Soil microbial response to nitrogen fertilizer and tillage in barley and corn

Cereal crops usually take up less than 50% of the N fertilizer applied. We hypothesized that, depending on application rate, the inorganic N in the soil could affect soil organisms. We investigated the effect of N applied to barley (Hordeum vulgare L.) at soil-test recommended rates (50-80 kg ha(-1)) in 2004-2006 in a Black Chernozem, and to corn (Zea mays L.) at recommended (80 kg ha(-1)) and higher (160 kg ha(-1)) N rates under no till (NT) and conventional tillage (CT) in 2004 and 2006 in an Orthic Gleysol, on soil microbial properties. Rhizosphere and bulk soil samples (0-7.5 cm soil depth) taken at flagleaf stage of barley growth and tasselling stage of corn growth were analysed for microbial biomass C (MBC), bacterial functional diversity and community-level physiological profiles (CLPPs). In barley, N increased MBC by 39% in bulk soil in one year. In corn, the response of MBC to N rate was quadratic, peaking at 80 kg N ha(-1). The functional diversity of soil bacteria in the corn trial was not reduced even at the highest N rate. No-till in corn increased MBC by 30-102%, and also tended to increase bacterial functional diversity. The tillage effect was detectable in CLPPs although year-to-year differences were more prominent. Therefore, N applied at agronomically recommended rates either did not affect MBC and bacterial diversity or increased these microbial characteristics; N applied at higher rates reduced MBC. These results from two soil types, crops and climatic regions support our hypothesis that non-target effects of N fertilizer depend on the application rate. Soil testing to apply recommended amounts of fertilizer would reduce non-target fertilizer effects. (C) 2011 Elsevier B.V. All rights reserved.