Soil microbial biomass and activities in a Japanese Andisol as affected by controlled release and application depth of urea

This experiment was conducted in maize field plots to study the effects of controlled release and application depth of urea on soil microbial biomass and activities at two depths of surface soil of a Japanese Andisol from June to September, 2001. Three N amendment treatments and a Control were included in this experiment: deep application (8 cm) of controlled release urea; deep application (8 cm) of conventional urea; surface application of conventional urea; Control, without N application. Prior to this experiment, the field plots received the same N fertilizer treatments for two consecutive years under maize/barley rotation. Soil microbial biomass, dehydrogenase and nitrification activities exhibited great vertical and temporal variations during the maize growth season, and the microbial biomass was significantly correlated to soil water-filled pore space (p < 0.01). N fertilization did not significantly affect the microbial biomass, but greatly increased the dehydrogenase and nitrification activities. The increase in the microbial activities following N fertilization was not attributed to the increase in microbial biomass but to the increase in intrinsic microbial activities. Controlled release urea was found to continuously affect the dehydrogenase activity over a shorter distance, while conventional urea could greatly increase the enzyme activity for a shorter period of time. Both controlled release and deep application of urea had potentials to reduce the nitrification activity and suggested that the nitrate production might be decreased in 0-10 cm surface soil. Deep application of urea increased aboveground N uptake by maize and then the recovery rate of N fertilizer, whereas controlled release of urea greatly increased grain yield and N uptake by grain.