First approximation of soil quality critical limits in land use systems in the Brazilian Amazon

Soil quality study as an assessment tool can contribute to soil management and guide land use decisions. We developed and evaluated soil quality indices (SQI) for land use systems in the Brazilian Amazon and identified the critical limits of soil quality as a first approximation. Additionally, we proposed an equation to determine Denardin’s Erodibility factor by a simpler and accessible method. We performed 21 physical, chemical, and biological analyses of 225 soil samples (0–20 cm depth) from five land use systems (LUS). We integrated and compared 12 SQIs using two data sets (total and minimum), two scoring methods (linear and non-linear), and three SQI integration approaches (Additive, Weighted additive, and Nemoro). The results indicate that in all LUSs, the most significant and best-estimated integration of soil indicators was the Additive SQI using the minimum data set and linear scoring method. Only clay content and Denardin’s Erodibility factor from the minimum data set showed significant linear regressions and high coefficients of determination (r² > 0.50, P < 0.05) with the Additive SQI. According to them, we determined the critical limits of the SQI in each LUS. Clay content threshold values to diagnose low soil quality were 225.28 and 171.89 g kg⁻¹ for the Agroforestry Systems, 210.74 g kg⁻¹ for the Silvopastoral System, 201.23 g kg⁻¹ for the Pasture System, and 198.29 g kg⁻¹ for Natural Forest. Erodibility threshold values to diagnose low soil quality were 0.0657, 0.0656, and 0.0596 t h MJ⁻¹ mm⁻¹ only for the Silvopastoral, Natural Forest, and Pasture systems, respectively. These findings suggest that the soils from the Pasture System and Natural Forest are vulnerable to anthropogenic soil degradation because they present steep linear slopes between the Erodibility factor and the soil quality critical limits. In these LUSs, we identified significant and mathematically plausible linear regression equations for determining Erodibility factor.