黄土高原干旱半干旱区生物结皮覆盖土壤水汽吸附与凝结特征

Characteristics of water vapor sorption and condensation in biocrusts covered surface soil in arid and semiarid areas of the Loess Plateau,China

生物结皮是一种广泛分布于干旱半干旱地区土壤表层的特殊复合体,为揭示其对土壤水汽吸附与凝结过程的影响,该研究通过室内定量水汽吸附试验和野外对水汽凝结的连续观测,对黄土高原典型生物结皮(藻结皮、藻藓混生结皮、藓结皮)与裸沙的水汽吸附和凝结特征进行对比研究。结果表明:生物结皮的覆盖显著提升了表层土壤的水汽吸附能力,其平均水汽吸附量比裸沙高66.7%。不同类型生物结皮水汽吸附能力差异显著,表现为藓结皮最高,混生结皮次之,而藻结皮最低。GAB(Guggenheim-Anderson-de Boer)吸附模型能较好的描述生物结皮土壤水汽吸附与解吸附过程,模拟结果决定系数R2>0.99、均方根误差RMSE<0.001 2 g/g及平均相对偏差百分比E<16.0%;此外,生物结皮加剧了土壤水汽吸附与解吸附曲线之间的滞后效应,其滞后指数平均是裸沙的2.0~2.9倍。水汽凝结结果显示,水汽凝结过程均受气温与相对湿度等气象因子制约,且生物结皮覆盖下表层土壤的水汽凝结和蒸发过程相较于裸沙更为迅速。同时,生物结皮的日均水汽凝结量是裸沙的1.6~1.8倍。综上,干旱和半干旱地区生物结皮覆盖显著提高了表层土壤的水汽吸附能力、并增加了水汽凝结量,对区域表层土壤的水分运动过程产生了重要影响。

Water vapor movement is one of the several important components of total(hydrological)water flux in the vadosezone of drylands.When the soil water content is relatively low,the vapor sorption and condensation naturally become vitalwater sources for plants,insects,and small animals,particularly in arid andsemiarid drylandswith quite small rainfall.Biocrusts(biological soil crusts)covera large portion of the terrestrial soil surface of drylands,and they greatly changesurface soil water and heat movement through regulating the energy balance of surface soil.However,the influences of the critical upper layerof soil,biocrusts,on water vapor sorption and condensation have not yet been fully understood.Therefore,it is necessary toinvestigate the characteristics of water vapor sorption and condensation in biocrustsand their influential factors to quantifyvapor water supplement from biocrusts,for a better understanding of biocrust effects on vapor movement in drylands.This studywas conducted to explore the essential features of vapor sorption andcondensation in cyanobacteria crusts(cyano-crusts),cyanobacteria and moss mixed crusts(mixed crusts),moss crusts(more than 30 yrs),and bare sand(aeolian sand).Anautomatic measurement was performed to analyze the vapor sorption(adsorption and desorption directions)of biocrusts and bare sand in laboratory,and a field measurement was implementedtomeasure the vapor condensationof the different treatment,including the condensation process and daily amount of vaporcondensation.The results showedthat the vapor sorption amount increased with the water activity(aw),and the biocrusts significantly increased vapor sorptionamount as compared withthe bare sand.The vapor sorption amount of the biocrusts was averagely 66.7%higher than thatof the bare sand;and as an example,that of the moss crusts was 1.0-2.2 times higher than that of bare sand.The simulation results implied that the GAB(Guggenheim-Anderson-de Boer)model was capable ofdescripting vapor sorption isotherms of the biocrusts,with R2>0.99,RMSE<0.0012 g/g,and E(mean relative percentagedeviation modulus)<16.0%.The significant hysteresis effects were observed according to the hysteresis index in Water vaporSorption Isotherms(WSIs)of the different treatment,which was in the decreasing order of moss crusts,mixed crusts,cyano-crustsand bare sand.The hysteresis indexof the biocrusts was 2.0-2.9 timeshigher than that of the bare sand.Moreover,the results of vapor condensation showed that the watervapor mostly condensed from sunset to sunrise for all the treatments,and then it gradually evaporated and reached theminimum at about 15:00.The condensation and evaporation rates of water vapor in the biocrusts were much faster than that ofthe bare sand.Especially in September,the amount of vapor condensation in the biocrusts was averagely 1.5 times higher than thatin the bare sand.The reason for this resultwas that the vapor condensation depended strongly on the meteorological factors,including airtemperature and relative humidity.Furthermore,the daily condensation amount of the biocrusts was averagely was 1.6-1.8 times higher than that of bare sand.Particularly,the condensation amount of different types of biocrusts was varied,and thecondensation amount of the moss crustswas the highest and which was 81.8%,11.1%,and 5.3%higher than that of the bare sand,cyano-crusts,and mixed crusts,respectively.Therefore,the vapor sorption and condensation of biocrusts play an importantrole in soil vapor movement,because that i)the biocrusts cover soil surfaceand enhance vapor sorption amount due to the higherclay and organic matter contents;and ii)the biocrusts increase vapor condensation amount through improving soil properties andgenerating mulching effect(e.g.,moss crusts).In conclusion,biocrusts should be highly considered in studying water transport in arid and semiarid drylands of Chinese Loess Plateau and similar regions around the world.