| In recent years, urban modern agriculture has gradually turned into the main development direction of agriculture in Beijing. Along with the adjustment of planting structure in Beijing suburb, vegetables industry, which is commonly taken as the basic living needs and "food basket" of the urban residents, plays a particularly significant role in urban modern agriculture. A wide range of anti-season vegetables can not only improve people’s life quality, but also bring farmers higher economic benefits, however, there is the reduction of water and nitrogen use efficiency, the degradation of soil environment quality as well as the groundwater pollution, which thus seriously restricts the sustainable development of modern agriculture in Beijing suburb. In this study, the greenhouse soil in Beijing suburb was taken as the research object. By using tomato as tested crops, the influences of different fertilizer N rate and irrigation rate on the balance of water and nitrogen on the greenhouse farming system were studied. In the meanwhile, the effects caused by planting sweet corn on fallow soil to the reduce nitrogen leaching were also quantitatively evaluated. Beyond that, the simulation analysis of characteristics and influencing factors of transport of water and nitrate on the seepage face that was generated after installing Zero-tension lysimeter(ZTL) in the field was also studied. Finally, the collection efficiency of ZTL installed with different heights of divergence barriers under different weather conditions over many years was analyzed by solute transport model. The main results were listed as follows:A greenhouse tomato experiment with different N fertilizer management, including four nitrogen application rates (conventional N fertilizer, N1; 25% conventional N fertilization, N2; 44% conventional N fertilization, N3, and 63% conventional N fertilization, N4) was conducted in Beijing suburb from 2008 to 2009. As for the irrigation rate, it was based on the common practice utilized by most farmers, receiving 215,219 and 271mm for three growning seasons, respectively. The results showed that, under the same amount of irrigation, the yields of tomato have no significant reduction even if the amount of N fertilizer decreased by 63%. Under the condition of fertilizing the same amount of N and adding about 54mm amount of irrigation in spring-summer season, the tomato yield was increased by 55.5% and 36.1% compared with that in autumn-winter season and winter-spring season, respectively. Also, the increase of irrigation amount gives rise to the increase of water drainage in spring-summer season. The amount of water drainage under spring-summer season was increased 24 and 18mm compared with autumn-winter season and winter-spring season, respectively. Water use efficiencie (WUE) under three growning seasons were 38,41,44kg m-3, respectively. The amount of nitrogen loss in spring-summer season accounted for 58-97kg N ha-1 (i.e.,20.1-22.6% of the total inputs N), and the amount of nitrate leaching under autumn-winter season and winter-spring season reduced to 20-55kg N ha-1(i.e.,7-12% of the total inputs N) compared to spring-summer season. Furthermore, the amount of nitrate leaching under N4 treatment was decreased by 40.2-60.5% compared to N1 treatment, and the nitrogen use efficiency (NUE) under N4 treatment increased 11.6-17.5% compared to N1 treatment.Thus, reducing the N fertilizer input remains a more effective and immediate way to lower N losses.A fallow experiment with two tillage treatments (fallow and planting catch crop) and two initial soil residual mineral N treatments, which were using two fertilizer treatments for fore-crops (380kg N ha-1 and 260kg N ha-1) was conducted in Beijing suburb. By simulating the characteristics of soil water drainage and nitrate leaching under varying weather conditions, we found that the upward flux of water and nitrate below the 100cm soil depth under catch crop treatment can be up to 1.0mm d-1 and 0.7kg N ha-1 d-1, respectively. Thus, the downward movement of water and nitrate leached into deeper soil layers was reduced. The model simulation results also show that the amount of nitrate leaching out of the 180cm soil profile during the three fallow seasons were 54.1,113.3,58.4kg N ha-1 for fallow treatment and 32.3,63.8,22.5 kg N ha-1 for catch crop treatment, respectively. The leaching percentage under catch crop treatment reduced 35.3-57.9% compared with fallow treatment.The simulation performed for the nitrate leaching under two rainfall intensity scenarios (R1:80mm d-1 and R2:150mm d-1) in 