| Excessive fertilizers are often applied on vegetable fields in order to pursue higher yields. In some areas, the amounts of chemical fertilizers and manures so applied are several times of that needed by vegetables. Nitrogen (N) and phosphorus (P) have been obviously accumulated in the vegetable soils, resulting in excessive nitrate and poor quality of vegetables. Furthermore, a series of environmental problems are produced, i.e., the deterioration of physical and chemical properties of vegetable soils, the nitrate's pollution of vegetables, groundwater and the eutrophication of surface water. A series of field investigations over the vegetable fields in the suburb of Fuzhou City, Fujian Province and studies in laboratory were carried out to deal with the above environmental problems caused by the exessive fertilization for vegetables. The fertilization status quo of vegetable fields, accumulative characteristics of nutrients in vegetable-field soils, the concentration of nitrate in vegetable, non-point source pollution of surface-field water and groundwater in vegetable fields were analyzed. Pot experiments with 7 different ratios of nitrate-N and ammonium-N and 3 kinds of nitration inhibitors (dicyandiamide, imidazole and pyridine, respectively) were conducted to test the effects on the yield, nitrate's concentration of vegetable and the available metabolism of endogenous nitrate of vegetable. An experiment with 7 different fertilization models (none fertilization, basal application of chemical fertilizer, half-basal application and half-top-dressing of chemical fertilizer, basal application of chemical fertilizer and dicyandiamide, half-basal application and half-top-dressing of chemical fertilizer and dicyandiamide, half-basal application and half-top-dressing of chemical fertilizer and organic manure, basal application of organic manure) was carried out to verify the effects of different fertilization modes on the growth of vegetables, nitrate's content, N and P-accumulation of vegetables, the utilization ratio of fertilizer, and the loss of N and P from the field. The aim of this dissertation was to establish an optimized fertilization model which is suitable for the production of higher yield & quality of vegetable, and friend with the environment. The main results were as follows:1. Fertilizers applied on the vegetable fields in the suburb of Fuzhou were mainly chemical fertilizers with a small part of organic manure. The average fertilization rate (N,P2O5 and K2O) of different kinds of vegetables was between 493.61 212.2 kg·hm-2·stubble -1. The proportion of N:P2O5:K2O was 1:0.580.98:0.550.92 with an average of 1:0.77:0.75, indicating the inbalanced fertilization of N:P2O5:K2O and apparent over-fertilization of phosphorus. The range of average fertilization rate (N,P2O5 and K2O) for vegetable was 2 002.33 455.2 kg·hm-2·a-1 under various kinds of rotation systems. The average contents of total phosphorus (2.04 g·kg-1), Oslen-P(182.9 mg·kg-1) and CaCl2-P (1.018 mg·kg-1) of the vegetable soils were 3.16,6.87 and 12.3 times higher than those in the natural soils sampled from the neighbouring hills, respectively. The average organic matter content (37.4 g·kg-1) and total nitrogen (2.178 g·kg-1) were increased by 33.43% and 17.16%, respectively; The content of total potassium changed a little. However, the average contents of alkaline hydrolyzable nitrogen (200.2 mg·kg-1), available potassium (243.8 mg·kg-1), CEC (14.7 cmol·kg-1) and pH (5.97) were decreased by 15.01%, 38.2%, 3.14 % and 9.7%, respectively.2. N and P non-point source pollution status quo of the vegetable fields in the suburb of Fuzhou city: (1) The proportions of the vegetable samples of which the nitrate contents were above the very severely polluted level (NO3-≥3 100 mg·kg-1), severely polluted level (NO3-≥1 440 mg·kg-1) and moderately polluted level (NO3-≥785 mg·kg-1) were 13.33%, 32.5% and 50.83%, respectively. (2) The proportions of the surface-field water samples of which the ammonia-N contents were above theⅢclass surface water limitation (1 mg·L-1) andⅤclass (2 mg·L-1) were 62.5% and 56.25%, respectively. 12.5% of the surface-field water samples of which the nitrate-N contents was above the limitation of Chinese surface water of collective living-drinking water (10 mg·L-1). The average and maximal N concentrations of the vegetable surface-field water samples were 10.99 and 33.80 mg·L-1, respectively, and 5.5 and 16.9 times of the theⅤclass limitation of the surface water quality standard (N 2 mg·L-1), respectively. The average and maximal total P contents of the vegetable surface-field water samples were 4.75 and 12.75 mg·L-1, respectively, and 11.9和31.9 times of theⅤclass limitation of the surface water quality standard (P 0.4 mg·L-1), respectively. (3) The proportion of the vegetable field groundwater samples of which the ammonia-N content was beyond the V class water quality limitation (0.5 mg·L-1) was 18.18%. The Proportions