| Characteristics of nitrogen(N) mineralization, nitrification and denitrification in six pairs of soils grown with vegetables and food crops (irrigated paddy rice - upland winter crops), three pairs of soils grown with vegetables and upland food crops in Nanjing suburbs, respectively, were investigated by incubation experiments in laboratory. The soils grown with vegetables had been converted from food crops around 20 years ago. Mineralization and nitrification incubation experiments were conducted under 65%FC(field capacity) or 100%FC and 28 C, and for nitrification study 200mgN/kg of urea was added to the soil. In denitrification experiments, the soils were incubated with and without KNO3 at a rate of 200mgN/kg under 100%FC or waterlogged and 28 C. Moreover, N2O emission and denitrification loss in three pairs of soils with and without 200mgN/kg of urea were investigated under 100%FC and 28 C.Field experiments with green cabbage (Xiao Qing Cai) in 2002 and Chinese cabbage in 2003 were conducted in Nanjing suburbs. Ammonia volatilization was measured by an enclosure method, and N2O emission and denitrification loss were measured by without and with acetylene inhibition-soil core incubation technique, respectively. In the green cabbage field experiment, 3 treatments were control, low rate N and high rate N with a total application rate of 0, 75 and 150 kgN/hm2, respectively. N was applied as basal and top dressing in the ratio of 0.60 to 0.40. In the Chinese cabbage field experiment, 4 treatments were control, slow-released urea, low rate N and high rate N with a total application rate of 0, 180, 300 and 600 kgN/hm2, respectively. In the slow-released urea treatment, whole amount of N was applied as basal dressing. In the other 2 N treatments, N was applied as basal and 2 top dressing in the ratio of 0.25:0.25:0.50.Results from laboratory show that at 65%FC incubation, ration of mineralized N to total soil N in six vegetable soils were lower than those of the corresponding paddy soils. Amounts of mineralized N and ration of mineralized N to total soil N in vegetable fieldswere higher than those in upland soils grown with food crops, hi comparison with the corresponding paddy soils, nitrification rates in soils of vegetable fields were lower due to the lower soil pH value in most cases. On the contrary, in comparison with the corresponding upland soils, nitrification rates in soils of vegetable fields were higher. Moreover, mineralization and nitrification rates from paddy soils and the corresponding vegetable soils incubated by 100%FC were higher than those by 65%FC. In contrast, mineralization and nitrification rates from upland soils and the corresponding vegetable soils incubated by 100%FC were lower than those by 65%FC. Results show that denitrification rates in all of 5 vegetable soils were lower than those of the corresponding paddy soils, and denitrification rate in one of three vegetable soils was lower than the corresponding upland soil. Results from another incubation experiment show that N2O emission and denitrification loss from two of three vegetable soils were significantly higher than those of the corresponding paddy soils after incubation of 21 days (P<0.05) . The amounts of the denitrification loss were not significantly increased by application of nitrogen fertilizer in the three pairs of soils. The relation between the accumulated N2O emission or denitrification loss and the incubation time, in the three pairs of soils either with or without applied nitrogen, was matched with the revised Elovich equation of y=bln(t)+a.Results from field experiment show that ammonia loss was very low after either basal or top dressing to the green cabbage due to the low soil pH of 5.4. In Chinese cabbage field, no ammonia volatilization occurred after basal dressing due to a heavy rainfall after fertilization even though the soil pH is 7.7. There was small amount of rainfall during 2-4 days after 1st top dressing, and ammonia volatilization was 0.52%, 5.39% and 7.7% of applied N for the slow-released urea, lo... |
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