4.3 Drivers of soil organic matter fractions
The uniform decline in POC and MAOC in the same relative proportion
among forest states except arid land indicated similar effects of
degradation on both fractions. POM and MAOM are assumed to have
different C turnover rates and degrees of physical-chemical protection
owing to distinct origins (Lavallee et al., 2020; Nicolás et al., 2012).
Comparing these fractions between different ecosystems, Yu et al. (2022)
found that 90% of grassland/shrubland soils stored more SOC as MAOM
than POM, whereas 40% of forest soils exhibited a higher POM fraction,
which was attributed to the contribution of woody vegetation. Similarly,
POC was shown to be highly sensitive to soil changes resulting from the
conversion of dry forest to crops in the semiarid Chaco (Villarino et
al., 2017). Focusing on the C component in POM and MAOM, we thus
expected a proportionally larger fraction of POC in less degraded SDTF
states given its origin primarily from plants (Baldock and Skjemstad,
2000). Contrary to our expectations, however, the POC/MAOC ratio was
constant across natural, semi-natural, simplified and shrub-dominated
forests in both seasons, despite large differences in vegetative biomass
and structure among these states. This suggests that POC and MAOC were
equally susceptible to forest degradation and, in general, degradation
resulted in SOC loss despite mineral protection in soil aggregates. In
contrast, studies from other ecosystems show a decoupling of POC and
MAOC. For example, soils with high SOC tend to store more C as POC given
that MAOC reaches saturation (Cotrufo et al., 2019). In disturbed soils,
MAOC is more stable and less sensitive to disturbance (Sokol and
Bradford, 2019). However, our results support the hypothesis that the
two fractions are coupled, suggesting that microbial and plant residues
contribute to both components to a similar extent and with the same
dynamics (Angst et al., 2021; Córdova et al., 2018; Yu et al., 2022). It
is also possible that soil texture, SOC content, and precipitation play
important roles in the distribution of organic C within physically and
chemically defined pools (Haddix et al., 2020; Yu et al., 2022). Despite
significant progress toward understanding the intrinsic mechanisms of
POC and MAOC distribution and functions in soils, further research is
needed to identify the primacy of biotic versus abiotic controls on
their dynamics in SDTFs subjected to different degrees of disturbance.
The higher POC/MAOC ratio in arid land compared to the other forest
states implies a functional tipping point with extreme degradation
(Ghazoul and Chazdon, 2017). This anomaly in arid land is likely the
outcome of several related factors. First, the microbial C pump
conceptualized by Liang (2020) that transforms plant residues to POC and
finally to MAOC through microbial anabolism was probably disrupted by
the harsh biotic and abiotic conditions of this state. Functioning of
this mechanism occurs via microbial transformation of POC through
microbial biomass and ultimately conversion into MAOC. Low inputs of
leaf litter and root exudates as well as water stress inhibit microbial
activity and biomass and constrain the C pump (Liang 2020), which could
lead to changes in the proportions of POC and MAOC. Likewise, low
vegetative cover means that arid land is prone to erosion, which leads
to nutrient leaching and depletion of SOC stocks (Villarino et al.,
2017). Second, lower quality litter tends to favour the formation of POC
over MAOC (Cotrufo et al., 2013). Therefore, low litter quantity and
quality combined with water stress in arid land likely contributed low
decomposition rates (Machado et al., 2019) which, in turn, inhibited the
conversion of POC into MAOC. Third, sandier soils in arid land could
inhibit the formation of MAOC given that it tends to associate with fine
silt and clay particles (Lavallee et al., 2020; Machado et al. 2019;
Schulz et al. 2016). Any or all of these factors could result in a lower
proportion of MAOC compared to less degraded states, altering the
delicate balance between these two fractions and disrupting an important
function of SOC storage and turnover. Given the implications of this
tipping point for the resilience of the STDF ecosystem, we urgently need
to better understand the mechanism underlying this result.