Studies were conducted to determine changes in organic matter and microbial biomass carbon in comparison with structural stability at the surface soil (0-5 cm) of a Charlottetown fine sandy loam, an Orthic Podzol, at three tillage and grassland sites situated in Prince Edward Island. The tillage experiments, established for 3-5 years, included comparisons of mouldboard ploughing, direct drilling, shallow tillage, and chisel ploughing. Two indices of soil structural stability were used: mean weight diameter (MWD) and aggregation index (AI). The latter index assigns a weight factor to aggregate size ranges based on their value for plant germination and root growth. Direct drilling and reduced tillage increased the level of soil organic carbon by 10-17%, relative to mouldboard ploughing. Organic carbon was more enriched in 1-2 mm and 4.75-9.00 mm macroaggregates, especially the former, compared with whole soil. The MWD of aggregates after wet sieving was 33% and 55%, relative to the grassland sites, for mouldboard ploughing and direct drilling, respectively. On these soils of similar mineralogy and particle size, a close linear relationship (r = 0.942) was observed between organic carbon and MWD. A linear relationship was also found between MWD and both microbial biomass carbon (r = 0.947) and nitrogen (r = 0.923). The AI was significantly correlated to both organic carbon and microbial biomass carbon using power regression. In contrast to MWD, the AI under direct drilling and reduced tillage, associated with an organic carbon level of 2.5%, approximated the AI under grass. Overall, the study showed that minimum tillage systems in humid climates can improve structural stability at the soil surface of fine sandy loams over a relatively short time frame.
