Soil organic and inorganic carbon interactions under tillage and cover cropping determine potential for carbon accumulation in temperate, calcareous soils

The global soil carbon pool comprises soil organic carbon (SOC), found in almost all soils, and soil inorganic carbon (SIC), in calcareous soils. Despite their agricultural significance, calcareous soils, mostly prevalent in drylands and often alkaline, are historically understudied. Using soils obtained from a decade-long, fully factorial field experiment located on temperate, near neutral pH, calcareous soils, this study examined the influence of cover crops (no-cover vs radish) and three levels of tillage intensity: shallow (10 cm) and deep (20 cm) non-inversion, and plough (25 cm inversion) on SOC and SIC stocks. Further, considering recent experimental and observational evidence indicating the interactions of SOC and SIC pools and their likely microbial control, we also investigated how SOC, the soil microbial biomass pool, and SIC are correlated. For SOC stock, there were significant interactions with total SIC and SOC:SIC ratio that differed by tillage intensity. Across the whole soil profile (0-60 cm), there was a significantly positive relationship between SOC content and SIC stock that was only present with ploughing. Further, at low SOC:SIC ratios (~0.5-3.0), while SOC stock was marginally lower under plough, at higher SOC:SIC ratios (~3.1–10.0), SOC stock was predicted to be up to ~4–fold greater (4 kg m-2) with ploughing than the lower intensity tillage treatments. This result highlights a critical SOC-SIC interaction that, depending on tillage intensity, may offset anticipated disturbance-related loss of SOC. SOC stock was also ~40% (0.42 kg m-2) greater at 0-10 cm and ~30% (0.2 kg m-2) greater at 30-40 cm under radish cover crop than without. SIC stock differences were determined by SOC content, tillage intensity and cover cropping. SIC stock was strongly correlated with SOC, with a predicted ~0.3–1 kg m-2 increase in SIC stock for ~1% increase in SOC. Under radish cover crops and with ploughing, there was ~0.7 kg m-2 more SIC than under all other conditions. Microbial biomass was positively correlated with SIC stock suggesting a causality that needs experimental testing. Given that reduced tillage is a frequently recommended practice to increase soil carbon storage and given the limited attention that has been paid to the influence of cover cropping on the SIC pool, our results indicate the need for further investigation around the dynamics of SOC and SIC interactions and stabilization processes in calcareous soils and highlights the pitfalls of a one-size-fits-all approach to soil carbon management.