Seasonal Variability of Soil Biochemical Indicators on a Long-Term Crop Rotation Experiment.

Researchers employ various soil biochemical measures as indicators of treatment effects in agricultural management experiments (tillage, fertilization, cover crops, organic amendments and crop rotations etc.). The effectiveness of these analyzes are contingent on appropriate sampling frequency to allow for treatment trends to emerge. The objective of the study was to measure several soil biochemical parameters on 13 crop rotation treatments over 9 sampling periods (May – October) in 2015 and 2016 and evaluate the seasonal stability of these parameters as well as treatment differences. The long-term rotation study was initiated in 2001 in Woodslee, Ontario, the crop rotations selected in this study all contained soybean (Glycine max) within the cropping sequence, ranging from monoculture soybean, a corn (Zea mays)-soybean rotation to more complex rotations such as soybean-soybean-wheat (Triticum aestivum) + red clover (Trifolium pretense) rotation. The parameters measured included; water extractable organic matter, soil microbial biomass, soil inorganic nitrogen (N), particulate organic matter, total N, soil organic carbon and pH. Treatment difference between monoculture soybean and soybeans within a crop rotation were often consistent throughout the sampling periods. However, comparisons between similar crop rotation treatments, for example soybean-wheat and soybean – wheat + red clover, only exhibited significant differences during certain sampling periods in the growing season, as crop growth and crop residue dynamics changed. This highlights the concern that single sampling or repeated sampling at the same time of year could result in treatment differences or lack thereof whereas these effects may vary over the growing season and temporal changes may also vary between cropping sequences.