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Isotope dilution methodology offers the advantage of an integrative technique for measuring N2 fixation in the field, but may have limitations in grass‐legume mixtures because of the possibility of transfer of fixed N from legume to grass. A study was conducted in the greenhouse and in the field utilizing two soils, Hanford sandy loam and Yolo silt loam (Typic Xerorthents) in which the organic matter was labeled by previous application of either 15N‐depleted or 15N‐enriched compounds. Contributions of atmospheric N2 to total plant N were calculated from the isotopic composition of plant tissue, using the composition of the non‐legume as a reference value. Ladino clover (Trifolium repens L.) and Wimmera ryegrass (Lolium rigidum L.) were grown alone and in mixture for purposes of comparison. In the greenhouse estimation of N2 fixation by the isotope and difference methods were in good agreement. There was essentially no transfer of fixed N2 from the clover to the ryegrass in any of the pot experiments. In the field experiment, clover obtained 85 to 100% of its N from the fixation process. In the clover‐ryegrass mixture, % total N and % 15N excess comparisons both indicated substantial transfer of N from clover to ryegrass after the stand had been established after about 6 months. Up to 79% of the N in ryegrass was calculated to be derived from such transfer. It is concluded that the isotope dilution method gives estimates of N2 fixation at least as good as those obtained by yield‐dependent methods such as the difference method, but it is not suitable for grass‐legume mixtures.

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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 KNO3 (≅ 285 kg N ha−1) enriched with 56 to 60% 15N to 1‐m2 plots. The surface flux of denitrification gases was measured from the accumulation of nitrous oxide (N2O) and 15N2 beneath airtight covers placed over the soil and from measurements of N2O using the acetylene (C2H2) 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 N2 produced was much greater than N2O. 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 NO3‐ 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.