Soil carbon dynamics in Canadian Agricultural Ecoregions: Quantifying climatic influence on soil biological activity

Climate is the major determinant of soil biological activity, including the decomposition rates of soil organic carbon (SOC) components. The objective of this study was to estimate the influence of climate on SOC dynamics, using long-term standard weather station data for Canadian Agricultural Ecoregions and the Introductory Carbon Balance Model for regional applications (ICBMregion). Mean daily temperature, total precipitation and potential evapotranspiration data were used in pedotransfer, soil water balance and biological activity functions to calculate a climate factor, re_clim, used to describe both inter- (1970–1999 data, daily re_clim) and intra-annual variation (1903–2000 data, mean annual re_clim) of the effects of climate on SOC decomposition rates across the country. When re_clim = 1.0 the SOC decomposition rate is equal to that at a reference site in Central Sweden: when it is <1.0 the relative SOC decomposition rate is lower, and when it is >1.0 it is higher. The results show that the cool and humid eastern Canadian Agricultural Ecoregions are characterized by higher SOC decomposition rates (average re_clim = 1.20), compared to the semi-arid regions of western Canada (average re_clim = 0.95). In other words, more C input is needed to maintain a certain SOC level in eastern than in western Canada. Inter-annual variation (the difference between minimum and maximum values) in re_clim for a given agricultural ecoregion was ≈±20%. The patterns of variation in intra-annual re_clim values differed between Agricultural Ecoregions: for eastern Canada re_clim reached a peak of ≈3.5 in mid-summer, and that of western Canada peaked at ≈2.5. This concept of re_clim was tested and used in other climate zones such as northern Europe and Sub-Saharan Africa, and enabled us to integrate large data sets concerning climate and SOC dynamics and to characterize Canadian Agricultural Ecoregions by one variable, re_clim.