Epsilon Open Archive

Abstract

Water is the most limited factor for crop production in dryland farming. In the work underlying this thesis soil hydraulic properties and water balance under various soil management regimes in the Chinese Loess Plateau were evaluated in laboratory studies, field experiments and simulations. Several methods for measuring soil hydraulic properties were also tested. The study was carried out at three sites on the Loess Plateau: Mizhi in the northern part, Heyang in the southeast and Yangling at the southern edge of the plateau. The saturated hydraulic conductivity of the loess soil at these sites was similar. However, the unsaturated hydraulic conductivity, K(), of the soil was higher at the northern site (Mizhi) than at the southern sites (Heyang and Yangling), and soil water retention showed the opposite pattern. Laboratory studies demonstrated that soil compaction significantly influenced soil hydraulic properties, reducing the saturated hydraulic conductivity and altering the shape of the soil water retention curves at two of the sites (Mizhi and Heyang). At the Yangling site long-term application of organic manure increased water retention in the low tension range and decreased the unsaturated hydraulic conductivity, K(), at relatively high water contents in the surface layer (0-5 cm). However, it did not affect the saturated hydraulic conductivity. These, and other, effects of soil management practices on the soils’ hydraulic properties alter the partitioning of the water balance. To assess these effects in a winter wheat system, an integrated ecosystem model was calibrated using the results of a three-year field experiment, and a long-term (45-year) simulation was run to evaluate the sustainability of wheat production under various soil management regimes. The simulations showed that mulching optimized the partitioning of the water balance components, decreasing soil evaporation and increasing both transpiration and deep percolation, leading to increased wheat yields and WUE. Furthermore, mulching significantly improved the quantity and frequency of deep percolation, which should enhance the groundwater recharge potential. Incorporating mulching into the winter wheat-summer fallow system could be a sustainable management strategy for the Loess Plateau. Increasing the organic matter content of the soil could be another beneficial approach, if sufficient resources were available. Soil compaction caused the most unfavourable partitioning of water balance, leading to the lowest yield and WUE of all the treatments. Considering all of the factors involved, the duration of bare fallow should not be less than 30 days to maximise the benefits of producing green manure or fodder without significantly reducing the wheat yield and WUE.