4.2 Enzymatic activity declines with ecosystem degradation
The likely driver of differences in soil quality among forest states was
enzymatic activity, which decreased at a steady rate along the gradient
of SDTF degradation. In a global meta-analysis, enzyme activity was also
found to decrease due to less substrate availability in intensive
management practices (Sinsabaugh et al., 2008). The decline in enzymatic
activity found in this study paralleled that of soil C and N, which
again can be attributed to differences in vegetation biomass and
diversity among forest states. The largest declines were in
dehydrogenase and β-glucosidase activity, indicating their usefulness as
indicators of soil quality, whereas urease activity also decreased but
not significantly, except in arid land. Likewise, Oliveria Silva et al.
(2019) found that β-glucosidase activity was the best indicator of dry
forest conversion to agriculture in the Brazilian Caatinga. However,
Sandoval-Pérez et al. (2009) found that urease and dehydrogenase were
not good indicators of land-use effects on soil fertility in a Mexican
dry forest. Thus, while enzymatic activity can provide a useful
indicator of soil function in response to anthropic disturbance, these
studies underscore the need for further research to generalize results
for SDTFs.
The activity of all three enzymes was similar between natural and
semi-natural forest, indicating minimal loss of soil function with
moderate degradation of SDTF. In contrast, activity of dehydrogenase and
β-glucosidase was significantly reduced in shrub-dominated forest and
arid land, and urease activity was significantly lower in arid land.
Loss of canopy cover and woody species richness in shrub-dominated
forest and arid land likely caused this severe reduction in soil
function (Singh et al., 2018). This may result from the direct effects
of plant species loss or indirect effects like increased soil erosion
and water stress due to removal of the canopy (Jara-Guerrero et al.,
2021; Singh et al., 2018). However, the results also showed that the
transition to simplified forest that occurs when livestock are excluded
from shrub-dominated forest and natural regeneration is allowed to
proceed (Jara-Guerrero et al., 2019) facilitated the recovery of
moderate levels of dehydrogenase and β-glucosidase activity. Given that
simplified forest is species poor relative to natural and semi-natural
forest, these results suggest that a threshold of canopy cover and tree
density is more important than species diversity to recover the
functionality associated with these enzymes (Singh et al., 2018).
Higher enzymatic activity in the dry season than the rainy season was
counterintuitive, but could be explained by the exact moment soils were
sampled in each season. For the rainy season, samples were collected
near the end of the season in May when the leaf litter and SOM had
largely been decomposed and leaf fall had not yet occurred (Anaya et
al., 2007). The low quantity of substrate may explain why enzymatic
activity was significantly reduced in the rainy season. For the dry
season, soils were sampled in October following the peak of leaf fall
and after several months of accumulation of plant litter and exudates
(Anaya et al., 2007), which increase the OM content of the soil and in
the process mitigate the conditions to which soil microbes are exposed,
particularly soil moisture. Both urease and β-glucosidase are
extracellular enzymes, meaning they can continue to function as long as
some moisture remains, even after the cells that synthesized them are
gone (Barbosa et al. 2023). Strong correlations between most soil
properties and the three enzymes indicated the importance of these
parameters for enzyme function in the dry season, whereas total N and
WSC were highly correlated with enzyme function in the rainy season,
which were likely related to OM flush events caused by rainfall (Murphy
and Lugo, 1986; Oliveria Silva et al., 2019).