Sources and relationships of inorganic carbon and organic carbon in arable soils of the west Loess Plateau

Abstract

There is evidence of high levels of soil inorganic carbon (SIC) in low- to mid-fertility loess soils of north China. However, little is known about the dynamics of SIC fractions and their relationship with soil organic carbon (SOC) in cropland. Here, we hypothesize that more pedogenic carbonate (PIC) is accumulated in mid-fertility soils, and PIC has a positive relationship with SOC. We select two groups of (low-fertility and mid-fertility) soils in the cropland of the west Loess Plateau. SOC and SIC contents and their ¹³C isotopic compositions were measured down to 100 cm or deeper. Our data show that the contribution of C4 crops to SOC (31–35 %) is approximately half of C3 crops’ contribution over 0–40 cm in both low- and mid-fertility soils, reflecting a low retention rate of C4 crops. SIC content is higher in low-fertility soils (16.0–17.2 g kg⁻¹) than in mid-fertility soils (13.2–15.2 g kg⁻¹) over the 0–100 cm layer, while δ¹³C values are more negative in mid-fertility (–4.53 ‰ to –3.89 ‰) than in low-fertility soils (–3.10 ‰ to –2.78 ‰). The fraction of PIC in SIC is ∼30 % in low-fertility soils but ∼50 % in mid-fertility soils. Despite relatively lower levels of SIC in mid-fertility soils, PIC stock is much greater in mid-fertility soils (5.9–7.8 g kg⁻¹) than in low-fertility soils (3–6 g kg⁻¹) over 0–100 cm. There is a significantly positive correlation (P < 0.05) between PIC stock and SOC stock in spite of a negative SIC-SOC correlation in the west Loess Plateau. Our analysis suggests that soil fertility enhancement may lead to the accumulation of PIC but the dissolution of lithogenic carbonate in loess soils. This study underscores the complex and nuanced relationships between SIC and SOC in arid/semi-arid croplands, which holds vital implications for carbon sequestration strategies and sustainable soil management in fragile loess environments.