Soil Microbial Community Response to Controlled-Release Urea in Western Canada.

Soil microorganisms mediate many important biological processes for sustainable agriculture.  The effects of controlled-release urea (CRU), applied at recommended rates, on soil microbial biomass and diversity were studied at six sites across western Canada from 2004 to 2006.  Fertilizer treatments were CRU, regular urea and an unfertilized control.  A treatment on timing of fertilizer application (fall vs. spring) was included in 9 of the 18 site-years. A barley (or wheat)-canola-wheat (or barley) rotation (continuous silage corn at Agassiz) was established 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 the functional diversity of soil bacteria were determined in crop rhizosphere and bulk soil at the flagleaf stage of cereal growth (anthesis stage for corn) or flowering stage of canola growth.  In the rhizosphere, there were no fertilizer effects on MBC or bacterial diversity in 94% and 72% of the site-years, respectively.  In bulk soil, the corresponding percentage was 78% in both cases.  Where fertilizer had significant effects, both CRU and urea increased soil MBC or bacterial diversity.  CRU increased MBC or bacterial diversity more than urea in three site-years, but the reverse was observed in one site-year.  Time of fertilizer application had no effects on MBC or bacterial diversity in 89% and 78% of the site-years, respectively, in the rhizosphere, and no effects at all in bulk soil.  Fall-applied fertilizer increased MBC more than spring-applied fertilizer in one site-year, but the reverse occurred for bacterial diversity in two site-years.  Tillage had no significant effects on soil microbial biomass or diversity in 78-94% of the site-years in the rhizosphere or bulk soil, and significant effects were usually in favour of ZT.  Interactions between fertilizer and tillage on MBC or bacterial diversity were observed in only one and two site-years, respectively.  Shifts in bacterial community structures due to fertilizer type, time of application, and tillage were sometimes observed where the treatment effects described above were not significant.  Therefore, in a wide range of environmental conditions in western Canada, fertilizer application had no significant effects on soil microorganisms in most cases, but CRU tended to increase soil MBC or diversity more than urea when effects were significant.  This means that CRU applied at recommended rates probably does not have negative effects, and may even have positive effects, on soil biological processes.