Carbon (C) that is stored in soils is the principal terrestrial C pool. The soil stores twice as much C compared to that which is stored in the atmosphere. Therefore, even a slight change in soil C stock can have a huge effect on atmospheric carbon dioxide concentration and global climate. Agricultural soils play a key role in this system: they cover about 38% of the land area world-wide and are intensively managed. The soil C also greatly contributes to sustainable food production as soil organic matter is to large extent made of C.
The aim of the research carried out for this thesis was to determine the nationwide soil C stock in Finnish arable mineral soils, study historic trends of C stock in soils and examine possible factors that affect those trends. Knowing the past can also give us insights into future trends in soil C and its climatic impact. The data presented in this thesis were obtained from the Finnish national soil monitoring network and long-term field trials in Uppsala Sweden and Pushchino Russia. In addition two process-based soil C models, namely: Yasso07 and RothC, were used and their findings were compared with those of long-term field trials.
Finnish arable lands were found to be rich in soil C. Mineral soils in Finland store between 41 and 67 Mg C ha-1 (0-15 cm) depending on the management, soil type and region. Nationwide the C stock in arable topsoil is about 117 Tg and although the deeper soils layers are poorly known the total soil C stock in mineral soils of Finland can be estimated to be about 300 Tg. The C stock of mineral arable soils has decreased. The decrease was found to be 0.22 Mg C ha-1 yr-1 (0-15 cm soil layer) according to the national soil inventory network of Finland and 0.29-0.36 Mg C ha-1 yr-1 (0-100 cm soil layer) according to results obtained by the Yasso07 model. The annual C emissions from agricultural mineral soils are about 0.5 Tg, which represents about 2.5% of the total greenhouse gas emissions in Finland. Process based modelling in which the past land use history was taken into account clearly indicated that the nationwide decrease can be linked to the past change in land use from forest to agricultural land, which has created the ongoing soil C loss.
Finnish arable lands are relatively young and it is likely that they are still losing the soil C that had been accumulated when these lands were part of boreal forest systems. Likewise the thesis indicates that the soil C loss partly results from the intensification in cultivations that took place in the past decades and that cultivation of annual crops has become more common. Cultivation of annual crops increases the soil disturbance due to tillage and decreases the below-ground C influx into the soil. The composition of litter quality was also shown to have considerable effects on the ease of decomposition of organic matter.
Previously, the Yasso07 model had been mainly tested and used in forested soils. Findings in this thesis demonstrate that this model can also be applied to agricultural mineral soils under boreal conditions. The fact that the model works equally well under various environmental conditions indicates that the accumulation of soil C is largely controlled by litter input, climate and litter chemical quality. The comparison between simulations and experimental data obtained from field trials showed that the results of Yasso07 model are comparable with those of the RothC model, which is currently one of the most widely used soil C models for agricultural applications.
The decreasing trend of soil C stocks found in this thesis has undesirable ramifications for climate, environment and sustainable food production. The changes in topsoil C might reflect the condition of soil C in deeper soil layers. Therefore, it would be crucial to investigate the storage of deep soil C and the long-term effects of agricultural practices on it.