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Spatial Variability of Soil Phosphorus in Relation to the Topographic Index and Critical Source Areas

Sampling for Assessing Risk to Water Quality

^a Environmental Science Department, University of Lancaster, Lancaster, LA1 4YQ, UK
^b Soil Science and Environmental Quality Team, Institute of Grassland and Environmental Research, North Wyke Research Station, Okehampton, Devon, EX20 2SB, UK
^c National Soil Resources Institute, Cranfield University, North Wyke Research Station, Okehampton, Devon, EX20 2SB, UK
^d National Soil Resources Institute, Cranfield University, Silsoe, Bedfordshire, MK45 4DT, UK

View larger version (61K): [in a new window]   Fig. 1. Plan view of the Den Brook catchment showing (a) a schematic layout of main catchment features including subclassified areas (see sector key in Table 1) and (b) soil sampling transects and locations overlaying a topographic index map of the catchment. Codes refer to transect identification (see Table 3). The term ln(/tan ß) denotes the topographic index, where is the upslope contributing area to a given point in the catchment and ß is the local surface slope angle.

View larger version (56K): [in a new window]   Fig. 2. Plan view of the Drewston catchment showing (a) a schematic layout of main catchment features including subclassified areas (see sector key in Table 1) and (b) soil sampling transects and locations overlaying a topographic index map of the catchment. Codes refer to transect identification (see Table 4). The term ln(/tan ß) denotes the topographic index, where is the upslope contributing area to a given point in the catchment and ß is the local surface slope angle. The wetland is located downstream of the arrow identifying transect MFG1–MFG8.

View larger version (8K): [in a new window]   Fig. 3. Box and whisker plots for Den Brook and Drewston showing distributions of Olsen P (0- to 7.5-cm depth) for all spatial samples. The boxes show the lower quartile, median, and upper quartile and the whiskers show the minimum and maximum concentrations measured.

View larger version (53K): [in a new window]   Fig. 4. Spatial plot of Olsen P (0- to 7.5-cm depth) results for Den Brook. Note that the size of the circles indicates relative magnitude of P concentration (see legend for minima and maxima, Table 1 for sector descriptions, and Table 3 for full results).

View larger version (56K): [in a new window]   Fig. 5. Spatial plot of Olsen P (0- to 7.5-cm depth) results for Drewston. Note that the size of the circles indicates relative magnitude of P concentration (see legend for minima and maxima, Table 1 for sector descriptions, and Table 4 for full results).

View larger version (22K): [in a new window]   Fig. 6. The variation of concentration with depth for Olsen P, water-soluble total phosphorus (WSTP), and water-soluble reactive phosphorus (WSRP) for (a) Den Brook and (b) Drewston. See Tables 5 and 6 for standard deviations.

View larger version (12K): [in a new window]   Fig. 7. Scatter plots showing topographic index [ln(/tan ß)] vs. Olsen P (0- to 7.5-cm depth) concentration for (a) Den Brook and (b) Drewston.

View larger version (15K): [in a new window]   Fig. 8. A comparison of distributions of Olsen P (0- to 7.5-cm depth) concentrations measured in the present study (black columns) and concentrations from the National Soil Inventory (NSI) database for similar land use and soil type for (a) Den Brook and (b) Drewston.