Contrasting soil microbial responses to fertilizers and herbicides in a canola–barley rotation

The combination of high input costs and low commodity prices is forcing some farmers to consider reducing crop inputs like seed, fertilizer and herbicides. In a field trial in which different canola ( Brassica napus L.) and barley ( Hordeum vulgare L.) inputs were subtracted from a full package, or added to an empty package, we studied the effects of full or reduced fertilizer and herbicide inputs on soil microbiological characteristics at two sites from 2005 to 2008. The full package consisted of a high-yielding crop variety seeded at an optimum rate, with fertilizers and herbicides applied at recommended rates. The empty package consisted of a less expensive, low-yielding crop variety seeded at a low rate, with no fertilizer or herbicide applied. Between these two extremes were treatments in which fertilizers or herbicides were applied at 50% of recommended rates or not at all. Each treatment was repeated year after year in the same plot, i.e., treatment effects were cumulative. Fertilizer effects on soil microbial biomass C (MBC), β-glucosidase enzyme activity and bacterial functional diversity (based on community-level physiological profiles) were usually positive. Reduced fertilizer application rates reduced the beneficial fertilizer effects. Significant herbicide effects on soil microbiological properties occurred less often, were smaller in magnitude than fertilizer effects, and were mostly negative. Reduced herbicide rates reduced the deleterious herbicide effects. These significant fertilizer and herbicide effects were observed in canola more than barley, and mostly in the final year of the study, indicating the cumulative nature of treatment effects over time. Therefore, repeated applications of agricultural inputs like fertilizers and herbicides can have more significant effects on soil biology and biological processes than single applications indicate. ► Significant fertilizer effects usually increased soil microbial biomass, enzyme activity and functional diversity. ► Significant herbicide effects usually reduced soil microbial biomass, enzyme activity and functional diversity. ► Significant effects were mostly observed in the 4th year of a 4-year trial, indicating cumulative effects.