Results and discussion
Along the natural salinity gradient, the SIC was higher at low salinity,
< 6‰, and declined with further salinity elevation (Fig. 2a),
suggesting that salinity stress influences SIC cycling (Wang et al.,
2019). SIC has two potential determinants: (1) uptake and deposition of
atmospheric CO2 by base cation (Xie et al., 2009; Raheb
et al., 2017), and (2) re-distribution of respired CO2among microorganisms and roots (Huber et al., 2019). pH markedly
influences the magnitude of SIC, soil acidity at low pH results in SIC
loss, and soil alkalinity at high pH increases atmospheric and soil
CO2 absorption of base cations (Liu et al., 2020a; Raza
et al., 2020). Slight pH alterations under different saline conditions
indicate that SIC alterations are not attributed to regional atmospheric
CO2 deposition (Fig. S2). Furthermore, SOM accumulation
due to improved plant communities may increase microbial and root
respired CO2, retaining SIC via base cation absorption
(Lal and Kimble, 2000). Highly photosynthetic crops andPhragmites in low-salinity soil results in higher SIC and SOC
than with low-photosynthetic Tamarix chinensis and Suaeda
salsa in high-salinity soil (Xia et al., 2019; Fig. S3, Fig. S4). Thus,
SIC is increased through changes in organic matter decomposition,
further verified through the direct effect of SOC on SIC (Fig. 2c).
Low-salinity soil contains more SOC, displaying a substantial decline at
high salinity of > 6‰ (Fig. 2a). The current view has
pointed that plant- and microbially-induced C drive SOC storage via
litter decomposition and OM transformation (Schmidt et al., 2011; Ding
et al., 2019). Plants displaying high-to-low photosynthetic C fixation
and the consistent trend between microbial residues and SOC indicate
that plant and microbial residues increase SOC storage with increasing
salinity (Fig. 2a). However, it is still unclear whether other soil C
components linking inorganic or organic biogeochemical processes as
potential determinants of SOC accumulation, e.g., SIC (primarily
carbonate) account for soil C in approximately 70% of these regions. We
assume that SIC not only provides C source to autotrophic microorganisms
(e.g., cyanobacteria) (Moore et al., 2020), but also serves as a
“reserving SOM source” via bio-transformation (Miltner et al., 2004).
Significantly linear associations among microbial residues, SOC, and SIC
indicate that SIC is a potential determinant of SOC components (Fig.
2b). Furthermore, SEM helped determine the causality and underlying
mechanism of SIC-microbial residues-SOC (Fig. 2c), suggesting microbial
residues as an indicator for SIC transition to SOC, and verifying our
hypothesis.
Despite regulation of terrestrial C cycles through two microbial
metabolic processes, i.e., atmospheric and soil C release through
microbial catabolism and respiratory CO2 fixation into
carbonate via base cation absorption (Zhao et al., 2020), microbial
anabolism-derived C contributes to the SOC pool (Liang et al., 2017). To
better understand soil C cycling, this study describes microbial
SIC–SOC conversion, which is important to understand the role of SIC in
C cycling in terrestrial ecosystems. SIC generally serves as a potential
C sink and limited information is available regarding its effect on
terrestrial C-energy process because of its long-term persistence, high
stability, and limited bio-availability (few biomes use SIC as
resources) (Zamanian et al., 2018; Liu et al., 2020b). We report the
contribution of SIC to SOC stocks, indicating that SIC provides
inorganic C-source for other soil biomes after conversion from
autotrophic microorganisms. We could not determine the relevant
microbial communities or determine the magnitude of the SIC-to-SOC
transition. Future studies need to investigate the association between
genomic and metabolomic factors in vitro and in situ ,
especially on the molecular and metabolic process involved in the
microbial SIC metabolism and assimilation and the mechanisms followed by
specific microbial species, using 13C-labeled
CO2 or carbonate. Moreover, understanding the role of
microbial necromass in SIC transition would enhance SIC applications for
C biogeochemical cycles in global saline and dry lands, accounting for
the provision of “missing C” to enrich the organic C reservoir through
interactions among SIC-microbial metabolites-SOC.
This work was supported by the National Natural Science Foundation of
China (41971119, 41871089), the Natural Science Foundation of Shandong
Province (ZR2019MD024), the Youth Innovation and Technology Foundation
of Shandong Higher Education Institutions (2019KJD010), and the
Doctoral
Research Start-up Fee of Binzhou University (2019Y18).