CO2 emission from soil inorganic C in coastal croplands of the Yangtze Delta

Soil inorganic C (SIC) has a huge stock and is supplemented by the bicarbonate-rich groundwater, irrigation water and seawater in the coastal Yangtze Delta area, where soil acidification may induce high CO2 efflux by neutralizing SIC. This study assessed whether and to what extent SIC generally contributed to surface or subsurface (50 cm) CO2 collected in typical croplands (a rice-wheat rotation paddy and an upland field) of this coastal region, and further investigated factors regulating SIC-derived CO2 efflux (E IC) by soil incubation. The E IC was separated from CO2 of organic sources (E OC) based on their distinct 13C isotopic signatures. Our results indicated that SIC contributed around 16 % to surface CO2 effluxes in the paddy field and 52 % to subsoil CO2 in the upland field. Despite the alkaline soil environments (with pH of 7–9), adding bicarbonate induced high CO2 effluxes comparable to those under glucose amendments, suggesting high reactivity of the dissolved SIC (HCO3 ) in E IC generation. Within a month, the 13C-labeled HCO3 (1 mg C g 1 soil) was almost completely converted to CO2 and induced extra E IC emission (84 % of the HCO3 input) from unlabeled sources. Bicarbonates also contributed > 40 % to total CO2 emission in soils amended with groundwater. However, the HCO3 -amended soil did not emit but strongly adsorbed CO2 when soil organic matter and microbial communities were destroyed by heating at 550 °C, implying the involvement of these organic components in HCO3 CO2 conversion. The E IC effluxes increased with temperature, N fertilization (due to acidity released in nitrification), microbial respiration (associated with straw addition or reinoculation rate of sterilized soil), and bicarbonate addition rate, but decreased with soil moisture. Multiple regression indicated that bicarbonate addition and N fertilization were most effective in increasing E IC. In conclusion, the SIC pools, particularly the bicarbonates, were active players in coastal soil C cycling, releasing CO2 fluxes susceptible to environmental, biological, and management influences. With the widespread acidification of calcareous soils under intensive N fertilization, special attention should be paid to not only the accelerated carbonate dissolution and leaching, but possible CO2 emissions resulting from the increased soil bicarbonates, in coastal agroecosystems.