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Several long-term agroecosystem experiments (LTAEs) across Canada have been maintained for periods of up to a century. Much scientific knowledge of changes in soil properties through time has been learned from these few, highly productive LTAEs. We determined the effects of land management changes (LMC) on soil organic carbon (SOC) by re-sampling 27 LTAEs across Canada using identical sampling and laboratory protocols. Seven LTAEs were sampled comparing perennial to annual cropping and it was found that SOC stocks (0-30 cm) were 9.0 +/- 1.5 Mg C ha-1 higher under perennial cropping after an average of 16.9 +/- 2.1 yr. This yielded a SOC stock change factor of 0.6 Mg C ha-1 yr-1, comparing favourably to a modelling assessment and the Intergovernmental Panel on Climate Change (IPCC) default factor. In six LTAEs in western Canada, no-tillage increased SOC storage by 3.2 +/- 1.3 Mg C ha-1 in the top 15 cm over a period of 23.3 +/- 2.7 yr relative to conventional tillage, a rate of SOC storage of 0.14 Mg C ha-1 yr-1. This rate was also similar to that derived by simulation modelling and was slightly lower than the default IPCC rate for subhumid and semi-arid regions. In eastern Canada, where tillage is much deeper than western Canada, SOC storage was not significant differently between the two tillage systems. In six LTAEs in western Canada, removing fallow periods every second or third year in favour of continuous cropping increased SOC storage by 5.2 +/- 1.1 Mg C ha-1 yr-1 over 21.8 +/- 4.0 yr or an average SOC stock change factor of 0.23 Mg C ha-1 yr-1 to 15 cm depth. This was slightly higher than two independent meta-analyses and rates derived from simulation modelling. The results determined from a re-sampling of LTAEs across Canada provided an invaluable method of validating rates of SOC change concluded by other means.
