Effects of Plant Community Type on Soil Methane Flux in Semiarid Loess Hilly Region,Central Gansu Province,China

Methane(CH_(4))is an important greenhouse gas second only to CO_(2)in terms of its greenhouse effect.Vegetation plays an important role in controlling soil CH_(4)fluxes,but the spatial variability of soil CH_(4)fluxes during vegetation restoration in Loess Hilly Region(LHR)is not fully understood.The effects of different plant community types[Medicago sativa grassland(MS);Xanthoceras sorbifolium forestland(XS);Caragana korshinskii bushland(CK);Hippophae rhamnoides shrubland(HR);and Stipa bungeana grassland(SB)]on soil CH_(4)flux in LHR were studied via the static chamber technique.The results showed that the five plant community types were sinks of soil CH_(4)in LHR,the plant community type significantly affected the soil CH_(4)flux,and the average CH_(4)uptake from high to low was in SB,HR,CK,MS,and XS.During the whole study period,the soil CH_(4)flux showed similar interannual variation.The maximum absorption of soil CH_(4)appeared in the growing season,while the minimum appeared in winter.Soil CH_(4)uptake was positively correlated with soil temperature and soil moisture.Soil temperature and moisture are important controlling factors for the temporal variability of soil CH_(4)flux.In LHR,the Stipa bungeana grassland is the more suitable plant community type for reducing soil CH_(4)emissions.In the process of vegetation restoration in LHR,the soil CH_(4)absorption potential of different plant community types should be considered,ecological benefits should be taken into account,and vegetation more suitable for mitigating the greenhouse effect should be selected.

Physical layer authentication of MIMO-STBC systems based on constellation dithering

Most of the existing physical layer watermarking authentication schemes are based on a single-input single-output system and require pre-issue of shared keys.To address these problems,in this thesis,a physical layer authentication scheme without the distribution keys is proposed based on the constellation dithering physical layer authentication watermarking mechanism with a multiple-input multiple-output(MIMO)system,and space-time block coding(STBC)is used to improve the robustness of transmission.Specifically,the legitimate node obtains channel state information(CSI)through channel probing and couples CSI with the message signal using a hash function to generate an authentication tag,which is then embedded through constellation dithering.The receiver extracts the tag and authenticates it using hypothesis testing.Performance analysis shows that the scheme is resistant to various attacks such as replay,interference,tampering,and forgery.Simulation results show that the use of MIMO multi-antenna diversity with STBC coding technique reduces the bit error rate(BER)of message signals and tag signals and improves the detection rate of legitimate signals.