Introduction
Soil inorganic carbon (SIC), usually as a “missing C sink”, accounts
for 60–80% soil C stocks, and essentially influences soil C cycles in
saline or dry lands (Mi et al., 2008; An et al., 2019). SIC can be
converted to soil organic carbon (SOC), providing organic C source to
soil organisms (Groshans et al., 2018). Although plant assimilated
inorganic C is the major SOC source that atmospheric and soil-released
CO2 fixed via photosynthesis contributes to the SOC pool
through litter and root input (Figure 1, Le Quéré et al., 2018),
microorganisms potentially contribute to the conversion of SIC to SOC
(Miltner et al., 2004; Liu et al., 2020b). Thus, it is important to
speculate the potential mechanisms in the microbially driven SIC-to-SOC
transition, thus facilitating soil C efflux and improving soil fertility
in resource-limited saline lands.
Most microbes use organic matter as energy for reproduction and growth,
contributing to the SOC pool through microbial cell residues
(Kögel-Knabner, 2017; Ni et al., 2020). However, certain microbes, e.g.,
photosynthetic bacteria, potentially use SIC (mainly carbonate) as C
source to synthesize cell (Zamanian et al., 2016; Moore et al., 2020).13CO2 incubation experiments have
reported that soil autotrophic microorganisms can utilize inorganic C to
synthesize microbial biomass (Perez and Matin, 1982; Miltner et al.,
2004). Although living microbial biomass constitute only 2-4% of SOC
(Dalal, 1998), rapid, iterative microbial cell growth and death generate
sizable microbial necromass entombing in soils, accounting for
> 50% SOM (Kallenbach et al., 2016; Liang et al., 2019).
Furthermore, Liang et al. (2017) proposed the concept of a microbial
carbon pump (MCP), providing novel mechanistic insights into SOM
formation involved in microbial anabolism. Such knowledge and the MCP
concept would help predict microorganisms converting SIC to SOC via
microbial necromass (Fig. 1).
A systematic field survey was performed to explore the shift in soil C
components (i.e., SIC, SOC, and microbial residues) along a natural
salinity gradient. Microbial residue mediation in soil C transformation
was evaluated using biomarkers including amino sugars. We aimed to
examine whether the trends in soil C components were consistent from low
to high salinity determine the mechanism underlying SIC—microbial
residues—SOC.