Effects of straw mulch on soil hydraulic properties and water status in sloping red soil farmlands

Although straw mulch is widely used to improve soil water and heat in farmlands, the comprehensive effects of application of straw mulch depend on environmental conditions such as soil and climate. In subtropical China, productivity of sloping red soil farmlands is limited by severe rain-induced erosion in the wet spring-summer period and frequent seasonal droughts in the dry summer-autumn period. There were possible hydrological connections between soil erosion and drought. At present, it has remained unclear how temporal variations in soil hydraulic properties due to local soil and water loss affect soil water storage in wet and dry seasons. In addition, little attention has been paid to comparison of the different effects of straw mulch on soil water storage in the two seasons. However, related knowledge is important to increase water use in local fields. The objectives of this study were to explore the role of straw mulch on preserving soil water in the wet and dry seasons in sloping red soil farmlands. To this end, a field experiment was performed in Xianning City of Hubei Province with red soils (Ultisols) originating from Quaternary red clays. In an experimental field of low hill with 8° slope, two treatments, including rice straw mulch over surface soil (SW) and no mulch (CK, control), were used to analyze the effects of straw mulch on variations in soil water retention, water availability, water supply, hydraulic conductivity, water content and water storage. Results showed that SW improved soil water retention and water availability, with increases in field water capacity, wilting coefficient and available water holding capacity of respectively 6.0%, 7.3% and 4.4% over CK. SW also greatly lowered the decline in saturated hydraulic conductivity of the surface layer of the red soil, especially under intense rainfall events. After 102.8 mm of rainfall event, mean soil saturated hydraulic conductivity under SW was on average 2.7 times that under CK. After straw mulch, water supply and unsaturated hydraulic conductivity of the soil increased when soil water suction was low, and decreased as soil water suction was high. Compared with CK, SW had a smaller rise in soil water storage in high-intensity precipitation events and less decline in soil water storage during no rain periods. In general, there was more soil water storage under SW than CK. Straw mulch increased soil water storage in wet and dry seasons respectively by increasing field infiltration and reducing evapotranspiration. Under SW, soil water storage increased only in limited volume in the rainy season and water preservation capability was less than that in the dry season. It was concluded that the ways, features and capabilities of soil water preservation by straw mulch were different for the wet season and dry season in sloping red soil farmlands in subtropical China.