Relative denitrification rates in surface and subsurface layers of a mineral soil

The effect of soil depth on denitrification was investigated on 57 dates du ring two time periods (I: March to August; II: October to March). Two repli cate grazed grass plots, that received 362 kg N/ha in seven equal increment s over the calendar year, were used for the study. Intact soil cores, 2.95 cm diameter and 10 cm deep, in successive 10 cm layers to a depth of 50 cm were collected at five random locations within each plot on each date. The cores were incubated outdoors at ambient temperatures and denitrification, as nitrous oxide emission, was measured using the acetylene-block technique . Soil moisture and soil mineral N were determined on the bulked cores at e ach sampling date. Concentrations of total organic carbon and water-soluble organic carbon were determined on a number of the bullied cores during the year. Multiple regression models of denitrification, fitted by backward el imination of the variables, soil moisture, soil nitrate N, soil ammonium N and total soil mineral N for each soil depth in periods I and II, showed th e importance of soil moisture, soil ammonium N in period I and soil ammoniu m N in period II at particular soil depths. Denitrification in the 0 to 10 cm layer was much greater than in the lower layers in both time periods. Of the total denitrification (0 to 50 cm) for the 12-month period (16.56 kg N /ha), 80% occurred in the 0 to 10 cm layer, with 62% of total denitrificati on occurring in the second time period.