4. Discussion
Our results showed a consistent and steady reduction in soil quality and function along the gradient of ecosystem degradation in SDTF. While previous studies have shown loss of soil organic carbon (SOC) (Andrade et al., 2020; Maestre et al., 2022; Menezes et al., 2021) and total nitrogen (Andrade et al., 2020) resulting from the conversion of SDTF to different land uses, few studies have focused explicitly on the soil effects of SDTF degradation (Schulz et al., 2016). We show for the first time a significant and uniform decrease in POC and MAOC along a broad gradient of ecosystem degradation in SDTF. The relative proportion of these two components of SOC was constant along the gradient except for the most degraded state (arid land), where POC was higher in proportion to MAOC, suggesting that a functional tipping point may be crossed in the transition from shrub-dominated forest to arid land (Ghazoul and Chazdon, 2017). The patterns of soil C were corroborated by a reduction in the activity of dehydrogenase and β-glucosidase, while those of N led to a decrease in urease activity. These findings address an important knowledge gap for SDTFs by showing a consistent loss of SOM and soil functionality with degradation and suggest that extreme degradation can result in an alternate state with compromised resilience (Oliver et al., 2015).