Effect of tillage and rotation on organic carbon forms of chernozemic soils in Saskatchewan

The effects of selected tillage and rotation systems on soil organic carbon and its fractions were studied on Chemozemic soils in south-western and east-central Saskatchewan. After practicing a no-till fallow unfertilized-wheat rotation for 7 years on an Orthic Brown Chemozem in southwestern Saskatchewan, total soil organic carbon (TOC) in the 0 - 5 cm and 5 - 10cm layers was slightly lower than the tillage fallow-unfertilized wheat comparable treatment. However, light fraction of organic carbon (LFOC) was similar in the two treatments. Comparison of the tillage fallow-unfertilized wheat to a treatment involving conversion to a fertilized continuous cropping system for 10 years showed TOC increased slightly in the two depths and LFOC increased by 24% and 29% in the 0 - 5 cm and 5 - 10 cm layer, respectively, of the continuous cropping treatment. Microbial biomass carbon (MB-C) was increased significantly at the 5 - 10cm depth. After conversion of fallow-wheat to alfalfa as perennial forage for 10 years, TOC increased by 80% and 27%, LFOC by 245% and 286%, and HFOC by 63% and 20% at 0 - 5 cm and 5 - 10 cm depths, respectively, compared to the tilled cereal-fallow system. Meanwhile, water soluble organic carbon (WSOC) was not affected but MB-C increased significantly. In an Orthic Black Chemozem in east-central Saskatchewan, the depletion and restoration of organic carbon was observed when native sod was changed into cropland and then back to grassland. For example, the TOC of cropland under cereal-fallow rotation for 62 years decreased by 42% and 33% at 0 - 5 cm and 5 - 10 cm depths, respectively, compared to native sod. The LFOC decreased by 79% and 74% in the layers, and reductions in WSOC and MB-C were even greater. After cropland was re-seeded to grassland for 12 years, the concentration of total organic carbon was increased by 16% and 22% while the mass of organic carbon was the same as the cropland in the two layers. The LFOC and MB-C amounts in the grass seed-down were double that of the cropped land, but the amounts of TOC, LFOC, and MB-C in grass seed-down were still significantly lower than the native sod.