| The Loess Tableland is in the warm temperate semi-humid continental monsoon climate zone, which is also an important grain-producing areas in the Loess Plateau. During the recent three decades, the climate change and continuous improvement of agricultural productivity have significantly changed the regional wet and dry conditions, the watercarbon processes and the water cycle. Based on field observations, this dissertation explored the temporal dynamics of soil water and soil temperature conditions of the deep soil profile, analyzed the variation of water-carbon flux and their coupling relationship, and discussed the impact of climate change on winter wheat yield of this area. The main results are listed as follows:(1) The average soil moisture for alfalfa meadow, fallow field, high yield farmland and low yield farmland were 15.1±0.5 %、22.0±0.4 %、19.6±0.5 % and 21.1±0.4 %, respectively(0~15 m and annual averaged). The soil water content in the shallow profile exhibited seasonal and annual wetting and drying cycle during March~June and July~October, the depth of this profiles were 0~2 m, 0~4.6 m, 0~3 m and 0~4.2 m, respectively. Soil water content in the deep soil profile had better temporal stability, and was influenced by land use types. The temporal stable depth for the four landuse types were 9.8 m、5.4 m、8 m and 5.6 m, respectively, the soil water content at these depth were similar to that of the average of the whole profile. The water consumption of the alfalfa meadow had been increased, which caused the increase of deep soil desiccation, the 2~10 m soil profile for the alfalfa meadow had become stable dry layer, blocking the way of precipitation recharge. During the winter wheat and spring maize growing seasons, soil water storage showed negative balance. Soil temperature in the shallow profile had a clear seasonal trend, which showed gradual increase with the increase of depth during autum and winter seasons, however, it showed gradual decrease during the spring and summer seasons. The temperature below 8 m depth had little changes with time.(2) In allusion to different types of land use, the time-dynamic characteristics of carbon flux in different ecosystems during 2013 to 2014 were analyzed. For the observation period the WUEe in alfalfa meadow, high yield wheat, low yield wheat, high yield maize and low yield maize were 3.08、1.74、1.25、1.78 and 0.88 g C kg-1 H2 O, respectively. The alfalfa meadow is a strong carbon sink, the farmland is weak carbon sink, and the fallow field is carbon source all the time. The photosynthetically active radiation(PAR) air temperature(Ta), soil temperature(Ts) and vapor pressure deficit(VPD) were the main influencing factors for carbon flux, however, their contributions were different among years.(3) The seasonal and interannual characteristics of evapotranspiration(ET) and carbon fluxes in both cropland ecosystem and agro-fruit complex ecosystem were closely correlated with crop growth and phenology. The water-carbon fluxes for both ecosystems showed positive correlation within and without the year, with the WUEe of 1.84 and 1.77 g C kg-1 H2 O, respectively. The carbon flux in cropland ecosystem was lower than that of the complex ecosystem, thus, the observation only in cropland scale may cause underestimation of the regional carbon sinks status. PAR and VPD were the main factors influencing the water carbon flux, WUEe decreased due to the increase of the two factors.(4) During 1957 to 2013, winter wheat yield had a fluctuating upward trend, however, the meteorological factors played an unfavorable role in the yield enhancement, with the speed of-76.3 kg ha-1 10a-1. The negative contribution of meteorological factors was up to 12.3 %, the increasing Ta was the main reason of the decrease of winter wheat meteorological yield, and the reduced precipitation also had nagtive contribution to it. |
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