Relationship between calcium forms and organic carbon content in aggregates of calcareous soils in northern China

Abstract

The agricultural soil organic carbon (SOC) pool is one of the most active C pools in terrestrial ecosystems, and soil aggregates are the main site of SOC fixation. Mineral sorption plays a crucial role in SOC accumulation. Calcium (Ca) is a typical soil mineral element that plays a crucial role in SOC accumulation and is fundamental for the formation of soil aggregates. Although Ca has been shown to regulate the stability of SOC, the relationship between different forms of Ca and SOC within aggregates in northern China remains unclear. In this study, we selected five soils (black soil, light chernozem soil, fluvo-aquic soil, sierozem soil, and loess soil) with different calcium carbonate (CaCO₃) contents as the target materials. Water-stable aggregates were separated using the wet-sieving method. Soil respiration was measured using the static alkali absorption method (carbon dioxide [CO₂] absorption by NaOH solution). The CaCO₃ content was quantified using the CO₂ gas volume method, and the continuous leaching method was used to analyze the different forms of Ca (water soluble-Ca, exchangeable-Ca, acid soluble-Ca, and bound to organic-Ca). In-situ field sampling was used to reveal the associations between different forms of Ca and SOC within aggregates. The results showed that besides the CaCO₃ form, exchangeable-Ca (20.06 %–57.86 %) and bound to organic-Ca (26.69 %–73.91 %) were major Ca forms in calcareous soils. There was a nonsignificant negative correlation between CaCO₃, acid soluble-Ca, and exchangeable-Ca and SOC in the bulk soil. However, there was a significant negative correlation among CaCO₃, acid soluble-Ca, and exchangeable-Ca with SOC in macro-aggregates (> 0.25 mm) and micro-aggregates (0.25–0.053 mm). A 90-day incubation experiment was conducted to verify the previous speculations, which revealed that CaCO₃ increased SOC mineralization and reduced the content of SOC, while enhancing the acid soluble-Ca and bound to organic-Ca content. In the treatment with added CaCO₃, there was a negative correlation between acid soluble-Ca and bound to organic-Ca and SOC in bulk soil and macro-aggregates (> 0.25 mm). The effect of Ca²⁺ on SOC should not be ignored. Excessive Ca²⁺ may be converted into different forms of Ca to occupy the bonding sites for SOC on the surface of soil particles with silt and clay particles, forming a pseudo-aggregate structure, and the SOC traveling between soil particles, which is not conducive to SOC stabilization and accumulation.