| In the face of nitrogen and phosphorus loss and decline in soil sustainable production capacity and agricultural non-point pollution to demolish environment in vegetable field in South China, selected typical farmland in Guangxingzhou Junshan Yueyang Hunan ,which is typical vegetable-producing areas in South China. We studied the space-time variation of the available nutrient of the vegetable field, monitored the dynamic evolvement trend of the in-site soil in farmland and soluble nutrients in groundwater. This paper aimed at hunting mechanism and approach of nutrient loss in the vegetable field in South China, exploring the technique and measure of improving the using efficiency of nutrients in vegetable field, decreasing the nitrogen and phosphorus pollution.We hung the following three methods together, farmer investigation, soil sample collection and field test. We arranged three experiments:1) We launched farmer investigation in spring and autumn, including the fertilizer type, fertilizer application period, fertilizer quantity, fertilization methods as the main content. Summed up the rule of fertilize.2) At the rear harvest period of the spring and autumn crops, using the grid navigation function of GPS collected 144 soil samples topsoil in 15 ha core research areas. Using the classic statistics and geo-statistics methods to research the nutrients space-time variation rule. 3) According to local common management, selected cabbage and Chinese cabbage as representative crops , in the local fertilization and recommended fertilization to layout the nutrients content dynamic monitoring experiment. We took fresh soil samples at different levels in farmland with interval of 10 days, if the groundwater buried depth was low less than 120 cm, collected the groundwater samples either.Analyzed the data of above experiments, we got following results in four aspects.Farmer fertilization characteristics.1) The main crop of the first season was pumpkin, and the planting area was 87.9% of the total area.The planting area of the main breed was 50.4% of total pumpkin area. The main crop of the second season was cabbage, and the planting area was 88.8% of total area, The planting area of the main breeds was 86.8% of total cabbage area.2) The regular application fertilizer was 15%N-15%P-15%K complex fertilizer, calcium phosphate and ammonium bicarbonate. Mostly base fertilize only while ploughing. The nitrogen surplus rate of the Chinese cabbage, capsicum and cabbage were 81.75%,71.02%,29.27%,120.19%,the phosphate surplus rate were 120.96%,152.8%,567.86%,186.88%,The potassium insufficient rate were separate 39.33%,60.39%,57.05%,49.44%, and the potassium fertilizer dosage was serious shortage.Space-time variation of the available nutrients in soil.3) Available N, P, K in autumn and winter obeyed normal distribution. Available N presented right partial distribution and other nutrients presented left partial distribution. The average and standard deviation of the three kinds of nutrients had little difference between different months. Three nutrients had middle level variations.4)Different available nutrients in the same reason were significantly positive correlation; the spatial synergies relationship were significantly in the observed area ,and the spatial synergy between available phosphorus and nitrogen was the strongest whether in summer or autumn.5)The same nutrients in different seasons were significantly positive correlation, the correlation coefficient were: 0.561 of available potassium,0.422 of available phosphorus, 0.265 of available nitrogen, seasonal changes of three kinds of nutrients were basically symmetrical distribution, the average changes of available N,P,K were: 0.7,1.1,1.9 mg/kg. seasonal variation of nutrients were significant correlation.6) The difference of ratio of available nutrients were unconspicuous in different seasons, NPK ratio of the average was 1:0.242:0.752 in August and 1:0.244:0.782 in December.7) In December the optimal model of available potassium was spherical model, while the other models were exponential models; in different seasons, the longest correlative range of available nitrogen changed little and maintained at 450 meters, the longest correlative range of available phosphorus, potassium were obviously different between summer and winter, which reduced from 413 meters, 497 meters in August to 141 meters, 157 meters in December; the nugget proportion in summer significantly were higher than in winter, except available phosphorus were weak in August, and the spatial correlations of others were moderate, the spatial correlations of the available nitrogen were stable in different seasons, the spatial correlations of available phosphorus and potassium in August were less than in December.8)The spatial distribution trends of nitrogen, phosphorus and potassium were obviously different in different seasons; compared with August, the area weighted average of available nitrogen, phosphorus and potassium changes were 0.47mg/kg,0.62mg/kg and 1.96mg/kg, the proportions of increased area were: 46.7%, 51.5% and 61.2%, average increased content were: 5.7mg/kg, 4.8mg/kg and 10.3 mg/kg; the proportions of reduced area were: 53.3%, 48.5% and 38.9%, reduced averages were: -4.1mg/kg, -3.8 mg/kg And -11.2 mg/kg.Dynamics of water and available nutrients in soil. 9) Water of soil totally declined in autumn. Water of soil increased with depth decreasing, and reduced with time extend. 