Soil erosion, diffuse source pollution and sediment problems associated with maize cultivation in England
Date: 3 February 2010
Thesis or dissertation
University of Exeter
PhD in Geography
This thesis reports an investigation of soil erosion problems associated with maize cultivation in England. To place the investigation into a broader context, the study commences with a review of soil erosion problems more generally, before focusing on the specific on-site and off-site problems associated with maize cultivation. ...
This thesis reports an investigation of soil erosion problems associated with maize cultivation in England. To place the investigation into a broader context, the study commences with a review of soil erosion problems more generally, before focusing on the specific on-site and off-site problems associated with maize cultivation. Agricultural statistics are used to quantify the recent expansion of maize cultivation in England and attention is directed to both the temporal trends and spatial patterns involved. A major expansion of maize cultivation occurred in England between 1990 and 2000. Particular attention is then directed to the expansion of maize cultivation in Southwest England, since this is a very important area for cultivation of forage maize. Against this general background, a more detailed investigation of soil erosion associated with maize fields and its impact on the local streams and rivers was undertaken. This focused on two river catchments, namely the River Culm above Cullompton, Devon, and the River Tone above Taunton, Somerset. These two basins were selected as having a high density of maize fields within their catchments. A detailed field survey was undertaken to identify all the fields used for maize cultivation in the two catchments during 2004 and the previous two years and to provide a map of their location. More detailed work, aimed at quantifying both gross and net rates of soil loss, was undertaken on six fields selected to be representative of maize fields in the two catchments. Beryllium-7 measurements were used to estimate the erosion associated with a period of heavy rainfall in late December 2004 and early January 2005, when the harvest fields were left in a bare compacted conditions, with little or no vegetation cover and field observations indicated that significant erosion occurred. The results obtained from the beryllium-7 measurements which related only to the short period in late 2004 and early 2005 were complemented by caesium-137 measurements in the same fields which were used to obtain an estimate of the longer-term (i.e. ca. 45 years) mean annual erosion rates associated with the more traditional land use that had characterized these fields prior to the introduction of forage maize cultivation. These results indicated that the introduction of maize cultivation increased gross and net rates of soil loss by ca. 4 and 8 times, respectively and significantly increased sediment delivery ratios, resulting in more efficient delivery of sediment from the eroding fields to the streams. An assessment of the likely impact of sediment mobilised from the maize fields within the catchments of the River Culm and River Tone during winter 2004-5 was made by establishing a sediment monitoring and sampling programme at the downstream gauging stations on these two rivers over the period November 2004 to March 2005. Estimates of the suspended sediment loads of both rivers were obtained for this period and these were compared with an estimate of the total amount of sediment delivered to the water courses in the two catchments from maize fields based on an upscaling of the results obtained from the beryllium-7 measurements undertaken on the six representative fields. Uncertainties regarding both field to channel and within channel and floodplain conveyance losses precluded definitive comparison of the estimates of the amounts of sediment delivered to the water courses from maize fields with the measured sediment loads. However, the results obtained demonstrated the likely importance of the contribution from eroding maize fields to the suspended sediment loads of the Rivers Culm and Tone during winter 2004-5. The geochemical properties of suspended sediment collected from the two rivers were also compared with the equivalent properties of soil collected from the surface of maize fields within the two study catchments, in order to provide further evidence of the impact of maize cultivation on their suspended loads. The available geochemical data confirmed that much of the sediment transported by the Rivers Culm and Tone could have been mobilized from maize fields, but the lack of detailed geochemical data, precluded a definitive conclusion regarding its source. The results obtained from the field-based component of the study have been combined with the information on the regional and national patterns of maize cultivation and synthesized to provide a general assessment of the likely environmental impact of maize cultivation in England. This information has in turn been used to consider the potential for developing improved land management practises to reduce the environmental impact of maize cultivation within the context of the EU Common Agricultural Policy (CAP) and the Water Framework Directive (WFD). Finally, recommendations for the further development and extension of the study are provided.
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