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).