毛乌素沙地南缘沙丘生物结皮对凝结水形成和蒸发的影响

在水分极度匮乏的荒漠生态系统,凝结水是除降雨之外最重要的水分来源之一,它对荒漠生态系统结构、功能和过程的维持产生重要的影响。为探明半干旱沙区生物结皮表面的凝结水形成和蒸发特征,采用自制的微型蒸渗计(直径7 cm、高5 cm的PVC管)实验观测了不同类型地表(裸沙、浅灰色藻类结皮、黑褐色藻类结皮和苔藓结皮)对凝结水形成和蒸发的影响。结果表明:(1)观测期间共有20次凝结水形成记录,除降雨天气外,几乎每天都能观测到水分凝结现象;(2)不同类型地表凝结水总量依次为(1.998±0.075),(2.326±0.083),(2.790±0.058)和(3.416±0.068) mm,生物结皮表面的凝结水总量显著大于裸沙(P < 0.05);随生物结皮的发育,不同类型生物结皮表面的凝结水总量呈增加的趋势,凝结水总量之间差异显著(P < 0.05);观测期间不同类型地表日平均凝结水量依次为(0.100±0.003),(0.116±0.004),(0.140±0.002)和(0.171± 0.003) mm,不同类型地表日平均凝结水量之间差异极显著(P < 0.01);(3)凝结水形成过程的观测结果显示,凝结水19:00开始形成,23:00-凌晨1:00形成不明显,1:00-7:00继续形成,除浅灰色藻类结皮外,太阳升出后在黑褐色藻类结皮和苔藓结皮表面继续形成少量的凝结水;凝结水7:30开始蒸发,10:30到11:00之间结束蒸发,凝结水在裸沙和浅灰色藻类结皮中的保持时间显著大于黑褐色藻类结皮和苔藓结皮中的保持时间(P < 0.05);(4)凝结水的形成受大气温度、地表温度、空气相对湿度和大气地表温度差等气象因素的影响,但其形成过程不与某一个气象因素呈简单的线性关系。

Effects of biological crusts on dew deposition and evaporation in the Southern Edge of the Mu Us Sandy Land, Northern China

Dew is one of the most important moisture sources except rainfall for extremely water limited desert ecosystems and they have a significant effect on the maintenance of ecosystem structure, function and processes. To better understand the influence of biological crusts on dew deposition and evaporation processes in a semiarid sandy ecosystem, dew quantities of different surface types (bare sand, light algae crusts, dark algae crusts and moss crusts) were measured in the southern edge of the Mu Us sandy land in northern China using micro-lysimeters with a diameter of 7 cm and a height of 5 cm. The main conclusions were as follows: (1) There are 20 records in the experimental periods where dew was deposited almost every day except rainy days; (2) The total dew amounts of different soil surface types were (1.998±0.075), (2.326±0.083), (2.790±0.058) and (3.416±0.068) mm for bare sand, light algal crusts, dark algal crusts and moss crusts, respectively. Moss crusts were characterized by having the greatest amount of dew at dawn, whereas bare sand yielded the lowest amount of dew and the total dew amounts of biological crusts were significantly higher than that in bare sand (P < 0.05). Therefore it can be concluded that the formation of biological crusts benefited dew deposition. Dew quantities of biological soil crusts increased with crust development and the total dew amounts of different types of biological crusts varied significantly (P < 0.05). Daily average dew amounts of different soil surface types were (0.100±0.003), (0.116±0.004), (0.140±0.002) and (0.171±0.003) mm for the bare sand, light algal crusts, dark algal crusts and moss crusts, respectively. The maximum amount of dew for each type of crust studied was several times greater than the minimum and the daily average dew amounts of different soil surface types were significantly different from each other (P < 0.01); (3) Results of dew formation processes at 2 hour intervals indicated that dew formation begins at 19:00, inconspicuous dew was deposited from 23:00 to 1:00 which then proceeded from 1:00 to 7:00. From 7:00 to 7:30, continuous dew was deposited on the surface of dark algae crusts and moss crusts even after sunrise but not on light algae crusts. Results of dew evaporation processes at 30 min intervals indicated that dew evaporation begins at 7:30, and ends from 10:30 to 11:00. Dew duration time in bare sand and light algae crusts was significantly higher than that in dark algae crusts and moss crusts (P < 0.05). It can be concluded that well developed biological crusts may potentially enhance hydrological circulation in the upper sand layer in sandy land. (4) Dew formation processes can be affected by air temperature, land surface temperature, relative air humidity and temperature differences between the atmosphere and land surface. Dew formation processes are not simply linearly related to any meteorological factors. These data further the understanding of dew formation characteristics of biological crusts in semi-arid sandy areas and assist in evaluating the significant ecological roles of dew in semi-arid environments.