Denitrification as affected by irrigation frequency and applied herbicides

Denitrification from a Yolo loam field profile was studied in relation to the frequency of irrigation water application. Field plots (1 m2) cropped with grass were intensely instrumented with soil atmosphere samplers, soil solution samplers, tensiometers and neutron access tubes. Equal irrigations of 1.15 of evapotranspiration were applied by a spray irrigation system at three frequencies consisting of three irrigations weekly, one irrigation weekly and one irrigation every two weeks. Fertilizer (300 kg N/ha) was applied uniformly during the first irrigation as KNO3 enriched with 60% 15N. The flux of the volatile denitrification products at the soil surface was measured from the accumulation of N2O and 15N2 beneath air-tight covers placed over the soil for 2-4-h time periods at frequent intervals after each irrigation. The largest fluxes of N2 and N2O at the soil surface generally occurred within 3-6 h after the end of irrigation. Fluxes decreased rapidly to near zero values within one to three days after the end of irrigation. As rates and total denitrification were very small for this soil with limiting carbon, differences in denitrification for the three irrigation frequencies were slight. In vitro experiments were carried out as follows: Several herbicides at concentrations of approximately 25 ppm were incubated with glucose (C/N=1.0) and NO3- in Yolo soil under He atmospheres at 250C. Chlorbromuron, atrazine, neburon, linuron and diuron all inhibited NO3- reduction with inhibition percentages ranging from 27 to 46%. (author)