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).