of groundwater samples of which the nitrate content was super-scale (≥10 mg·L-1) and seriously super-scale (≥20 mg·L-1) were 54.55% and 27.27%, respectively. The total N of all the groundwater samples were beyond the V class water quality limitation (N 2 mg·L-1,GB3838-2002). The proportion of the vegetable field groundwater samples of which the total P was beyond the V class water quality limitation (P 0.4 mg·L-1, GB3838-2002) was 81.82%.3. The optimized fertilization models for controlling nitrate pollution of vegetable: (1) Within the ranges of ratios of NO3--N / NH4+-N (3/11/3), with the decreasing ratios of NO3--N / NH4+-N, the plant's tallness, weight and yield of vegetable were increased firstly and decreased afterwards while the content of nitrate in the vegetable was increased firstly, decreased after and increased again. As compared with the ratio of NO3--N / NH4+-N of 1/3, the ratio of NO3--N / NH4+-N of 2/3 resulted in the increase in the tallness, weight and yield of Pakchoi by 15.57%,45.05% and 13.67%, respectively, and in the reduce in the nitrate of Pakchoi by 37.49%. Therefore, the ratio of NO3--N / NH4+-N of 2/3 was better for the growth and the decrease of nitrate of Pakchoi. (2) When three kinds of nitration inhibitors, dicyandiamide, imidazole and pyridine, were applied with the fertilization model (NO3--N / NH4+-N = 2/3), the yields of Pakchoi were increased by 6.06%28.55%, the nitrate concentration was decreased by 2.69%19.66%, the N amount of Pakchoi was increased by 2.38%38.42%, the leaf's NRA (Nitrate Reductase Activity) was increased by 24.28%77.32%, MPS (Metabolic Pool Size) was increased by 29.45%272.17%,, SPS (Storage Pool Size) and MPS/SPS were increased by 2.78%17.38% and 0.04%0.59%, respectively. Consequently, the availability of endogenous nitrate in Pakchoi was improved.4. The optimized fertilization models for controlling the leaching of N and P: (1) The phosphrous adsorption characteristics of the vegetable field soils can be well fitted by Langmuir isotherm equation (R2=0.991**0.998**). The range of the instructive phosphorus application rates estimated from the Langmuir equation were between 11.6267.37 (P)kg·hm-2 with an average of 27.18 (P)kg·hm-2. The contents of the available and total phosphorus of the vegetable soils were much higher than the leaching thresholds (56.96 mg·kg-1 and 1.146 g·kg-1) estimated by regression equation. The average DPS of the vegetable soils was 23.12%, close to the easily leaching threshold (25%) of the soil. The DPS of 4 pieces of the vegetable fields investigated had already been beyond the threshold. It is obvious that the phosphorus lost potential of the vegetable soils in the suburb of Fuzhou City is rather high. (2) The soil colum experiment in greenhouse showed that"Basal application of chemical fertilizer and dicyandiamide"and"Basal application of organic fertilizer"were better than the other models because they not only improved the vegetable's agronomic properties, increased the yields (by 113%301% and 238%250% compared with none fertilization) and the N accumulation (by 194%336% and 208%227% compared with none fertilization treatment), P accumulation (by 93.5%133% and 144%229% compared with none fertilization treatment) by the vegetable and therefore the utilization ratio of the fertilizer, but also reduced the nitrate concentration of the vegetable (by 10.9%39.6% and 6.8%34.3% compared with the basal application of chemical fertilizer treatment) and the leaching loss of NO3--N and NH4+-N (by 53.4% and 46.6%% compared with the basal application of chemical fertilizer treatment) and water-soluble P (by 17%% and 11.3% compared with the basal application of chemical fertilizer treatment ) during the vegetable growth period and hence reduced the risk of agricultural non-point source pollution.5. The optimized fertilization models for controlling the loss of N and P along with the surface runoff:"Basal application of chemical fertilizer and dicyandiamide"and"half-basal application and half-top-dressing of chemical fertilizer and organic manure"were better than the other models because they not only improved the vegetable's agronomic properties, increased the yields (by 93%226% and143%154% compared with none fertilization teatment) and the N accumulation (by 231%320% and 153%216% compared with none fertilization treatment) and P accumulation (by 131%417% and 169%1167% compared with none fertilization treatment) by the vegetable and the utilization ratios of fertilizer by the vegetables, but also reduced the loss of the NO3--N and NH4+-N (by 46.46% and 48.10% compared with basal application of chemical fertilizer treatment) and water-soluble P (by 21.02% and 10.73% compared with basal application of chemical fertilizer treatment) from the field along with the surface runoff during the vegetable growth period and hence reduced the risk of agricultural non-point source pollution. |
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