The Effect Of Dew Condensation By Land Use Cover Change In The Sanjiang Plain

Dew is a crucial input for water balance and nutrients cycling of wetland orfarmland ecosystem. Dew could play important role in spaying leaf fertilizers inpaddy and drought-resisting and preventing the disease and pest in rainfed land andwetland. In the1960s, Sanjiang Plain was the largest concentrated area of freshwaterwetlands in China. Over the last60years, Sanjiang Plain was reclaimed four times.Most of its marsh wetland areas were turned into cultivated land. But it was still notclear that the process of dew formation, character of dew and vapor source of dewfrom different land types. Dew was monitored in situ on stem and leaf of plant inCarex lasiocarpa, Calamagrostis angustifolia, soybean or corn, and paddy fieldsduring the growing season from2008to2010. Method on dew condensationmonitoring in Wetland and farmland ecosystem was explored for the first time. Themethod including to set up a series of parameters about dew characterization, to selectthe proper collector, to identify the dew duration and to deduce the dewfall formula.The conclusions was list as following:1)Dew began to condense from one and a half hour after sunset and I reachedthe peak at about half an hour before sunrise. Dew duration was about6-8hours frommid-May to mid-October in mire wetland at Sanjiang Plain.2)Dew intensity reached peak at August. The higher relative humidity, dew pointtemperature, and vapor pressure were the important factors improving the dewintensity. In addition, wind speed is also a non-neglected factor. The dew productionin Carex lasiocarpa marsh more frequently occurred under the conditions of windspeed around2.0m/s, the water vapor pressure above9hPa; the relative humiditybetween90%and95%and dew point temperature above6℃.3)The annual Dewfall showed a distinct variety of the amount between paddyand rainfed land. The annual dewfall in rainfed land ranged from10mm to15mm. Inthe paddy Dewfall was about two to three times larger. The Dewfall increases rapidlywhen rainfed land reclaimed into paddy.4)pH of Carex lasiocarpa dew was6.42±0.23and the acid dew never occurredin the research area. Therefore, dew can not damage protective surfaces on leaves, interfere with guard cells and poison plant cells from the aspect of pH. The types ofmetal element in Carex lasiocarpa dew were abundant. K, Mg, Ca are the nutrientelements and Mn, Cu, Cr, Zn, Na, Mo, V, As, Fe, Ni are the trace elements. They wereall can be monitored in Carex lasiocarpa dew. The mean concentrations of Cd, Be, Co,Se, Mo, Th, U and Tl were below1.0μg/L. The mean concentrations of Pb, As, Ni, Cr,V, Ag, Cu and Zn were between1.0μg/L and100μg/L. The mean concentrations of00Na, Al, Fe and Ba were between100μg/L and1000μg/L. The meanconcentrations of K, Mg, Ca and Mn were higher than1000μg/L. Carex lasiocarpadew can provide nutrient elements and trace elements to plant. Dew can reveal the airpollution status and it is significant to monitor the chemistry character of dew. Thetrace amount of Pb, Ba, Se, As, Co, Cr, Cu in Carex lasiocarpa dew implied that thereis no automobile exhaust, coal combustion or industrial pollution in this area.5)pH was between5.36and7.0with a mean pH of6.36. Therefore, dew cannotdamage leaves in the paddy. The NO₃^--N concentration in dew was significantly(p<0.05) greater than NH₄⁺-N and PO₄^3--P concentrations. Dew can offer moreNH₄⁺-N, NO₃^--N and PO₄^3--P than rain. The deposition amount of NH₄⁺-N, NO₃^--Nand PO₄^3--P from July to October in paddy dew was0.10,0.22and0.04kg/hm²,respectively. This amount was much larger than the volume of foliar fertilization, withthe deposition amount of essential nutrients in dew close to100times as much as thatin foliar fertilization.6) Based on isotopic mass conservation to partition quantitatively thecontribution of different vapour sources of dew, the guttation and transpiration fromthe plant accounted about30%and the evaporation of surface water mixed withvapour in air accounted the other partition of paddy dew in rainy season. As towetland dew, the evaporation of surface water took the most patition. The δ18O andδD values of dew exhibited the similar trend to the stable isotope in rain. Therefore,the stable isotope value of dew is a useful indicator as vapour resource of rain.7)The Dewfall did not change too much when wetland reclaimed into rainfedland but increase rapidly when rainfed land reclaimed into paddy. LAI was thedetermining factor that affected the annual dewfall. N and P content was lower in paddy dew than it in wetland. The main vapor of dew in wetland and paddy was boththe vapor evaporated from surface water.