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.