Denitrification and nitrate leaching during the fall and winter following dairy cattle slurry application

Conditions in soils of south-coastal British Columbia during the fall and winter favor both NO3− leaching and denitrification. Estimates of NO3− leaching were made on a coarse-textured soil having a high water table (Sumas) and a well-drained, medium-textured soil (Agassiz) in each of 1991 and 1992, following application of 0, 300 and 600 kg total N ha−1 as dairy cattle slurry at the end of September. Leaching was calculated by measuring NH4+ and NO3− concentrations biweekly to 90 cm depth and subtracting denitrification losses. Denitrification estimates were made using the acetylene blockage technique on intact cores taken to 43 cm depth at biweekly intervals from the 0 and 600 kg N ha−1 treatments. Most of the NO3− disappeared from the 90 cm soil profile by January in both years. At the high rate of manure application, denitrification losses were 29 and 20 kg N ha−1 at Agassiz in 1991 and 1992, and 48 and 17 kg N ha−1 at Sumas in 1991 and 1992, respectively. Denitrification losses in the non-manured treatments ranged from 3 to 16 kg N ha−1 Significant denitrification losses occurred at the 15- to 28-cm and the 30-cm to 43-cm depths, indicating that denitrification measurements at these depths are important. Leaching losses ranged from 60 to 188 kg N ha−1 from the control treatments, and from 102 to 241 kg N ha−1 following the highrate of manure application. Denitrification accounted for up to 17% of the NO3− loss from the soil profile, indicating that the majority of the NO3− was leached. Key words: Acetylene blockage technique, denitrification rate, air-filled porosity, manure

Denitrifïcations in Sediments from the Hyporheic Zone Adjacent to a Small Forested Stream

Denitrification was assayed by the acetylene blockage technique in hyporheic sediments. Samples were obtained along transects perpendicular to the stream at two sites: (1) the base of a slope dominated by old-growth redwood and (2) the base of a slope dominated by alder regenerating from a clearcut in 1965. Denitrification was evident at in situ nitrate concentrations at all locations tested. Activity was stimulated by nitrate but nitrate plus glucose had no additional effect. Denitrifying potentials increased with increasing distance from the stream channel. Dissolved oxygen was 100% of the concentration expected in equilibrium with the atmosphere in water obtained from monitoring wells immediately adjacent to the stream but was as low as 7% of the expected value in water 11.4 m inland. Both nitrate and dissolved organic carbon decreased over summer in wells at the base of the alder-forested slope. A 48-h injection of nitrate-amended stream water into hyporheic water 8.4 m inland stimulated nitrous oxide production in the presence of acetylene. Nitrous oxide was generated as nitrate and acetylene were co-transported to a well 13 m down-gradient. The acetylene-block experiments coupled with the chemistry data suggest that denitrification can modify the chemistry of water during passage through the hyporheic zone.

SEASONAL PATTERNS OF DENITRIFICATION AND LEAF NITRATE REDUCTASE ACTIVITY IN A CORN FIELD

Denitrification in soil around the bases of corn stalks, determined by the "acetylene blockage technique," exhibited a general trend of decline from June to September. Leaf nitrate reductase activity, determined by an in vivo assay procedure, was low in June and July, and then exhibited a pronounced maximum at the time of tasselling.