Abstract
Land use and land cover changes (LUCC) can drastically alter various components of the critical zone, including soil thickness and soil chemical weathering processes. Often these studies, however, tend to focus on extreme cases, not representing what actually happens on average at larger, regional scales. Here, we evaluate the impact of LUCC on soil thickness and soil weathering degree at the regional scale, where we use soil spectroscopy to derive weathering indices. In a subtropical region in Southern Brazil, we collected calibration/validation soil samples (n = 49) from 4 different locations for which we measured the mid-infrared (MIR) spectral reflectance and 3 soil chemical weathering indices: chemical index of alteration (CIA), the total reserve in bases (TRB), and the iron ratio (Fed/Fet). We used partial least square regressions on this calibration/validation dataset to relate the MIR spectra of the soil samples to these weathering indices, resulting in good calibration relationships with R2 values of 0.97, 0.91 and 0.84 for CIA, TRB and Fed/Fet, respectively. Applying these relations to MIR spectra of regionally collected soil samples allowed us to calculate soil weathering degrees for a large number of soil samples (n = 229), without requiring costly and time-consuming chemical analyses. We collected these soil samples at 100 mid-slope positions: 50 under forest and 50 under agricultural land use. Land use explained only a minor part of the variation in soil thickness and weathering degree. Thus, while local water and tillage erosion rates might be considerable after deforestation, this has not led to significant reductions in average soil thickness and has not affected soil weathering degree. Slope gradient is the main factor influencing the spatial variability in soil thickness and weathering degree on mid-slope sections in our study area. Human activities over the last century did not fundamentally alter these patterns.
Highlights
The critical zone (CZ) is the outer layer of the Earth’s surface, extending from the upper limit of the vegetation down to the ground water
We evaluated the calibrations using the coefficient of determination (R2) and the Akaike information criterion (AIC) that was calculated for the validation sub-dataset
We first showed that mid-infrared spectroscopy can be used to successfully estimate different weathering indices, thereby confirming that this technique can be used to rapidly obtain information about soil weathering degree at a relatively large spatial scale
Summary
The critical zone (CZ) is the outer layer of the Earth’s surface, extending from the upper limit of the vegetation down to the ground water. Increasing demand for agricultural land, due to the growing world population and changing food patterns, has drastically increased deforestation and land use and land cover change (LUCC) (Foley, 2011; Godfray et al, 2010; Lal, 2001) These changes have been documented to strongly impact different components of the critical zone, leading to reduced soil organic carbon content Physical soil properties, and soil chemistry and weathering are influenced by forest conversion to cropland, which can decrease the acidity of soils (Ameijeiras-Marino et al, 2018; Lucas, 2001), especially due to the disturbance of the acidic upper layer of the forested soil profiles (Lundstrom, 1994), and the application of fertilizers and lime (Cornu et al, 2007; van Breemen et al, 1983). Such external inputs have been linked to changes in soil acidity (Cornu et al, 2007; Goulding, 2016; van Breemen et al, 1983) and clay mineralogy (Bortoluzzi et al, 2012; Caner et al, 2014) and could influence soil formation and weathering dynamics
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