analysed spatial patterns of 137Cs inventories for an agricultural hillslope that was subjected to land consolidation. The study area is representative for field consolidation in intensively cultivated areas of the southwestern Parisian basin (France). We used a spatially-distributed model to convert these observations into soil redistribution rates. Emphasis will be put on the impact of small-scale topographical features, especially lynchets and undulations landforms associated with local soil accumulation along current and former field borders. Finally, we discuss the wider implications of our findings in relation to agricultural landscape evolution and model development.
Section snippets
Location and physiographical settings
The study was conducted on a 3.7 ha field located at the downslope part of a south-east facing hillslope in the southwestern Parisian Basin, France (47°08.31′N, 0°10.97′E) (Fig. 1). The area is part of the Quincampoix watershed and is characterised by an undulating topography commonly observed in terrains underlain by Cretaceous chalks in this region. In the study field, the elevation ranges between 43 and 60 m with slope between 0 and 8.8%. Two types of linear anthropogenic landforms – lynchet
Recent soil redistribution patterns and relations with topographical settings
The observed 137Cs reference inventory was 1367 ± 30 Bq·m− 2 in 2009 in Seuilly (Fig. 5a). About 40% and 65% of the total 137Cs inventory was concentrated in the uppermost 5 cm and 10 cm, respectively. As observed at many other undisturbed locations, the 137Cs content declined almost exponentially with soil depth in these profiles (Walling and Quine, 1992).
The 137Cs residuals (Eq. (2)) ranged from − 1030 Bq·m− 2 to 980 Bq·m− 2 in the 68 analysed cores. The largest variations were observed along the
Conclusions
This study focused on the impact of land consolidation and field borders on medium term soil erosion (1954–2009) and landscape morphology within a 3.7-ha field consolidated in 1967. The spatial patterns of 137Cs inventories were analysed and a spatially-distributed conversion model simulating the respective implication of water and tillage erosion was applied. The model simulated tillage erosion as the most dominant process across the study area, by generating 95% of soil redistribution
Acknowledgements
This project was funded by ANR (Agence Nationale de la Recherche) in the framework of the LANDSOIL project (ANR-08-VULN-006). The authors would like to gratefully thank Jean-Paul Bakyono and Isabel Pene-Galland for their role on field data collection and sample preparation. This paper was much improved thanks to the comments of two anonymous referees. This is LSCE contribution no. 5013.
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