干旱区生态环境知识资源中心

Water vapour adsorption (Eva) becomes one of the most important sources of available water for annual vegetation in arid area. However, studies on Eva in the soil or the mulch are greatly limited, and there is no information about comparison of Eva amounts under rice-straw and gravel mulch. This study evaluated the effects of rice-straw and gravel mulch on Eva in lysimeters and determined the environmental parameters affecting Eva. The weight of the lysimeter, soil moisture, air temperature, dew point and relative humidity were measured every 15 min. The phase of water vapour around the mulch might change to liquid water adsorbed by the soil under mulch, so the weight of the lysimeters for mulch regimes increased in the afternoon since the condensation water (the liquid phase of water) could be detected and weighed by lysimeters. No matter fine or cloud days, Eva or condensation might have occurred below the mulch due to the high oscillation of air humidity, which might result in a high vapour pressure gradient from the atmosphere to the soil, bringing about adsorption of water vapour by the soil. Eva increased as the daily fluctuation in air humidity and temperature increased. The high daily fluctuations of temperature could favour the high temperature gradient in the mulch, which caused that the water vapour fluxed downwards and it was probably adsorbed by the relative cooler soil under the mulch. The duration of adsorption is about 7 hours per day. A total amount of 200.4 mm of water vapour was adsorbed for rice-straw mulch during the experiment period (corresponding to 1.06 mm/day), which can satisfy 74.3% of the pan evaporation. Soil under straw mulch is able to adsorb vapour water in significant amounts, although previous studies have overlooked this fact. Eva is beneficial to plant since the liquid phase of water can be used by plant. In the morning, evaporated water from the adsorbed water used energy that would otherwise be used to evaporate soil water. The Eva might reach a peak value under the moderate soil moisture condition. Rice-straw mulch tended to adsorb more vapour than gravel mulch probably because rice-straw has larger surface area and the vapour concentration under straw mulch might be higher. The significance of Eva or condensation would be enhanced if condensation water could be stored by some methods, such as preventing condensation water evaporation by much thicker straw mulch.