Results are presented from a 3 year investigation into nitrate leaching from isolated 0.4 ha grassland plots fertilized with 250, 500 and 900 kg N ha-a-l . Cumulative nitrate leaching over the 3 years was equivalent to 1.5%, 5.4% and 16.7% ofthe fertilizer applied at 250,500 and 900 kg N ha-rates respectively. Over a whole drainage season, mean nitrate leachate concentrations at 250 kg N ha-did not exceed 4 mgl-', although maximum values of 13.3 mg1-l were observed. In contrast, at 900 kg N ha-the mean nitrate leachate concentration in two of the years exceeded 90 mg I-'. Mineral nitrogen balances constructed for the 1979 growing season indicated that leaching at 250 kg N ha-* was low because net mineralization of soil organic nitrogen was small, and crop nitrogen uptake almost balanced fertilizer application. Although the pattern of nitrate leaching suggested that by-passing occurred in the movement of water down the soil profile, it was not possible to confirm this using simulation models of leaching. Possible reasons for this, including the occurrence of rapid water flow down gravitationally drained macropores, are discussed.
A pulse dilution ~SN technique was used in the field to determine the effect of the ammonium to nitrate ratio in a fertilizer application on the uptake of ammonium and nitrate by ryegrass and on gross rates of mineralization and nitrification. Two experiments were performed, corresponding approximately to the first and second cuts of grass. Where no substantial recent immobilization of inorganic nitrogen had occurred, mineralization was insensitive to the form of nitrogen applied, ranging from 2.1-2.6 kg N ha -1 d -1. The immobilization of ammonium increased as the proportion of ammonium in the application increased. In the second experiment there was evidence that high rates of immobilization in the first experiment were associated with high rates of mineralization in the second. The implication was that some nitrogen immobilized in the first experiment was re-mineralized during the second. Whether this was nitrogen taken up, stored in roots and released following defoliation was not clear. Nitrification rates in this soil were low (0.1-0.63 kg N ha -1 d-l), and as a result, varying the ratio of ammonium to nitrate applied markedly altered the relative uptake of ammonium and nitrate. In the first experiment, where temperatures were low, preferential uptake of ammonium occurred, but where >90% of the uptake was as ammonium, a reduction in yield and nitrogen uptake was observed. In the second experiment, where temperatures and growth rates were higher, the proportion of ammonium to nitrate taken up had no effect on yield or nitrogen uptake.
SUMMARY Results are presented from a three year lysimeter investigation, employing single (15NH4NO3) and double (15NH415NO3) labelled ammonium nitrate to study the uptake of soil and fertilizer nitrogen by cut ryegrass at 250, 500 and 900 kg N ha−1 a−1. Average annual recoveries of nitrogen were equivalent to 99,76 and 50% of the nitrogen added at 250, 500 and 900 kg N ha−1, respectively. At 250 kg N ha−1 the difference between the overall nitrogen recovery and the fertilizer recovery was almost entirely attributable to pool substitution resulting from mineralization/immobilization turnover (MIT). At 900 kg N ha−1 both the low overall recovery of nitrogen and the low fertilizer recovery reflected the large excess of available nitrogen over crop requirements. No evidence of ‘priming’ was obtained. Analysis of the results from single and double labelled lysimeters using simultaneous equations indicated that at 250 kg N ha−1,∼70% of the nitrogen in the crop was derived from the ammonium pool. At 500 kg N ha−1 this dropped to 64%, while at 900 kg N ha−1 the figure was 59%. There was a suggestion that at the lower application rates, preferential uptake of ammonium was occurring but that as N supply exceeded crop requirements, nitrate was the major N source. Despite the preferential exploitation of the ammonium pool, at 250 and 500 kg N ha−1 pool substitution resulting from MIT resulted in lower recoveries of fertilizer ammonium compared with fertilizer nitrate.
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