Influence of urea fertilization and urease inhibitor on the size and activity of the soil microbial biomass under conventional and zero tillage at two sites

Fertilizers are frequently used in agriculture to enhance crop yield and quality. Increasingly, microbial and enzyme inhibitors are being used to enhance the efficiency of fertilizer nitrogen use. The assumption being made is that the impact of these compounds is short-term or localized and thus does not adversely impact soil quality. This study investigates the implications of urea fertilizer with and without urease inhibitor [N-(n-butyl) thiophosphoric triamide, NBPT], conventional and zero tillage systems, and soil types on the soil biological characteristics including N mineralization. Microplot studies were conducted in clay loam and fine sandy loam soils in Manitoba. Soil type had a significant effect on the soil organic C but tillage did not have any significant effect. Soil microbial biomass C content ranged widely (131–1215 µg g −1 soil). The clay loam soil generally tended to contain higher biomass C than the sandy loam soil. The potential soil mineral N production (N 0 ) was higher in clay loam soil (92.6 µg g −1 soil) than sandy loam soil (40.2 µg g −1 soil). Tillage systems, urea and urease inhibitor application had no significant impact on the soil biomass C, N 0 , arylsulfatase or acid and alkaline phosphatase content. At these sites considerable fluctuations occurred in the microbial and biochemical properties due to the variation of soil type. In general, these fluctuations were mainly related to soil organic matter and soil moisture content. Tillage system, urea with and without NBPT application did not contribute any notable impact on the soil biological properties reported in this study. Key words: Soil biological quality, soil type, urease inhibitor, conventional tillage, zero tillage