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There is evidence of higher levels of pedogenic carbonate (PIC) than soil organic carbon (SOC) in cropland, and a positive relationship between PIC and SOC in salt-affected soils of arid and semi-arid regions. This study is designed to test the hypothesis that PIC is influenced by intensive cropping and salinization in semi-humid regions, in which soil carbonate (SIC) often exceeds SOC. We select a typical cropland with a maize-wheat rotation in the North China Plain, which covers two distinct regions, i.e. the Hebei Plain (HBP) under intensive cropping and the Yellow River Delta (YRD) under soil salinization. Our data show large variations in soil carbon stocks, with slightly higher values for PIC (3.9-14.5 kg C m -2 ) relative to those of SOC (2.2-9.2 kg C m -2 ) in the top 1 m. On average, SOC stock is 5.65 kg C m -2 in the YRD, which is slightly lower than in the HBP (6.21 kg C m -2 ); SIC is significantly higher in the YRD (16.9 kg C m -2 ) relative to the HBP (13.7 kg C m -2 ).However, PIC stock is smaller in the YRD (8.67 kg C m -2 ) relative to the HBP (9.41 kg C m -2 ). Despite no clear SIC-SOC relationship, there exists a significant positive correlation (P < 0.01) between PIC and SOC stocks in the study area. The PIC:SOC ratio is generally greater than one over a 0-100 cm layer in the majority of croplands in the north China, with larger ratios in the salt-affected soils. Our analyses suggest that the formation and storage of PIC are regulated by levels of SOC and Ca 2+ /Mg 2+ in soil profiles, and there is large potential for enhancing carbon sequestration as carbonate under intensive cropping through sound management in the cropland of arid, semi-arid and semi-humid regions..

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This study aims to improve our understanding of the mechanisms regulating the stability of soil organic carbon (SOC) in saline–alkaline soils. We hypothesized that desorption of SOC is greater in higher salinity/higher pH soils, and the higher level of soil inorganic carbon (SIC) leads to enhanced SOC stability (or weaker SOC desorption) in saline–alkaline soils like those of the Yellow River Delta (YRD). We collected soils from 23 cropland sites (from 0–100 cm) along three lines (upper, middle, and lower lines, with different salinity) across the Yellow River, which have high pH (7.97–9.02). We analyzed SOC, SIC, and water‐extractable organic carbon (WEOC) and salts. Our data showed a large range in SOC (2.3–11.7 kg C m−2) and SIC stocks (13.3–24.7 kg C m−2) over 0–100 cm. The lowest SOC and SIC stocks were found in the lower YRD where soil pH was highest (8.6–9.0). WEOC:SOC ratio (an indicator for SOC desorption) was greater in soils with higher pH, lower SIC, and/or water‐extractable Ca2+ plus Mg2+, indicating that SOC stability is weak in high salinity/high pH soils, but high levels of SIC or Ca2+/Mg2+ can increase SOC stability. Our study suggests that amelioration practice may lead to an increase in both SOC and SIC stocks in saline–alkaline soils.

Although various measurements (including organic amendments) have been used to ameliorate saline–alkaline soils, soil organic matter (SOM) remains low in most salt-affected agriculture soils. It was hypothesized that lower SOM level was partly attributable to weaker SOM stability (thus greater desorption) in the salt-affected soils. To test this hypothesis, we conducted a 16-week incubation experiment using low- and high-salinity soils to evaluate the effects of soil ameliorants (gypsum, CaCO3, rice straw and biochar) on SOM labile fractions, i.e., water extractable organic carbon (WEOC) and nitrogen (WEON), together with microbial biomass carbon (MBC) and nitrogen (MBN). Our results showed an increase in MBC and MBN under all amendments in both low- and high-salinity soils, reflecting improvements in soil properties. Gypsum amendment led to a decrease in both WEOC (by 15–21%) and WEON (by 14–31%). CaCO3 amendment only caused a decrease in WEON (by 14–27%), with a greater decrease found in the high-salinity soil. There was an increase in WEOC (by 13–66%) but a decrease in WEON (7.6–46%) under biochar and straw amendments in both low- and high-salinity soils. WEOC:SOC ratio (an indicator for SOC desorption) showed a decrease under gypsum and biochar amendments but an increase with CaCO3 and straw treatments. There was a decrease in WEON:TN ratio (an indicator for ON desorption) under all amendments, with the greatest decrease under biochar treatment. Our analyses demonstrated an enhancement in SOC or ON adsorption under all amendments, indicating that SOM stability might be enhanced in association with soil amelioration. Our study also highlights that there is strong decoupling between carbon and nitrogen cycles and further studies are needed to examine the impacts of such decoupling on SOM stability.