Tillage effects on soil microbial and biochemical characteristics in a fallow-wheat rotation in a Dark Brown soil

As the adoption of reduced tillage increases on the prairies, so too does the use of herbicides and society's concern regarding their possible negative impact on the environment. Two experiments were conducted on Regina heavy clay, an Orthic, Dark Brown Chernozem, at Regina, Saskatchewan, for up to 21 yr, to determine the influence of tillage on yields of hard red spring wheat (Triticum aestivum L.) in a fallow-wheat (F-W) rotation. The treatments used to control weeds in the fallow phase were conventional mechanical tillage (CT) vs. zero tillage (i.e., herbicides only or ZT). In one experiment, glyphosate was the herbicide used; in the other, paraquat (plus Buctril M) was used. In 1991 (the 21st yr), soil sampled from the 0- to 5-cm depth of each rotation phase was used to determine the possible long-term effect of these treatments on selected soil microbial populations and soil biochemical characteristics. The samples were taken in early June (prior to herbicide application), in mid July (3 wk after the 21st herbicide application), and in early September (11 wk after the 21st herbicide application). Generally, the herbicides had no long-term or short-term (in the 21st yr) deleterious effects on soil microbial populations (bacteria, actimonycetes, fungi, nitrifiers, denitrifiers), nor on microbial biomass or potential C or N mineralization. Compared with ZT, CT had a negative impact on most soil characteristics. Microbial activity was primarily a function of crop residue inputs and growing season weather conditions near the time of sampling.