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.