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 to120 kg ha −1 on rhizosphere and bulk‐soil microbial biomass carbon (MBC) and functional diversity (Hʹ), 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 Hʹ 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 Hʹ 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.