| In this paper we took the cornfields in the central sub-humid zone in Jilin Province as the research object. Combining field trials with micro-region test, we carried out crop fertilization test and observed the nitrogen balance for continuous 4 years. We studied the major processes such as nitrogen deposition input, crop uptake, soil residues and nitrogen leaching loss in the farmland ecosystem under the corn continuous cropping planting system, clarified the relationship between nitrogen agronomic effect and nitrogen balance. Basing on the results, for reducing the application of nitrogen to improve the environmental conditions, determining the reasonable applying amount of fertilizer to ensure the crop yield, we put forward the fertilizer type, quantity and optimum fertilization program. The main results obtained were as followed.(1) The average wet deposition of N was 18.3 kg/hm2, which was about 12.2% of the average amount of applied N (150 kg/hm2) in the central part of Jilin province during the corn growth period. Assuming the precipitation during the crop growth period is about two thirds of the whole year and the nitrogen concentration in the rainfall is the same, it is estimated that the annual farmland nitrogen deposition can reach 27kg/hm2, which is equivalent to about 18 percent of the local application amount of nitrogen fertilizer. The average concentration of the total N in the rainfall was 4.14 mg/L, in which the concentration of the NH4-N and NO3-N was 1.13 mg/L and 1.49 mg/L respectively, accounting for 57 percent of the total N, which showed that the atmospheric wet deposition of nitrogen had become an important source of N in local farmland. The amount of wet deposited N fluctuated greatly between years and seasons. The annual amount of wet deposition of N is significantly positively related to the amount of rainfall, which is higher in May and June. The amount of wet deposition of N for a single rainfall is significantly positively related to the amount of rainfall.(2) The relationship between corn yield and the application amount of N applied and absorbed can be expressed on linear plus platform and quadratic equations. Taking the nitrogen application amount when reaching 95 percent of the highest yield as an economic and efficient fertilization, the fertilization amount in the high yield and medium yield should be 72% and 56% of the current (250 kg/hm2). Nitrogen agronomy effect decreased exponentially with the application amount of nitrogen, and the apparent nitrogen balance increased linearly with the amount of nitrogen, and the relationship between nitrogen inputs and net income was in line with diminishing rate. When the nitrogen application was 211kg/hm2, the income, which was 19,815 Yuan/hm2, was the highest. Comprehensively considering the relationship between apparent equilibrium point of fertilization, fertilization amount of nitrogen and crop yield, and the relationship between fertilizer application investment and net income, a reasonable nitrogen fertilization range for local cornfield can be determined at 159 kg/hm2-187 kg/hm2, which can obtain relatively higher yield, higher net income and can reduce the risk of leaching. In terms of the kind of fertilizer, the yields in the plots applied organic manure only for three consecutive years or combining organic manure and inorganic chemical fertilizer are close to or even exceed the plots applied inorganic chemical fertilizer only, not only getting high-yield, but also reducing the cost, increasing the efficiency, and be good to the environment and ecology.(3) Total nitrogen leaching amount of farmland background(no fertilization, CK) was 5.00 kg/ hm2·a. The average loss of total N in the nitrogen-applying treatments was between 4.53kg/hm2·a and 7.17kg/hm2·a. The loss of farmland background accounted for more than 70% of them.The nitrogen leaching intensity was determined by the volume and concentration of the leachate. The 4-years average total nitrogen concentration in the treatments of without fertilization (CK), applying phosphorus and potassium (NO) was 14.08 mg/L and 16.22mg/L respectively, in other treatments with nitrogen fertilizer was between 17.07 mg/L and 22.72 mg/L. The total nitrogen concentration in treatments with nitrogen application was significantly higher than that in non-applying nitrogen treatments. The amount and concentration of leachate are closely related to the rainfall during the growth period. On average, when the rainfall is less than 42.8mm, leaching solution in each treatment is generally not more than 3.7mm. When the rainfall was greater than 42.8mm, the leachate increased 0.15mm with each additional 1mm rainfall.1Leachate can be collected 5-11 times per year, an average of 7-8 times, and occurs mainly in June and July. In fact, the number of leaching solution also showed a big difference among treatments. Deep leachate in each treatment accounted for 3.00%-5.53% of the rainfall in the same period; wherein, the leachate quantity in the treatment of CK and NO was large, approximately 29.0mm; in other treatments the amount of leachate is between 18.7mm-22.0mm. Since the data were gotten from the observation to the rainfall in the leakage micro region, the variation may be mainly affected by the soil uniformity of the pool.(4) The soil inorganic nitrogen content after harvest increased exponentially with the increase of nitrogen application. When the nitrogen applied was greater than 250 kg/hm2, the soil inorganic nitrogen content after harvest significantly increased with the increase of the rate of nitrogen application. The results of observations showed that seasonal nitrate leaching depth was about 60cm-80cm. Therefore, in order to reduce nitrogen leaching losses, lower production inputs, the appropriate application amount of nitrogen should not exceed 250 kg/hm2.(5) According to the concept of crop soil buffering capacity, we got buffer zones of nitrogen for different yield levels to avoid significant loss of soil nitrogen. Considering the relationship between com yield and nitrate loss, the buffer zone should be 159kg/hm2-161kg/hm2, namely nitrogen application should not exceed 161kg/hm2. Considering the relationship between corn yield and soil inorganic nitrogen residues the buffer zone should be 140 kg/hm2-250 kg/hm2. With the increasing of nitrogen, crop nitrogen apparent absorption rate decreased from 53.8% to 20.4%. The proportion of the apparent residual nitrogen and other losses ascended synchronously, while the amount of nitrogen applied at nitrogen apparent equilibrium point is 187 kg/hm2. From the investment-net earning curve, we obtained the nitrogen amount which should be applied for maximum economic benefits, which should be 211 kg/hm2; Taking nitrate limit (20 mg/L) in groundwater standards III as the upper limit of the leachate quality indicators, considering the relationship between the nitrogen application and nitrate nitrogen concentration, the maximum nitrogen applied should be 242 kg/hm2.In summary, in order to guarantee crop yield, prevent nitrogen loss, and reduce the load on the environment, the appropriate amount of nitrogen fertilizer applied in the corn farmland in central Jilin should be 159 kg/hm2-187 kg/hm2. When determining the application amount of nitrogen fertilizer, the nitrogen coming from the wet deposition of atmospheric precipitation should be considered together, thus reasonably control the nitrogen residues after harvest to minimize the amount of nitrogen leached into deep underground or the groundwater. As the local soil fertility was high and existed big difference, the application of nitrogen should be managed scientifically to adapt to the local conditions. The application of chemical fertilizer should meet the multi-requirements of high yield, low input, low pollution, sustainable development, achieving the harmony of economic, social, ecological and environmental benefits. |
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