| The North China Plain is the most important food production area in China, providing nearly 20% of the total national winter wheat-maize production with intensive management practices by receiving adequate irrigation water and excessive chemical fertilizers, and followed by intensive tillage, crop rotation, etc. Therefore, this kind of intensively managed cropping systems with low N use efficiency and high water consumption could be an important source of atmospheric CO2 and N2O, and usually be responsible for nitrate leaching into groundwater, leading to some severe problems, i.e., water shortage, global climate change, and heavy groundwater contamination, etc. An urgent demand for optimizing management practices to maintain high crop yields, to conserve diminishing natural resources, and to minimize environmental damage in North China Plain is emerging.Drip irrigation combined with split application of N fertilizer dissolved in the irrigation water (i.e. drip fertigation) is considered an efficient strategy for water and nutrient application during crop production. It could be a promising technique for N2O mitigation, water and fertilizer saving, because drip irrigation reduces surface evaporation and deep percolation, obtaining high water use efficiency, while fertigation is ideally suited for controlling the placement, time and rate of fertilizer N application, thereby increasing N use efficiency and decreasing N losses. However, the dynamic process of soil water and mineral N generated when drip fertigation is used is unclear, and the nitrification, maybe a major contributor to the emission of N oxides, is still ambiguous, due to the lack of field measurements from wheatnmaize rotation systems in North China Plain. Therefore, in order to increase the environmental benefits of drip fertigation without yield penalties in irrigated wheat and maize crops, by combining the field experiment methods with biogeochemical model, the objectives of this study were: (1) to monitor the water and N diffusion and movement mechanisms, releasing routes and transferring characteristics under drip fertigation; (2)to quantify the N2O emissions and evaluate the possibility of reducing these emissions by manipulating the mineral N applied by fertigation; (3) to validate and calibrate the DNDC model for simulating drip-fertigated water and N dynamics, and N2O emissions using the field monitored data (4) to better understand the main factors driving the N2O fluxes and assess the optimized management practices maintaining crop yield by using DNDC model in order to make general recommendations for farmers and policy makers. The main results were as follows:(1) Drip irrigation main affected the horizontal and vertical transports of water and nitrate N, while nitrogen application affected the horizontal movements of nitrate. The experiment showed that vertical migration depth of soil water and nitrate-N increased with the increase of drip irrigation amount. When the irrigation coefficient was 0.5 and 1, the vertical migration depth of soil water and nitrate N was 60 and 80 cm. Reduced amount of drip irrigation could reduce the risk of water and nitrate N leaching. The accumulation of nitrate N at the boundary of the wetted soil volume occurred when the nitrogen application was low. The highest nutrient homogeneity of wetted volume was under N2 treatment, which showed that optimal fertilizer application could improve the soil nutrient homogeneity. The accumulation of nitrate at the vertical boundary of the wetted volume was not observed within any treatment. After harvesting of winter wheat and summer maize, nitrate N accumulation in 0-100cm soil layer had positive correlation with the amount of fertilizer application, while the NO3"-N accumulation in 0-40 cm soil layer was significantly higher than other soil layers.(2) When the irrigation coefficient≥1, it could maintain the water content over 75%-80% of field capacity in 0-80cm soil layers during the whole cropping season. In the winter wheat season, the highest water use efficiency was observed (2.28 kg m-3) under the treatment with irrigation coefficient of 1. But the crop yield and water use efficiency were not increasing in the summer maize season because of plentiful rainfall. The apparent nitrogen loss among all drip fertigation treatments were between 0 and 21.59kg ha-1 during winter wheat season, having significantly positive correlation with nitrogen fertilizer. The N loss was 228.58kg ha-1 in conventional irrigation treatment which was far higher than that in drip irrigation treatments. Different from winter wheat season, the drip fertigation would not decrease the apparent nitrogen loss in summer maize season. The most optimal fertilizer treatment was N2 treatment by comprehensively assessing nitrogen recovery efficiency, nitrogen physiological efficiency, nitrogen agronomic efficiency and grain nitrogen recovery efficiency.(3) The drip irrigation reduced the surface evaporation and deep percolation, obtaining high water use efficiency, while increasing N use efficiency and decreasing N losses. The results of the water and NCV-N content of 0-10cm soil layer after drip fertigation in jointing stage of winter wheat showed that drip fertigation had better water and fertilizer retaining effects than that under conventional irrigation and fertilization. During the winter wheat season, when the rainfall was low, nitrogen surface accumulation phenomenon was observed in both drip fertigation and conventional treatment but in summer maize season with plentiful rainfall. In summer maize season, conventional treatment may cause a higher risk of nitrate N leaching. Soil water content fluctuated more evenly under drip fertigation. The nitrate N content in surface soil in N3 treatment was improved the most with the increasing of fertilizer application.(4) N2O fluxes in all the drip fertigation treatment were lower than that in conventional treatment. N2O fluxes showed arising tread within 3-5 days when drip fertigation was used, high peak of N2O emissions occurred in July due to the plenty of rainfall. Compared to conventional treatment, the drip fertigation treatment significantly decreased the N2O cumulative emissions by 43%with the same nitrogen amount. The average N2O direct emission factor and emission intensity in drip fertigation treatment decreased by 27%and 47%, respectively. The soil surface tempreture, NO3--N content and WFPS had significant effects on N2O emissions under drip fertigation.(5) The fertigation function in DNDC model was calibrated by introducing parameters of surface soil temperature. The proportional coefficient of NO3- and NH4+ transforming to N2O in DNDC model was modified improving the precision for simulating N2O emissions, soil NO3* and NH4+accumulation. The revised model version yielded better results in simulated crop yield, soil soil NO3- dynamics, N2O fluxes and cumulative N2O emissions. Based on comprehensive consideration of crop yield and N2O emission, the modified DNDC quantified the effects of different drip irrigation scenarios on crop yield and N2O emission, and put forward the optimal drip fertigation strategy, i.e., the drip irrigation amount was 130mm (irrigation coefficient=1), nitrogen application amount was 189kg N ha-1 in winter wheat season, while the drip irrigation amount was 19.2mm (irrigation coefficient=0.2), and nitrogen application amount was 231kg N ha-1 in summer maize season. At the same time, the fertigation should be canceled during the rainy season. This drip fertigation management practice could accommodate the needs to maintain crop yields, to conserve diminishing natural resources, and to minimize environmental damage. |
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