Soil Microbial Response to Nitrogen Rate and Placement and Barley Seeding Rate under No Till

Urea [(NH(2))(2)CO] applied in the seed row can damage seedlings and affect soil microorganisms. A field study was conducted in five site-years to compare the effects of seed-placed and side-banded N applied to barley (Hordeum vulgare L.) at 0 to 120 kg ha(-1) on rhizosphere and bulk-soil microbial biomass carbon (MBC) and functional diversity (H.), and to investigate if increasing barley seeding rate (200-400 seeds m(-2)) would modify the N effects. Nitrogen rate affected MBC in four and one site-years in barley rhizosphere and bulk soil, respectively. Two of the four responses in the rhizosphere were quadratic, and the other two were a linear decrease and a cubic response. The response in bulk soil was a linear decrease. One of the two responses in the rhizosphere depended on N placement. Responses of H. to N rate, which were mostly cubic and depended on N placement, were observed in three and four site-years in the rhizosphere and bulk soil, respectively. Increasing barley seeding rate increased MBC in one and three site-years in the rhizosphere and bulk soil, respectively, and increased H. in one and two site-years, respectively. In two of the three cases in bulk soil, seeding rate increased MBC only when N was banded. Banded N up to 60 kg ha(-1) had minimal adverse effects on soil microorganisms, but only 30 kg N ha(-1) or less of seed-placed N was relatively harmless. Increasing seeding rate did not usually alleviate the negative effects of seed-placed N applied at high rates.