Modelling of soil organic carbon changes and carbon balance under Conservation Agriculture and conventional cropping systems in Southern Finland

Conservation agriculture (CA) is a farming system that promotes maintenance of (1) minimum soil disturbance avoiding soil inversion (i.e. no-tillage or minimum tillage), (2) a permanent soil cover with crop residues and/or cover crops, and (3) diversification of plant species. The adoption of CA is promoted by FAO as a response to sustainable land management, environmental protection and climate change adaptation and mitigation. According to FAO, implementation of CA in Europe would reduce emissions by about 200 Mt CO₂ per year. The carbon credit system (1 credit = 1t CO₂ reduced) allows the compensation of the release of GHG generated by industries by means of funding emission reduction projects. Despite its potential for emission reduction, agricultural systems, however, are nearly beyond of carbon market.

The objectives of this study were (1) to assess the potential of CA for soil organic carbon (SOC) sequestration for the current climate conditions and for a future climate scenario; (2) to estimates carbon balance and possibility to obtain carbon credits in Southern Finland.

Five cropping systems were simulated by using the ARMOSA process-based crop model: conventional systems under ploughing with monoculture and residues removed (Conv–R) or residues retained (Conv+R); no-tillage; CA and CA with a cover crop, Italian ryegrass (CA+CC). In Conv–R, Conv+R and NT, the simulated monocultures were spring barley. In CA and CA+CC crop rotations were spring barley - oilseed rape - oats - spring wheat. Simulations were carried out for the current (1998-2017) and future climatic scenarios (period 2020-2040, scenario Representative Concentration Pathway 6.0).

We evaluated carbon balance by using SALM method (Verified Carbon Standard, VM0017), which is a method to quantify in terms of carbon credits the Sustainable Agricultural Land Management projects. The method takes into account the dynamics of carbon stored in soil and the direct emission of N₂O due to use of fertilizers (organic and mineral) and CO₂ emission due to chemical fertilizer production, the amount of fuel used in tillage and other field operations. For estimation, we used the value of carbon credit of 21€.

Under current climate conditions, Conv–R and Conv+R emitted totally about 4.7 and 2.0 t CO₂e ha^-1yr^-1, respectively, mainly due to SOC loss (1 and 0.34 t ha^-1 yr^-1, respectively). No-tillage emitted 0.4 t CO₂e ha^-1yr^-1, mainly, due to N₂O from fertilizers and chemical fertilizer production. In contrast, CA and CA+CC allowed to SOC sequestration of 0.315 and 0.650 t ha^-1yr^-1, resulting in emissions reduction of 0.420 and 1.62 t CO₂e ha^-1 yr^-1, respectively. By adopting CA and CA+CC in Finland, there is a potential to obtain 2.5 and 3.7 carbon credits with the value of 52 and 77 € ha^-1yr^-1 respect to baseline (Conv+R).

Under future climate scenario (+0.6 °C; –120 mm y^-1), SOC decline for conventional systems will be more pronounced compared to that under actual climate, and SOC sequestration will be possible to accomplish only for CA+CC.