Soil microbial community response to controlled-release urea fertilizer under zero tillage and conventional tillage

Soil microorganisms mediate many important biological processes for sustainable agriculture. The effect of a polymer-coated controlled-release urea (CRU, ESN®) on soil microbial communities was studied at six sites across western Canada from 2004 to 2006. Fertilizer treatments were CRU, urea and an unfertilized control. Timing of fertilizer application (fall vs. spring) was studied in 9 of the 18 site-years (combinations of sites and years). Wheat (Triticum aestivum L.), canola (Brassica napus L.) and barley (Hordeum vulgare L.) were grown in rotation at five sites, and silage corn (Zea mays L.) was grown in all 3 years at one site, under conventional tillage (CT) or zero tillage (ZT). The fertilizers were side-banded at 50–60 kg N ha−1 for wheat, barley and canola, and broadcast at 150 kg N ha−1 for corn. Microbial biomass C (MBC) and bacterial functional diversity and community-level physiological profiles (CLPPs) were determined at about the flowering stage of each crop. In situ CO2 evolution (soil respiration) was measured, and microbial metabolic quotient (qCO2) determined, at one site in 2 years. In the rhizosphere, fertilizer effects on MBC and functional diversity were observed in 1 and 5 of 18 site-years, respectively; and in bulk soil in 4 site-years each. These effects were usually positive relative to the control. CRU increased MBC or functional diversity more than urea in 3 site-years, but the opposite was observed in 1 site-year. Time of fertilizer application affected MBC in 1, and functional diversity in 2, of 9 site-years in the rhizosphere, and no effects were observed in bulk soil. Fall-applied fertilizer increased MBC more than spring-applied fertilizer, but the opposite was observed for functional diversity. Tillage affected MBC and functional diversity in 4 and 5 of 18 site-years, respectively, in the rhizosphere, and in 3 and 4 site-years in bulk soil. Tillage effects were usually in favour of ZT. There were no treatment effects on CO2 evolution, but an interactive effect of fertilizer and tillage on qCO2 was observed in 1 year when qCO2 in the control treatment was greater than that in either fertilizer treatment under CT, but urea increased qCO2 relative to the control under ZT. Shifts in CLPPs were sometimes observed where the treatment effects described above were not significant. Notwithstanding the limitations of culture-dependent CLPPs, most fertilizer effects on soil microbiological properties were not statistically significant. Therefore, these fertilizers probably did not adversely affect most soil biological processes.