Effects of tillage on CO2 fluxes in a typical karst calcareous soil

It is widely believed that soil disturbance by tillage was the primary cause of historical soil organic carbon (SOC) loss and high carbon dioxide (CO2) emission levels in the calcareous karst region. Nevertheless, the mechanisms underlying CO2 fluxes resulting from the soil property changes caused by tillage are poorly understood. A one-year simulation experiment using different tillage frequencies was conducted to quantify the impacts of tillage and soil property changes on CO2 flux. Treatments included conservation tillage (T0), semiannual tillage (T1), tillage every four months (T2), bimonthly tillage (T3), and monthly tillage (T4). The effects of tillage on soil CO2 flux had a strong seasonal pattern. CO2 fluxes in higher tillage frequencies of T3 and T4 were significantly higher than those of other treatments in spring, summer, and autumn. For viable tillage management, CO2 fluxes in T1 and T2 were significantly higher than those in TO in spring and autumn. No significant differences in CO2 flux were found among treatments in winter. Tillage also had a significant influence on soil biogeochemical properties. Aggregate stability, dissolved organic carbon (DOC), and microbial biomass carbon (MBC) significantly decreased in T2, T3, and T4, whereas SOC significantly decreased just in T3 and T4 after 1 year. A structural equation model analysis showed that the annual cumulative soil CO2 flux was directly affected by annual changes in SOC (Delta SOC), DOC (Delta DOC), and MBC (Delta MBC). Tillage frequency directly influenced annual changes in large macroaggregates (Delta AG) and Delta MBC. These results indicated that tillage practice indirectly lowered SOC by reducing large macroaggregates and microbial biomass, which in turn, enhanced CO2 flux. Our results suggested that tillage disturbance in the karst soil significantly increased SOC loss through enhanced CO2 flux compared with that in the non-karst soil in a similar climate. In contrast, reducing or eliminating tillage in the wet-hot season could lower CO2 flux rate by minimizing large macroaggregate disturbance and, by extension, microbial access to mobile carbon sources.