4.1 Soil quality declines with ecosystem degradation
The alignment of forest states in the PCA biplots indicates a clear gradient of declining soil quality that parallels the decline in vegetation biomass, complexity, and diversity from natural forest to arid land (Jara-Guerrero et al., 2021; Ribeiro et al., 2019). Similar to other studies that have found differences in total C and N between undisturbed and disturbed SDTF (Andrade et al. 2020; Menezes et al. 2021; Schulz et al., 2016), these two important constituents of SOM decreased steadily with degradation. SDTFs tend to have a larger proportion of biomass belowground compared to wet tropical forests (Murphy and Lugo, 1986), with high biomass of roots in the upper soil layers, especially that of fine roots (Castellanos et al. 1991). Thus, belowground biomass associated with the abundant woody vegetation found in natural, semi-natural and, to a lesser degree, simplified forest, can partly explain why these states have high C, N and WSC content as well as a high C/N ratio relative to shrub-dominated forest and arid land. Higher C and N stocks may also be attributed to the different quality and quantity of the litter input in less degraded forests (Machado et al., 2019). Furthermore, variation in the PC scores was higher in these three states than shrub-dominated forest and arid land, likely due to greater heterogeneity in species composition and functional diversity between sampling sites (Ribeiro et al., 2019; Sagar et al., 2003). The scarcity of woody vegetation in shrub-dominated forest and arid land likely increased water stress and had a homogenizing effect on environmental conditions (Andrade et al. 2020; Jara-Guerrero et al., 2019; Menezes et al. 2021), which was reflected as reduced variation in physical-chemical properties.
Plant growth flourishes in all SDTF states during the rainy season, which probably explains the apparent mitigation of the effects provoked by of the degradation gradient relative to the dry season. Most plants in SDTFs shed their leaves during the dry season, contributing a high flux of nutrients (Anaya et al. 2007) that likely exacerbated physical-chemical differences between natural and semi-natural forest and the other states in this season. WSC was especially high in these two states in the dry season, accumulating in substantial quantities due to limited leaching and decomposition in the absence of significant rainfall. In the rainy season, such differences abated because growth in all vegetative strata and functional types attenuated differences in biomass and fresh organic matter among states. This was especially evident for simplified forest, which overlapped with semi-natural forest in the rainy season. Increased variation in PC scores in the rainy season was also probably due to plant growth in all states, revealing heterogeneity among sites within each state (Waring et al., 2021). Despite these seasonal differences, the consistency in the order of forest states in both seasons reinforces the notion that soil quality declined along the gradient of SDTF degradation.