Crop rotation, tillage system, and precipitation regime effects on soil carbon stocks over 1 to 30 years in Saskatchewan, Canada
Maillard, É., McConkey, B.G., St. Luce, M., Angers, D.A., Fan, J. (2018). Crop rotation, tillage system, and precipitation regime effects on soil carbon stocks over 1 to 30 years in Saskatchewan, Canada. Soil & Tillage Research, [online] 177 97-104. http://dx.doi.org/10.1016/j.still.2017.12.001
Plain language summary
Changing agricultural practices on the Canadian Prairies have significantly impacted the amount of soil organic carbon. A 30-year experiment conducted at the Swift Current Research and Development Centre shows how a move from land tillage with and fallow years to a no-till, continuous cropping system has changed the amount of soil organic carbon. During the last 15 years, the use of pulse crops replacing fallow was incorporated into the experiment. The results show that for the first 5 years that eliminating tillage only increased soil carbon for a wheat-fallow rotation, while eliminating both tillage and fallow increased soil organic carbon in the first 7.5 cm of the soil for all rotations. The effect of eliminating fallow on soil organic carbon storage became noticeable at a depth of 15 cm after 10 years, and 30 cm after 19 years. This increase was related to the increase of crop residues with the elimination of fallow. No-till managed rotations tended to have more soil organic carbon than tilled rotations, although the size of the effect varied across years and was most consistently noticed only in the upper 7.5 cm of soil and in the fallow-wheat rotations. Replacing fallow with a pulse crop increased soil organic carbon in the tilled system increased soil organic carbon to 7.5 cm after 6 years and to 15 cm after 11 years. The soil organic carbon benefit of replacing fallow with a pulse crop was greatest in the tilled systems where initial soil organic carbon was lowest.
The experiment also showed the significant impact of weather on soil organic carbon. While soil organic carbon increased in years with more precipitation, its decomposition was proportionately more in wet years, causing a net decrease overall. During a prolonged period of wetter conditions, soil organic carbon was found to decrease. Therefore, while the practices of reduced tillage and fallow generally increase relative soil organic carbon, the absolute soil organic carbon change in this semiarid climate is more dependent on weather.
For both agronomic and environmental purposes, it is relevant to assess the effect of various land management practices on soil organic carbon (SOC). In the North American Great Plains, fallowing was used to increase soil water storage and production of succeeding crops. In addition, tillage was done to prepare seed beds and to control weeds. But these management practices led to important SOC losses, mainly due to increased mineralization with tillage and reduced plant C inputs with fallowing. In recent years, reduction of fallowing, use of minimum and no-tillage management practices and diversification of crop rotations with the inclusion of pulse crops have been studied. But the results are variable according to soil and climatic situations. In addition, long-term data are not always available. In this context, the objective of the present work was to assess over time the effects of cropping systems involving the use of various tillage management, the presence or absence of fallowing (1981–2011), and the presence or absence of pulse crops (1997–2011) on plant C inputs and SOC stocks along a 0–30 cm silt loam soil profile at Swift Current, SK. First changes between SOC stocks were apparent five years after the start of the experiment involving changed crop systems and mainly in the surface 0–7.5 cm soil layer instead of 0–15 cm or 0–30 cm soil profiles. This study confirmed the accumulation of SOC under continuous wheat systems in comparison to fallow-wheat rotations, probably related to larger C inputs in the continuous wheat systems. Although the effect of tillage was quite scattered in time, it seemed to be more pronounced in the fallow-wheat rotations than in the continuous wheat systems, with higher SOC stocks with no tillage. Then, the replacement of fallow phase by pulse crops offered promising potential to rebuild SOC stock after fallow-wheat rotations, even if its efficacy might depend on the initial SOC content and/or its combination with a tillage reduction. Finally, even if a kind of equilibrium could set after changes from a traditional fallow wheat rotation, the SOC dynamics might be highly influenced by precipitation regime and all the carbon accumulated for decades could be lost in few years (less than five in this study). Indeed, in this semi-arid prairie, the increase of the apparent decomposition with precipitation exceeded that of the plant biomass C inputs.