Temporal Change of Soil Organic Carbon Under Climate Change in Denmark: A Hundred Year Simulation Using Three Models.

The balance between inputs and degradation of soil organic carbon (SOC) has a critical influence on nitrogen (N) mineralization. SOC stock changes do not occur instantaneously but over a period of decades (IPCC 2006). The temporal change in SOC was analyzed over a 100 year period based on climate change projections of two regional circulation models (RCMs) under the IPCC A1B emission scenario in 21^st century. A 30 year (1960-89) of observed climate data from Aarslev, Denmark (55°31´N, 10°44´E) were used to form the baseline, and generate the synthetic data for the future scenario analyses via LARS weather generator (Semenov and Barrow 1997). With increasing CO₂levels, and under continuous winter wheat system with conventional tillage, three commonly used crop-soil models were implemented to study the temporal change of SOC. In these models, SOC turnover is influenced by climate, soil texture and crop management.

The results were analyzed using mixed-effect analysis. The models indicated that SOC will decrease over the simulation period except during the first 25 years. The models also indicated a slow increase in N mineralization, potentially leading to net N accumulation. However, the models predicted increased N leaching and decreased plant N uptake. This means that increase in mineral N in the soil will not support long-term stimulated C assimilation in response to elevated CO₂. The most important finding was that there was no significant interactive effect between N input and N mineralization rate. The models further indicated increased denitrification. The decreased available N in soil and its uptake, and CO₂ induced accumulation of C in plant tissues will likely lead to down-regulation of photosynthesis and a reduction in grain quality due to N dilution in the future. Under projected climate change, decline in SOC under continuous mono cropping systems suggest increased focus on N fertilization strategies and the use of catch crops.