Long-term effects of tillage and crop rotations on soil organic C and total N in a clay soil in southwestern Saskatchewan

Soil organic matter contributes to the productivity and physical well-being of soils. An 11-yr study was conducted on a clay soil in the Brown soil zone in southwestern Saskatchewan to determine the influence of tillage and cropping frequency on soil organic C and total N content. Carbon and N behaved in a similar manner. Cropping frequency did not affect soil organic C or total N content, but soil C and N were greater under no-tillage (NT) than under mechanically tilled continuous wheat (Triticum aestivum L.) (Cont W) and fallow-wheat (F-W) rotations. Effects were apparent in the 0- to 7.5- and 7.5- to 15-cm depths. Over the 11-yr period, F-W (minimum tillage) gained no additional C; Cent W (conventional tillage) gained 2 t C ha(-1), and both Cent W (NT) and F-W (NT) gained 5 t C ha(-1). Changes in organic C and N were greatest in the final 4 yr of the experiment when crop residue production was greatest. Using data from two similar experiments conducted during the same period on soils differing in texture, we demonstrated that C gains were directly related to clay content of the soils. Thus, when attempting to estimate C storage in soils, we must consider both residue input and soil clay content.