Abstract
There is evidence of connections between soil organic carbon (SOC) and inorganic carbon (SIC) in dryland of north China. However, fractions of SOC and SIC and the relationship are not well understood in the Loess Plateau that undergoes profound erosion and redeposition. A study was conducted in low-elevation cropland of Loess Plateau across two distinctive basins: Linfen basin (LFB) with lower soil pH (< 8.4) and subject to erosion–redeposition, and Yuncheng basin (YCB) with higher soil pH (> 8.6) and under the influence of the Yellow River. Soil samples were collected from 30 sites over 100 cm. We determined SOC, SIC, dissolved organic carbon (DOC) and other properties. Above 100 cm, SOC stock is significantly higher in LFB (10.0 ± 2.6 kg C m−2) than in YCB (6.9 ± 1.5 kg C m−2), but SIC lower in LFB (14.0 ± 2.5 kg C m−2) than in YCB (17.0 ± 5.7 kg C m−2). We find a significantly negative correlation between SOC and SIC stocks in LFB, but no clear relationship in YCB. DOC:SOC ratio (an indicator for DOC desorption or SOC stability) is significantly higher below 40 cm in YCB (1.9%) than LFB (1.2%), indicating stronger DOC desorption in YCB that has stronger hydrological process due to the influence of the Yellow River. Overall, SOC has a negative correlation with SIC and soil pH, and DOC:SOC ratio has a significantly positive correlation with soil pH. Our analyses suggest that erosion/re-deposition of topsoil is partly responsible for the negative SIC-SOC relationship in LFB, and high soil pH and stronger hydrological processes are attributable to relatively lower levels of SOC in YCB. This study highlights that soil carbon fractions in the lowland of Loess Plateau are influenced by many drivers, which leads to complex relationships between major soil carbon pools.
Highlights
The storage (~ 1500 Pg) of soil organic carbon (SOC) is greater than the sum of carbon stock in the atmosphere (~ 750 Pg) and terrestrial biosphere (~ 560 Pg), acting as both sources and sinks and contributing huge share to the regional and/or global carbon budgets (Amundson 2001; Jobbágy and Jackson 2000)
Our analyses show that dissolved organic carbon (DOC):SOC ratio is higher in Yuncheng basin (YCB) (0.76–1.88%) than in Linfen basin (LFB) (0.61–1.24%), with a significant difference below 40 cm (Table 2), which indicates stronger desorption of SOC in the subsoils of YCB
Our data show that soil inorganic carbon (SIC) stock is significantly higher than SOC stock over 0–100 cm in the whole study area; SOC stock is negatively correlated with soil pH, but positively correlated with EC
Summary
The storage (~ 1500 Pg) of soil organic carbon (SOC) is greater than the sum of carbon stock in the atmosphere (~ 750 Pg) and terrestrial biosphere (~ 560 Pg), acting as both sources and sinks and contributing huge share to the regional and/or global carbon budgets (Amundson 2001; Jobbágy and Jackson 2000). The pool of soil inorganic carbon (SIC) is comparable to that of SOC in both the global land (Lal 2004) and China’s land (Li et al 2007). There are some studies addressing SIC-SOC relationship, which report inconsistent findings. While there is evidence of a positive correlation between SIC and SOC stocks in the cropland of north China (Guo et al 2016; Shi et al 2017b; Wang et al 2015b), studies report a negative SIC-SOC relationship under various land uses in the North China (Li et al 2010; Zhao et al 2016). Zhao et al (2016) found that SOC had a negative relationship with SIC under mixed land uses Lett. (2020) 7:17 grass, shrub lands) in the Loess Plateau that undergoes profound erosion
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