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.
Acknowledgements
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).