The Photosynthetic Characteristics Of Leymus Chinensis And Soil Respiration Response To Different Grazing Intensities

The input of carbon has decreased from the aboveground to the underground, toreduce the carbon sequestration capacity of grassland. Photosynthesis and soilrespiration are the elementary component of global carbon cycle. The photosyntheticand physiological indexes of dominant species-Leymus chinensis, soil respiration andits controlled factors were measured in Hulunber meadow steppe. It can be analyzedby the photosynthesis of L. chinensis for the adaptation to grazing intensities on theindividual level, looking for suitable grazing intensity to reduce the adverse effects,providing basic data for the simulation and monitoring of carbon cycle.The photosynthetic characteristics of ungrazed and grazed Leymus chinensis andsoil respiration were studied by the method of infrared radiation gas analyzer underthe treatments of six different grazing intensities (R0, R1, R2, R3, R4and R5) among allexperimental plots in Hulunber meadow steppe. The photosynthetic pigments andbiomass (including aboveground and root) were studied by Acetone ethanol methodand drying method.The results have showed as follow.(1) Results show that daily variation of net photosynthetic rate (Pn), transpiration rate (Tr),stomatal conductance (Gs) appears single peak curve and double peak curve, but intercellular CO2concentration (Ci) demonstrates shape of U. Which responsed to different grazing intensitiesunder condition of continuous grazing. Pn, Tr of ungrazed and grazed L. chinensis have showedsingle peak curve during the period of experiment under different grazing intensities. Tr is moresensitive to different grazing intensities and seasonal change than Pn. In addition, Pn, Tr, Gs of L.chinensis shows the decrease with the increase of grazing intensities, but Ci presents the increase.Pn, Tr of ungrazed and grazed L. chinensis has presented the decline with the increase of grazingintensities, Ci reverses (except R1) in the period of growing season2011year. The photosyntheticvalues of early growth stage are better than late. The Pnof grazed L. chinensis were lower thanungrazed L. chinensis, while Tr higher. Air temperature (Ta), relative humidity (RH),photosynthetically active radiation (PAR) were combined the effects of each other rather thanoperated by a single factor. After grazing, Pn, Tr of damaged L. chinensis are severely sensitive toresponse to hydrothermic factors. R1has no significance with photosynthesis of L. chinensis, while R4and R5achieve the significant level. The maximum of net photosynthetic rate (Pmax), apparentquantum yield (AQY), and apparent dark respiration rate (R_d) of L. chinensis with the increase ofgrazing intensities (except R1) have decreased. It showed that the absorption and utilization abilityof L. chinensis was low of weak light, and was weak sensitivity of strong light. The content ofchlorophyll of grazed L. chinensis were not rising. With the increase of grazing intensities, thecontent of chlorophyll L. chinensis has changed dramatically, influencing on seasonal condition.(2) The diurnal variation of soil respiration rate (SRR) has correlated with soil temperature(ST), soil water content (SWC) and the growing period of herbage. The maximum has appearedbetween10and12o'clock. But the time of appearing minimum was inconsistent. The minimumappeared in the period of16to18o'clock on12June and10July, while it showed in the period of6to8o'clock on12August and14September. The diurnal variation trend of SRR has presented asingle peak trait curve, the seasonal change trend of SRR among experimental period haspresented a double peak trait curve under different grazing intensities in2010year, while hasexpressed the form of multi-peak fluctuations in2011year. The mean value of SRR decreased withthe increase of grazing intensities. The difference of SRR was more obvious with the increase ofgrazing intensity gap. In the peak of growing season, the diurnal changes of SRR fluctuatedfiercely with the increase of grazing intensities, then, the extent of fluctuation slowed down in thelate stage of growing season. Grazing intensity has altered the micro-enviroment of SRR in thesome extent. Such as, SWC, above ground biomass (AB), root biomass (RB) and litteraccumulation (LA) have declined with the increase of grazing intensities (except R₁). The gap ofgrazing intensities was larger, the difference of controlled factors being more sharply. ST has noregular changes under different grazing intensities, while the ST of R4and R5are distinctly higherthan R0or R1. When the temperature is lower than the minimum of growth (such as, on30thSeptember in2010year, on three experimental days September in2011year), the difference ofSRR has had no significance among different grazing intensities. SRR has decreased with theincrease of grazing intensities (except R1) on the rest of experimental days, therein, R4and R5aresignificantly lower than R0and R1. SRR of the adjacent grazing intensities did not exist significantdifference. In the series of time, ST has been the main controlled factor of SRR. The correlationcoefficient between SWC and SRR under R4and R5, has been extremely higher than R0. SRR waspositive correlation with biotic factors (including AB, RB and LA) and the hydrothermic factor (main including SWC, ST) in these experimental surroundings, negative correlation with grazingintensity and soil bulk density.In a word, R1has had no significance with the photosynthetic characteristics of L.chinensis, SRR and its controlled factors. While R4and R5have expressed appreciableimpact on above studies. R2and R3have influenced on them between R1and R4, R5.Hence, R2and R3are the reasonable grazing intensities from the viewpoint of yield.But R1could be suitable choice, from the perspective of ecology.