Microbial Enzyme Activity in Irrigated Canola Plots Receiving Different Nitrogen Applications.

Soil enzymes play an important role in controlling the reactions that govern nutrient cycling and organic matter decomposition. Management practices such as irrigation and the application of nitrogen (N) fertilizer have implications for soil microbial activities. This study investigates the impact of rate (110 & 220 kg N ha^-1) and timing (single application at seeding vs. split application with half at seeding and half prior to bolting) of N fertilizer application on N cycling enzyme activities, in a two-year experiment with irrigated canola. We hypothesise that the rate and timing of the N fertilizer application alters the activities of the enzymes responsible for N cycling in the soil. Potential for N mineralization, immobilization and denitrification was evaluated by urease, arylamidase and denitrification enzymes activities, as well as potential for nitrification assay on soil samples collected over two growing seasons. Soils were sampled before and after each N application and at the time of the first major moisture (irrigation) event following seeding. Enzymatic conversion of urea, arylamidase mediated N conversion and denitrification enzyme activity were not affected by the rate of fertilizer application, but timing of the N application significantly affected urease, arylamidase and denitrification enzyme activities. Potential for nitrification was affected by rate and timing of fertilizer-N application. These results suggest that N management practices can affect the biological conversion of fertilizer-N in the soil, including an increase in the potential for nitrification. Relating these results to canola yield and gaseous loss as N₂O is underway.