Effects of land use on soil erosion in a tropical dry forest ecosystem, Chamela watershed, Mexico

During the past few decades, Mexico has been converting tropical dry forest (TDF) into cropland and pasture, with land degradation expressed as soil erosion being the main environmental consequence. The factors and processes influencing soil erosion are related to scale. At a microscale, the stability of soil aggregates has a significant impact on soil credibility and strongly influences other soil properties. However, at plot and watershed scales, these relationships are less well known. The relationships between the distribution of soil aggregate size, soil properties and soil erosion were examined for two soil geomorphological units (hillslopes over granite and hillslopes over tuffs) and three land uses (TDF, unburned pasture and burned pasture) within the Chamela watershed of west-central Mexico. To evaluate soil aggregation as a parameter for upscaling soil erosion, the researchers measured microtopographic features at plot scales and interpreted 1:35,000 panchromatic aerial photographs at a watershed scale. Analysis of variance indicated significant differences in soil organic carbon (P < 0.05) and soil moisture (P < 0.01) contents between the two soil geomorphological units, and field tests showed differences in soil texture and structure. As a consequence, the macroaggregate content (> 1 mm) and the mean weight diameter (MWD) are significantly greater (P < 0.05) with the same land use on the hillslopes over tuffs. At a plot scale, this is reflected by subtle erosion features (surface litter movement), while at a watershed scale 8 km-km(-2) of drainage density occurred without the development of gullies. On the hillslopes over tuffs, burned pasture has the lowest macroaggregate content and MWD, which may explain the stronger erosion features observed at the plot scale (as splash pedestals, terracettes, surface crusting and prerills) and higher drainage density (8-13 km-km(-2)) with few gullies. Despite soil erosion differences, the macroaggregate content and MWD for TDF over granite are similar to those for burned pasture over tuff. The main conclusions of this study are that soil aggregation and MWD are useful indicators of the strength of erosion features at watershed and plot scales when evaluated in the same soil geomorphological unit. The soil aggregation differences between the two soil geomorphological units confirm the need for a geornorphological stratification of the landscape. Combining analysis of soil aggregation and soil erosion within a geomorphic framework helps identify landscape units susceptible to erosion and land degradation following land-use change. (c) 2005 Elsevier B.V. All rights reserved.