Comparison of soil organic carbon stock change with eddy covariance carbon balance at an ICOS crop ecosystem site reveals unexplained carbon losses

According to the latest estimates, soils globally store 1500 to 2400 Gt of carbon (C) at a depth of 1 m in the form of organic matter. Almost the same amount of inorganic C is estimated to be stored at a depth of 2 m. Soils contain about twice as much organic carbon as the atmosphere and three times as much as vegetation. Small changes in this large soil reservoir could therefore have major effects on atmospheric carbon dioxide (CO₂) concentrations. Soil organic carbon (SOC) stocks are strongly influenced by land use, and soils have lost an estimated 140-150 Gt C globally due to disturbance and cultivation since agriculture began 8000 years ago. Global warming is disrupting the carbon cycle and could lead to a decrease in SOC worldwide. Increased nitrogen (N) deposition and intensification of N use in agriculture since the 20th century are also affecting soil inorganic carbon (SIC) stocks through carbonate weathering on agricultural sites, a process that could counteract efforts to increase SOC by changing land management.

Long-term carbon observation sites, such as ICOS ecosystem sites, are unique networks for assessing soil carbon stock evolution by comparing changes in SOC stock over time through soil sampling with the annual ecosystem carbon budget combining CO₂ fluxes through eddy covariance, carbon imports and exports through organic fertilization and harvesting, and dissolved carbon leaching. In this study, we compared the evolution of the SOC stock over 14 years with the carbon balance over the same period on the French crop site ICOS FR-Gri near Paris. The site is a wheat-barley-maize rotation with occasional oilseed rape. We find that SOC decreased by 68 ± 18 g C m^-2 y^-1 over the 14-year period in the 0-60 cm layer, with 70% of the loss coming from the 0-30 cm layer. Integration of carbon fluxes at field boundaries over the period 2006-2011 led to an estimated total carbon loss of 130 ± 110 g C m^-2 y^-1 in this field, an estimate close to pan-European studies (138 ± 239 g C m^-2 y^-1). Carbon leaching was estimated over the same period at 28 g C m^-2 y^-1 of which 21 g C m^-2 y^-1 was inorganic. The difference between the carbon balance and the SOC stock change amounts more than 50 of g C m^-2 y^-1, suggesting an additional carbon loss that may partly be carbonate weathering at a site that contains carbonates in part of the field.