| Rice is one of the major grain crops in China, accounting for about 40% of the total grain production. Rice planting area accounts for about 28% of the total area of grain crops. Rice production plays a decisive role in grain production in China. The shortage of water resources in rice-planting areas has become an important issue in local agricultural production. The extensive implementation of rice-based water-saving irrigation, including alternate wetting and drying (AWD) irrigation, is of great significance in ensuring national water security. However, compared with the conventional continuous flooded irrigated (CFI) paddy field, the water and nitrogen transport in AWD paddy fields is more complicated, and changes in soil moisture and temperature are more frequent and with large magnitude. Water-saving and fertilizer-saving (nitrogen fertilizer) are two important targets in rice production, even though they may be mutually contradictory in AWD rice fields. It is necessary to study the mechanism of water and nitrogen transport and transformation in soil of AWD paddy fields and to determine a reasonable and integrated method for irrigation and nitrogen fertilizer application. In this thesis, for the soil-plant-atmosphere-continuum system of paddy fields, field experiments of water and nitrogen, soil hydrodynamic tests in laboratory and one-dimensional model simulations were conducted to study the effects of AWD irrigation on the water and nitrogen transport and transformations. The mechanism for changes in the field regime of water and nitrogen of paddy field under AWD with the irrigation and precipitation processes was undermined. The strategies for water and nitrogen management were explored to maximize water-saving, fertilizer-saving and high-rice-yields for rice production under AWD. The main conclusions of this thesis study are as follows:(1) Field experiments were conducted to observe the water and nitrogen leaching in paddy fields under AWD and CFI. The effects of AWD irrigation on rice growth and crop yield were investigated. AWD irrigation has no adverse effect on rice yields at the field level. Nitrogen application significantly increased grain yield, but nitrogen production decreased with increasing nitrogen application rate. Successful implementation of AWD irrigation reduces the input of irrigation water, due to reduced infiltration and leakage losses. However, due to the cracks formed in the drying stage, the preferential flow during the rewetting period after drying causes a large amount of water loss in the field. The soil layer, plow pan, is the key is to prevent leaching of water and nitrogen, so cracks formed in the dry period should not be extended to the bottom of plow pan. The increase of capillary rise of groundwater was 26.1-27.4% of the total water input. AWD irrigation greatly reduces irrigation water use, partly because it increases the capillary rise of groundwater. Therefore, the contribution of groundwater capillary rise to water saving irrigation should be considered in the design of irrigation schedule. The NO3-N concentration in the paddy field under AWD was 64% higher than that under CFI, and the NO3-N leaching in the AWD paddy fields was higher than that in the CFI paddy fields. Therefore, AWD irrigation can induce higher nitrogen leaching loss in paddy field. The source and fate of nitrogen in paddy fields between AWD and CFI were significantly different. Compared with CFI, AWD irrigation intensifid nitrogen transformation processes, including nitrification-denitrification, mineralization and volatilization, and enhanced gaseous nitrogen emissions and potential greenhouse gas (N2O and NH4) emissions. Successful implementation of AWD irrigation in sustainable rice production requires adequate consideration of this negative effect.(2) According to the field and laboratory long-term water and nitrogen experiments, the database including soil characteristic parameters, the field wate and-nitrogen regimes and the climate micro-environment was constructed. The HYDRUS-1D and the system dynamics model were studied. The system dynamic model can simplify the analysis of water and nitrogen balance in the field. The HYDRUS-1D model can handle the boundary condition of dynamic change in paddy fields under AWD irrigation and CFI. Although the HYDRUS-1D model can not fully describe all the nitrogen transformation processes through the first-order dynamic reaction chains, the nitrogen concentration and balance in the rice fields can be analyzed and simulated using the HYDRUS-1D model by assembling some similar nitrogen transformation processes. Because the HYDRUS-1D model can handle the changes of parameters with soil water content, the HYDRUS-1D model can be used to compare the water and nitrogen regimes in paddy field under AWD irrigation and CFI. Water and nitrogen balance in paddy field can provide the basis for water and nitrogen management.(3) The effect of water and fertilizer reuse on regional water-saving and agricultural non-point source pollution was studied in on-farm tanks (OFTs) system of rice irrigation area in South China. The results showed that OFTs can effectively improve agricultural drainage reuse and reduce pollutant discharge during rice growing season. Precipitation is the decisive factor in determining the reuse of water in OFTs. Full using of pond water storage capacity to collect the return water of upstream paddy field and regulating outflow of OFTs to maximize irrigation for downstream paddy fields can improve the return water reuse. The water quality monitoring in OFTs showed that 47.2% of the total phosphorus and 60.8% of the total nitrogen in the return water were removed by OFTs, and the average removal rate of the suspended solids was 68.4%. Ammonium nitrogen is the main nitrogen pollutant in paddy field return water, and particulate phosphorus is the main phosphorus pollutant in return water. Particulate phosphorus accounted for 90% of total phosphorus in OFTs. The longer the hydraulic retention time is, the better the removal rate of total nitrogen load is. The relationship between phosphorus load removal and hydraulic retention time is uncertain. Therefore, the reuse of return water and nutrient by integrated OFTs is of great significance to reduce the nitrogen and phosphorus emission from riceland, which is of great significance to the improvement of ecological environment of paddy fields. |
