Carbon budget of Finnish croplands — Effects of land use change from natural forest to cropland

• We simulated reasons for the loss of carbon from cropland soil in Finland.
• Croplands lost carbon because they produced less litter than pre-cropland forests.
• Climate warming has not been a significant reason for the carbon loss to date.
• Applying manure and avoiding fallow are means to retard the carbon loss.
• Our modeling approach appeared useful for studying soil carbon in croplands.

The pool of soil organic carbon (SOC) in Finnish croplands has declined during recent decades according to nation-wide soil inventories, but reasons for this trend remain unclear. We studied the possible reasons using an approach based on the Yasso07 soil carbon model. We evaluated also the suitability of this approach for estimating the pool and changes of SOC in boreal croplands at the regional scale. The simulated SOC pool declined in each of the four study regions we divided the country into over the entire study period from 1900 to 2009. During the last 35 years, the mean estimate of the decrease rate varied from 0.29 to 0.36 Mg ha− 1 year− 1 among the regions. The mean estimate of the SOC pool in the croplands varied from 92 to 124 Mg ha− 1 at the end of the study period. In a sensitivity analysis, the estimates of the decrease rate slowed down by 25% or accelerated by 38% at most, whereas the pool estimates increased or decreased by 18% at most. According to our simulations and the sensitivity analysis, the SOC pool declined because croplands produced less litter than pre-cropland forests and this agricultural litter decomposed more rapidly. On the other hand, climate warming has not been a significant reason for the decline yet. Increasing carbon input to the cropland soil by applying organic manure and avoiding bare fallow are means to slow down the loss of SOC. The simulated estimates were similar to measurement-based ones available for comparison. We concluded that our approach was suitable for studying the reasons for the declining trend of SOC and the major uncertainties were caused by inexact input values.