The gaseous losses of N (N 2 O + N 2 ) measured for 130 days (May-September 1983) from conventional fallow at Yorkton, Oxbow and Weyburn soil sites ranged from 9 to 11, 15 to 31 and 60 to 87 kgN∙ha −1 for upper, middle and lower slope positions, respectively. The corresponding values for chemical fallow were 18–28, 24–51, and 69–98 kgN∙ha −1 . In both tillage systems, gaseous N losses increased in the order of upper < middle < lower slope positions and were associated with the variations in soil moisture. The results obtained from additional widely scattered field studies on chernozemic soils further confirmed that the more dense surface soil and relatively higher soil moisture (lower air-filled porosity) were the major factors affecting increased denitrification under chemical fallow. Volumetric soil moisture was the only factor which showed a very highly significant correlation with N 2 O emmisions. Key words: Acetylene inhibition-soil core technique, chemical fallow, denitrification, nitrification
Rapeseed plant tissue and composite soil samples from field fertility plots were assayed for total N and inorganic N content, respectively, and corresponding 15N abundance. Lower δa 15N values were obtained for subsurface horizons containing residual fertilizer-N in both dryland (−2.0) and irrigated (ave. −3.5) plots compared to the surface horizons (3.4 and ave. 8.6). A systematic decline in the δa15N values of the plant tissue was observed with irrigation application (20 and 30 cm) and application of N fertilizer (220 kg/ha). This was attributed in both instances to the increased contribution of fertilizer N to the plant. Calculation of the mass balance of the two isotopes in the fertilized dryland treatment indicated that no significant fractionation had occurred during N uptake by plants but the differences in the δa15N values represented changes in soil N. While this study has clearly demonstrated the potential of utilizing variations in natural 15N abundance to trace the fate of applied fertilizer N, it has also shown that it is of utmost importance to recognize the N transformations that have taken place in the soil-plant system and the isotope effects that accompany them.
Nitrogen balance (fertilizer N accounted for in the soil–plant system) and standard isotope (obtained on above-ground plant parts) criteria were used to evaluate the efficiency of nitrogen sources for barley grown on a Chernozemic and a Solonetzic soil under greenhouse conditions. The isotope criteria, percent total N in the plant tissue derived from fertilizer (% N d.f.f.), "A" values, and uptake of fertilizer N by the crop, clearly indicated the superiority, in terms of plant availability, of the NO 3 − -N source, followed by NH 4 + -N, with urea the least effective. In contrast, loss of nitrogen from the soil–plant system was greatest for the NO 3 − -N and least for the urea (i.e., 67 vs. 26% on the Solonetzic soil). Such conflicting results can be explained on the basis of slow hydrolysis of the urea and rapid plant uptake of N from the NO 3 − -N form. It is concluded that, although isotope-derived criteria such as % N d.f.f., A values, and uptake by the crop of fertilizer N provide precise measurements of the performance of N sources, serious errors in causative factors may be made unless "nitrogen balance" data are available. The significance of primary and corrected (rate of fertilizer N application corrected for fertilizer N loss) A values are discussed.
Cool soil temperature regimes with initial soil temperatures of 5 °C rising to 20 °C at the heading stage reduced the rate of growth of barley by approximately one-third compared to 15–25 °C but did not change the barley yield or the fate of the applied fertilizer N in the soil biomass, roots, or tops of the plant or that lost by denitrification. The primary isotope data, % Ndff or ’A’ values remained relatively constant irrespective of whether the straw was placed on the surface or mixed throughout the soil. In contrast, the nitrogen balance data verified that fertilizer N loss, presumably due to denitrification, was as high as 35% in certain treatments, and further that up to 40% of the added fertilizer N was immobilized where the straw was uniformly mixed in the soil. The nitrogen balance data were used to correct the original rate of fertilizer N application. When this was done, A values calculated on the basis of the revised rates of application showed that the amount of soil N which was denitrified or immobilized was approximately double that of the applied fertilizer N. Thus, it is possible where a N balance is included in an investigation to quantitatively assess the effect of management practices on available soil N. It is further concluded that differential immobilization or denitrification of the 15 N fertilizer standard may invalidate yield-dependent isotope-derived data, such as dinitrogen fixation unless nitrogen balance data are available to permit the appropriate corrections to be made. Key words: Zero till, N-cycle, temperature, crop residues, barley
The δ a 15 N for the total and nitrate soil N from surface samples taken from recharge and discharge areas associated with a saline seep differ significantly. Suggested reasons for these differences are included. These data suggest that the nitrate moving towards the surface with the soluble salts is depleted in the heavier isotope; the very high δ a 15 N for total and nitrate N of surface samples suggests that denitrification has been a dominant process operating in the saline seep area. Incubation studies carried out on Ap samples taken from the recharge and discharge areas have verified that the δ a 15 N (and perhaps also the nature of the mineralizable organic nitrogen) from a "mature" and "recent" saline profile are different from each other, and distinctly different from that of the recharge area.
Four soils of different physical and chemical characteristics were treated with 10 ppm lead in the form of PbCl 2 labelled with lead-210 (2.5 mCi/g 207 Pb) and subjected to a 7-wk incubation period. The extractability of the added lead was assessed using 12 extractants. Almost complete recovery (93–98%) of the applied lead was obtained with 6 N HNO 3 . Large amounts of the applied lead [Formula: see text] could be extracted by chelating agents (DTPA and EDTA). Relatively small amounts were exchangeable with 0.5 M BaCl 2 (0.1–4%) and N NH 4 OAc (1–11%). Proportionally slightly higher amounts were extracted with 2.5% AcOH and 0.5 M NaHCO 3 . Only traces could be found in the 0.05 M CaCl 2 extract.
