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Soil organic carbon is a dynamic medium of carbon transfer and is a main pathway for the circulation of carbon through soils in Karst systems.Karst systems are typically ecologically fragile areas with limited environmental capacity and resistivity.In addition,the implementation of a state policy encouraging the return of farmland to woodland(grass) and labor output in recent years has led to considerable change in land use patterns over a short period in these areas,which is bound to have far-reaching impacts on the spatial distribution pattern of soil organic carbon.Extensive previous research has investigated the response characteristics of soil organic carbon under different land use patterns.However in general,most of these studies have only investigated the total soil organic carbon in whole soils,and have rarely studied the organic carbon and labile organic carbon in soil aggregates.Organic carbon and labile organic carbon in aggregates are largely isolated from microbial activity because of the physical protection of the aggregates,and this may have a major impact on their function as soil carbon sinks.Therefore,the purpose of this study was to explore the distribution and accumulation characteristics of organic carbon and labile organic carbon in aggregates through survey,sampling,and analysis of soils in selected Karst mountain areas in China. The experiment was conducted in a small watershed in Huajiang Karst areas in southwestern Guizhou(25° 38.988′—25° 41.322′N latitude and 105° 38.031′—106° 40.505′E longitude).Samples of topsoils(0—15 cm) were collected from five kinds of typical land use patterns(Pitaya orchard,grassland,Chinese prickly ash orchard,forest and shrub-grassland).Soil aggregates were separated by dry sieving.Aggregate composition was dominated by particles 0.5 mm which accounted for 82.57% to 94.79% of the whole soil under the different land use patterns.In all size classes of aggregate particles,forest had the highest values of organic carbon and labile organic carbon,followed by Chinese prickly ash orchard,while grassland and shrub-grassland were relatively lower.With decreasing aggregate particle size,the peak of organic carbon and labile organic carbon basically appeared in particles 0.25 mm in size.However,these aggregate particles contributed less than 6% and 4%,respectively,to the contents of soil organic carbon and soil labile organic carbon.The accumulation of soil organic carbon and soil labile organic carbon were influenced by increasing aggregate size,with 5—1 mm aggregates contributing 28.70%—49.47% and 34.13%—47.47% of the organic carbon and labile organic carbon contents,respectively.Therefore,it can be assumed that particles of aggregate size 5—1 mm hold the majority of soil carbon fixed in Karst mountain areas.In addition,the labile organic carbon content and total organic carbon content in soil aggregates were highly significantly positively correlated(r=0.8768),which suggests that labile organic carbon may be a sensitive index for measuring the dynamics of organic carbon in soil aggregates in Karst mountain areas.Our results will help to reveal the regional soil organic carbon sequestration in Karst mountain areas in China.

Three off-axis projection systems based on even aspheric surface reflectors and Zernike polynomial surface reflectors are designed using wave front aberrations analysis and modulation transfer function.The projection system consists of three reflectors,in which the number of Zernike polynomial surface reflectors is different.The first mirror M1 is concave to reduce the size of the second mirror and obtain high contrast ratio.The second mirror M2 and the third mirror M3 are convex to achieve a shorter projection distance and little system aberration.Optical performance analysis show that the system based on three Zernike polynomial surface reflector is the best,the MTF is more than 60% at 60 lp/mm and system distortion is less than 2.0% under the condition of F-number=2.8,the magnification of 80.Compared results with references show that the designed system based on three Zernike polynomial surface reflectors can more easily eliminate system aberrations,shorten system thickness and increase the visual angle and relative aperture.

According to the characteristics of semiconductor laser,beam plastic system of semiconductor laser is designed.First,the cylindrical lens is used to align and deflect the beam spread along the Y axis.Then the laser spread along X axis is aligned and expanded by the telescope system.Finally,transmitting laser beam is compressed using meniscus lens to realize beam shaping,reduce the beam divergence angle,improve the quality of the beam.Using ZEMAX software simulation system,the results show that the divergence angle of output beam along the X axis and the Y axis is 4.922 mrad after plastic,spot diameter is 1.2707mm,total length of plastic system is 65.6618mm,the maximum diameter of all element is 20.52mm.Both the quality of output beam and structure of the system are better than the similar products.

Based on the heat conduction theory,a two-dimensional axis symmetric transient physical model was established,of which GaAs was irradiated by Gauss laser.By solving the heat conduction equations using COMSOL Multiphysics analysis software,the curve of material surface temperature and the relationship between the temperature at the center of the light spot and material thickness were obtained,and then the relationship between threshold damage time of GaAs and the incident light power density was gained.The calculation results show that the laser induced damage is mostly decomposition damage in the areas where temperature exceeds decomposition temperature of material.The theoretical analyzing result is in accordance with the relative experimental result,so the laser irradiation effect model proves correct.