Field Measurement of Denitrification: III. Rates During Irrigation Cycles

Abstract

Abstract Absolute amounts and rates of denitrification from a Yolo loam field profile at Davis, Calif., were studied in relation to the influence of irrigation frequency and soil incorporation of crop residue. Two different carbon (C) treatments were established by using plots to which no crop residues had been incorporated within 1 year prior to the experiment and plots to which 10 metric tons ha −1 of chopped barley straw were incorporated into the top 10 cm of soil 2 months prior to fertilization. The same total amount of water was applied at frequencies of three irrigations per week, one irrigation per week, and one irrigation every 2 weeks to areas cropped with perennial ryegrass. Fertilizer was applied as KNO 3 (≅ 285 kg N ha −1 ) enriched with 56 to 60% 15 N to 1‐m 2 plots. The surface flux of denitrification gases was measured from the accumulation of nitrous oxide (N 2 O) and 15 N 2 beneath airtight covers placed over the soil and from measurements of N 2 O using the acetylene (C 2 H 2 ) inhibition method. Small fluxes of denitrification gases were measured in this well‐drained alluvial soil under normal cyclic applications of irrigation water. Total denitrification ranged from 0.7 to 5% for the least frequently irrigated (no straw) and most frequently irrigated (straw) plots, respectively. Surface denitrification gas fluxes were largest after the first irrigation, decreased to near zero values within 1 or 2 days after each irrigation, and generally decreased for subsequent irrigations. The amount of N 2 produced was much greater than N 2 O. The nitrous oxide flux at the soil surface varied between 5 and 27% of the total denitrification over a 40‐ to 50‐day period. Nitrous oxide mole fractions tended to be smallest immediately after irrigation and increased as the soil water redistributed and the soil profile became less anoxic. The irrigation frequency of three irrigations per week gave higher soil NO 3 ‐ concentrations within the root zone of the crop than those of the other two frequencies. Thus, frequent, small irrigations may result in less leaching losses than infrequent, large irrigations.