10) Nitrate-nitrogen in surface soil were greatly higher than in other soil layer, the overall change trend of nitrate-nitrogen was increase in beginning and then decrease, finally reached stability. Nitrate-nitrogen in surface soil got the highest value about 20 days after fertilization, then declined continually in the whole crop growth period, relatively stable until late crop growth period.11)The content of ammonium nitrogen in soil was not high and relatively stable throughout the growing season, there was little difference in various levels. Crop species and fertilizer rate had no significant influence to ammonium nitrogen. The content basically maintained at 1 mg/kg in 4 treatments, but increased significantly in the late period of harvest.12) Available phosphorus mainly concentrated in the surface, the contents of other levels were relatively low, the difference was unconspicuous; The content of available phosphorus in the surface decreased gradually, but the content in other levels changed a little, basically maintained at 5 mg/kg. Nearly all of phosphorus absorbed by crops came from the surface, The phenomenon of phosphorus downward was not obvious.13) Availabe potassium of surface soil was high and changed clearly, The change of availabe potassium in surface soil clearly affected the content of 20-40cm and 40-60cm soil layer. Change law of potassium of different fertilizer treatment were similar, but significant different between different crops.Dynamics of soluble nutrient in underground water.14) When the ground water burial depth achieves 40cm the soluble nutrient content was higher than other depths, the soluble nutrient contents was: 249mg/kg of nitric nitrogen, 0.96mg/kg of ammonium nitrogen, 0.75mg/kg of soluble phosphorus, 10.1mg/kg of soluble potassium.Summarizing the fore-mentioned result we got following conclusion:The fertilization characteristics of peasants.The vegetables planting pattern of Guangxingzhou ws relatively unitary, the difference of variety and the quantity of fertilizer were very small. between the peasant household, applyed fertilizer the variety to be quite unitary, the vegetable nutrients non-equalizing was serious, nitrogen and phosphorus was sufficient while the potassium was serious insufficient.The variation rule of available nutrients. Alkaline hydrolysis nitrogen, available phosphorus and available potassium in August and in December obeyed the normal distribution, mean value, the standard deviation of three kinds of nutrients were not significant differenr between two months, variation degree of three kinds of nutrients were medium. Different available nutrients in the same season , same kind of nutrients during the different seasons, the change of available nutrients between different season were remarkable corelative. The ratio among available nutients were not significent difference between different season. Most of the variance models were exponential model, the spatial relevant degree were moderate, the longest correlative range of alkaline hydrolysis nitrogen in different season maintained stable, but those of the available phosphorus and potassium were obvious difference in the summer compared with winter. Nugget of summer must were obviously bigger than the winter. The spatial distribution of available nutrients was obviously different in different months.Dynamic rule of soil moisture and available nutrients. The moisture and the available nutrients in the entire vegetative season reduced, the content of surface layer were generally high, while it unceasingly reduced with the time lapse or in the vertical direction by the surface layer to in-depth. The main existence shape of nitrogen in the soil body was nitrate nitrogen, the content was bigger than the ammonium nitrogen content by far, under the local management pattern the nitrate nitrogen content of the surface layer achieve to highest level nearly 20 days after fertilizing. The phenomenon of surface gathering of available phosphorus was most obvious, in the entire vegetative season the soluble phosphorus content of subsoil changed little. Surface layer content affect subsoil ,the order was potassium > nitrogen > phosphorus. Ground water soluble nutrient dynamic rule. The available nutrients content of ground water were correlative with the soil layer it achieved. The nitrogen was relatively quite easy to drain.Nutrients outflow mechanism. Unreasonable fertilizeration and the local specific hydrology condition were the primary cause for the nutrients outflow.
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