Soil Moisture Characteristics Of Three Different Vegetation Types In Loess Region Of Western Shanxi Province

The northwest part of Shanxi province is the transition from semi-arid to arid landscape of loess plateau with frequent drought and therefore the water content, especially the soil moisture, serves as the main limiting factor to local reclamation of vegetation, which plays the important role in water dynamics in soils. It has been noted that vegetation types impose different impacts on soil and water conservation. This addresses the significance of soil moistures in different vegetations and the implications for developing the plant communities in the area. This thesis monitored plots of three vegetation types:the poplars(Populus hopeiensis), the peashrub (Caragana korshinskii), and the grassland, as well as the abandoned farmland as control. The soil moisture was measured in the raining period of growing season (April to October) in 2012 for quantitative dynamics of water content in soil profiles in depth of 0-600cm, and for estimating the water availability. Meanwhile, the soil dryness and dryness range of each vegetation type was assessed for improvement proposal. The main findings of the thesis are as follows.1. Soil moisture dynamics over time was found highly dependent on vegetation types during the raining period of growing season. The soil water contents among all sampling plots varied generally in accordance with the seasonal changes in precipitation. The soil moisture showed the following trend grassland>Poplar>abandoned farmland>peashrub. The grassland had the highest soil moisture across the year. In the study period the soil water contents fluctuated differently depending on vegetation types. In poplar and grassland plots the soil moisture showed steady increasing trend; in peashrub plots it increased first and then decreased until reaching to the minimum level after raining season; and in the abandoned farmland it showed slight decrease throughout the year.According to the soil moisture dynamics of the plots over time the whole period is divided into three phases:the steady evaporation phase prior to rain season (April to the end of May); the soil moisture accumulation phase in raining season (June to the end of August); and the slow evaporation phase after rain season (early September to the end of October).2. The soil moisture in different types of vegetation in the 0-600cm soil profile showed significant spatial variation, showing the general trend of increase-then-decrease pattern along the growing depth along with fluctuations. However, the range of soil depth corresponding to the maximal/minimal soil moisture varied depending on vegetation plots, mostly due to precipitation, soil evaporation, plant transpiration, and water absorption by root systems. According to the variance in soil moisture the soil is divided into four layers along the depth:the variable layer, active layer, sub active layer and relatively stable layer. The spatial distribution of the layers also varies depending on the vegetation type.The analysis on the moisture availability showed none of ineffective-invalid moisture layer in 0-600cm soil profiles among all four vegetation plots. The soils of Poplar trees and grassland were partially with water infiltration layer, which was not found in peashrub and abandoned farmland. The abandoned farmland contained no highly-efficient moisture layer. In the rest plots, the highly-efficient and efficient moisture layer accounted the most part in the soil profiles.3. The dry layers were found among all vegetation plots in the soil in depth of 0-600cm. The peashrubs had the highest water consumption, followed by the abandoned farmland; while the grassland were the best in water conservation. The soil dryness is ranked as:peashrub>abandoned farmland>Poplar>grassland.The dry layers varied in all soil profiles in response to the vegetation types and rainfall. This study shows the moisture recovery in the dry layer in grassland and poplar plots after rainfall. In comparison, the peashrub plots contained severe dry layer in the middle part of soil profiles; the poplar plots contained slight to intermediate dry layer in the soil in 0-300cm depth; and the grassland plot contained slight dry layer only at the surface.