measured in a VARIAN 220FS SpectrAA atomic absorption
spectrometer (USDA, 1972). All atomic absorption spectroscopy
measurements were performed in triplicate and standard solutions
(Merck quality) were used for calibration.
The state of soil erosion and the erodibility factors were
determined by bivariate correlation analysis (Spearman correlation
coefficient, Rho) and multiple linear regression analysis (stepwise
method). The Bonferroni correction was applied to adjust the
significance level (alpha) of the 24 correlations obtained between
erosion and the independent variables included (alpha = 0.05/
24 = 0.002). The erosion threshold, in relation to erodibility factors,
was obtained by means of two types of variance analysis (Kruskal–
Wallis test for several independent variables and Mann–Whitney
testfor twoindependentvariables). Thefirstwas a generalanalysis in
which the results of erosion were distributed at different intervals,
usingthe soilerodibilityfactors as the grouping variable. Twocriteria
were applied for this purpose: (a) the distribution should present
regular intervals, that is, representing a constant rate of growth (for
example, at intervals of 10: 10–20, 20–30; or of 20: 10–30, 30–50),
because the variance is sensitive to the range if the results present a
high degree of dispersion; (b) the distribution should present a
minimum size (n) of 3 cases in each interval. The second type of
analysis performed concerned the variance between the erosion
and the erodibility factors found to be significant in the previous
analysis. This analysis consisted of examining pairs of consecutive
intervals. The erosion threshold was established at the boundary
betweenthe two intervals ofthepairs analysis for whichthehighest
levelof statistical significancewas obtained.Allthese analyzeswere
conducted using SPSS 22.0 software.
measured in a VARIAN 220FS SpectrAA atomic absorptionspectrometer (USDA, 1972). All atomic absorption spectroscopymeasurements were performed in triplicate and standard solutions(Merck quality) were used for calibration.The state of soil erosion and the erodibility factors weredetermined by bivariate correlation analysis (Spearman correlationcoefficient, Rho) and multiple linear regression analysis (stepwisemethod). The Bonferroni correction was applied to adjust thesignificance level (alpha) of the 24 correlations obtained betweenerosion and the independent variables included (alpha = 0.05/24 = 0.002). The erosion threshold, in relation to erodibility factors,was obtained by means of two types of variance analysis (Kruskal–Wallis test for several independent variables and Mann–Whitneytestfor twoindependentvariables). Thefirstwas a generalanalysis inwhich the results of erosion were distributed at different intervals,usingthe soilerodibilityfactors as the grouping variable. Twocriteriawere applied for this purpose: (a) the distribution should presentregular intervals, that is, representing a constant rate of growth (forexample, at intervals of 10: 10–20, 20–30; or of 20: 10–30, 30–50),because the variance is sensitive to the range if the results present ahigh degree of dispersion; (b) the distribution should present aminimum size (n) of 3 cases in each interval. The second type ofanalysis performed concerned the variance between the erosionand the erodibility factors found to be significant in the previousanalysis. This analysis consisted of examining pairs of consecutiveintervals. The erosion threshold was established at the boundarybetweenthe two intervals ofthepairs analysis for whichthehighestlevelof statistical significancewas obtained.Allthese analyzeswereconducted using SPSS 22.0 software.
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