Dynamics Of Soil Water And Deuterium And Oxygen-18Stable Isotopes In Soil-Crop System And Field Water Consumption Characteristics

Water shortage has direct influence on food security and the main pathway for agroecosystem to consume water is through evapotranspiration (ET). Investigating evapotranspiration, discovering the characteristics, transformation processes and sources of water consumption, and quantitatively distinguishing components and variations of ET in growth period will explain the use, distribution and transport of water in agroecosystem, and thus, provide a theoretical foundation to the sustainable utilization of water resource and to ensure the food security. This research focused on the laws of water movement in SPAC (soil-plant-atmosphere-continuum) system. We conducted isotope study of the farmland surface hydrological processes in the North China Plain (winter wheat-summer maize) and the Northeast Plain (spring maize) and analysed the structure characteristics of water consumption for multiple years. Research results showed that the dynamic variation of moisture and the distribution characteristics of hydrogen and oxygen stable isotopes in SPAC system, which could provide a scientific basis for developing the water-adapted cropping system and for improving the water productivity. The conclusions of this dissertation are as follows:1) The δ18O-δD content composition, distribution and influencing factors between various water sources (i.e., precipitation, soil water, groundwater and plant stem water) for winter wheat-summer maize and spring maize in growth period were systematically analyzed. Comparing with the meteoric water line of the δ18O-δD distribution graph of the study site, the line slope of that of soil water evaporation in0-5cm soil layer was observed to be smaller, indicating that heavy evaporation occurred in this near-surface soil layer (0to5cm). Isotope values of soil water increased from deep to shallow soil surface and the different variation intensity of hydrogen and oxygen isotope was found in different soil profiles, which followed the order:bare soil treatment> conventional treatment> straw treatment> coated treatment.2) The root water uptake can be considered as the mixture of water from different water sources. By using the direct comparison method and Iso-Source model (which based on the isotope mass conservation law). Winter wheat mainly uses soil water from10-20cm layer in the heading stage,20-40cm in flowering,0-10cm in milking stage and10-20cm in maturity stage. And summer maize mainly uses soil water from0-10cm layer in the jointing stage,0-20cm in tasseling stage,10-20cm in milking stage and40-100cm in maturity stage.3) The ratio of partitioning fluxes from the isotope mass balance was relatively steady compared to the AquaCrop model result during2011to2013. The ratio of transpiration has the same amount with the total evaporation flux, which was using the AquaCrop model at the seedling stage, jointing stage and grain filling, were about40%,90%and80%, respectively. However, the ratio of transpiration to evapotranspiration decreased slightly to80%at maturity using the1MB, the ratio of transpiration to evapotranspiration was about90%, indicating the comparability of the two methods in their partitioninginto E and T. 4) In order to investigate crop responses to long-term climatic factors and different groundwater depths influence to the water productivity (WP), we used the AquaCrop model to study the water consumption for summer maize in the north China from1980to2010. We set two groundwater depths (i.e., one is set at1,5m where groundwater in our field experiments can recharge crop in water-short periods, and the other one is that groundwater is deep enough where capillary recharge is impossible in water-short periods in most parts of the NCP) to examine the responses of crop yield and WP to differed water conditions. Results showed that the WP under no water stress conditions were2.49and2.24kg·m-3, respectively. The WP under water stress conditions were only1.92and1.97kg·m-3, respectively. Therefore, in dry years supplemental irrigation must be added if a higher yield and WP is desired.