2010, which indicated that sweet corn can partially reduce water drainage and nitrate leaching at depths of 100-180cm when the rainfall intensity reached to R2 scenario. Therefore, using sweet corn as a summer catch crop is an effective way to reduce the risk of nitrate leaching in greenhouse soil during fallow seasons.To improve the collection efficiency of ZTL, the study added divergence barrier to ZTL(ZTLd), influenced factors that can affect the collection efficiency of ZTL, such as soil hydraulic property, soil textures, irrigation rates, initial soil water content and height of divergence barrier, were simulated by HYDRUS-2D model. In the study, three soil textures including sandy loam, loam and silt were considered, and the values of Van Genuchten model parameters for test soils did not change in HYDRUS-2D model. Moreover, irrigation rates ranged from 100mm to 1000mm, and height of divergence barrier ranged from 0cm to 70cm. Two initial soil water contents (i.e.0.15 and 0.35cm3 cm-3) and two soil evaporation (i.e.0.2 and 0.5cm d-1) were adopted. According to the results, the collection efficiency of ZTL without divergence barrier (ZTLO) was low due to lateral diversion of water above the seepage face. As for the ZTLO under 1000mm-irrigation rates, the collection efficiencies were respectively 0-11%,5%-13% and 6%-12%in the sandy loam, loam and silt soil. Due to the fact that more water was accumulated above the seepage face, the measured soil water flux increased as the divergence barrier was installed. However, the height of divergence barrier reduced with the increase of irrigation rates as well as the decrease of water holding ability of soil and soil evaporation. When the height of divergence barrier was less than 20cm and irrigation rate reacheed 1000mm on the condition of 0.35cm3 cm-3 initial soil water content and 0.2cm d-1 soil evaporation, the collection efficiency of ZTLd was increased to 50%. Thus, coarser-textured soils, divergence barrier, lower soil evaporation and higher irrigation rates are preferred so as to measure water flux rate accurately by ZTLd. Higher initial water content (0.35cm3 cm-3) would lead to lower collection efficiency of ZTLd in sandy loam, and collection efficiency of ZTL at a certain burial depth might not be suitable for other depths. According to the assumption of study, all simulations are based on homogeneous soil without consideration of preferential flow, thus there was no effect of size on the collection efficiency. To conclude, the results of this research can exert significance to the improvement of ZTL.In the field experiment, ZTL was installed at the intensive greenhouse in Beijing subarb during summer open-roof fallow seasons of 2010 and 2011. Two treatments, high nitrogen residual (N1) and low nitrogen residual (N2), were set up according to the rate of nitrogen application on previous crops. On the basis of measuring and calculating water drainage and nitrate leaching in each treatment using ZTL, HYDRUS-2D model was used to conduct simulation analysis on the collection efficiency of ZTL installed with different heights of divergence barriers (DB) under different weather conditions over many years. The results showed that during the 2-year period of field experiments, no soil leachate was collected by ZTL in either treatment, which was associated with the lateral diversion of water formed when the water infiltration reached the contact surface between ZTL and soil (capillary barrier). Meanwhile, after the same treatment were installed with 60cm high DB, ZTL’s collection efficiency of water drainage (We) and collection efficiency of nitrate leaching (Ce) raised to 15.4% and 19.3%, respectively. However, no significance difference in We or Ce was observed between Ni and N2. Scenario analysis under different weather conditions over many years revealed that the installation of 10 cm high DB in rainy years was enough to increase ZTL’s We and Ce to 23.7% and 29.5%, respectively, while in dry and normal years, ZTL’s We and Ce were both 0 even 60cm high DB was installed, indicating that ZTL can only measure and calculate water drainage and nitrate leaching accurately when the soil water content at the capillary barrier approaches saturation due to heavy rainfall. ZTL has similar collection efficiencies of water drainage and nitrate leaching among different soil nitrogen residual treatments, which all increase with increasing DB height and precipitation. The results of this study can provide a reference for improvements of ZTL design. |
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