| Soil respiration (Rs) is one of the main processes of loss of organic C, and playcritical roles in affecting the SOC pool and atmospheric CO2. Globally, Rs isestimated to be50-75P g C yr-1. Because of the large flux, even a small change in Rsrate would have profound impact on the atmospheric CO2concentration. Arid andsemiarid regions represent30%of the earth’s land area, and soils from these areascontain huge amounts of inorganic C (750–950Pg C) in the form of carbonates thatcould potentially be an active participant in short-term CO2fluxes. What is more,because of the relatively small organic C pools that they contain, Rs is one of theecosystem properties most sensitive to climate change. The western Songnen Plain isone of the three largest soda saline-alkali areas in the world. It is also one of the mostimport meadow steppes in China. In this paper, the alkali-spot land (As) and fivetypical vegetation communities (Suaeda glauca community (Sg), Chloris virgatacommunity (Cv), Puccinellia distans community (Pd), Leymus chinensis community(Lc) and Phragmites australis community (Pa)) of this saline and alkaline land inwestern Songnen Plain were selected as study objects in order to understand thetemporal-spatial variations of Rs and their controlling factors. In addition, thenegative CO2flux from saline-alkaline soils as well as its impact factors andmechanism were also involved. Therefore, the main conclusions are as followings:(1) In the two years of measurement, soil CO2fluxes at all sites exhibitedpronounced diurnal and seasonal variations. The diurnal patterns of Rs could beexpressed as single peak curves, reaching to the maximum at11:00-15:00and fallingto the minimum at21:00-1:00(or before dawn). Negative soil CO2fluxes werefrequently observed during the night over the two growing seasons at the As and theSg sites. The seasonal variations of soil CO2flux at all sites overall corresponded toseasonal changes in soil temperature. The mean daily value of Rs was moderate in late spring (0.05-2.51μmol m-2s-1in May), increased sharply to a peak in summer(1.0-5.09μmol m-2s-1in July), and then decreased in autumn (0.02-1.51μmol m-2s-1in October). However, the Rs rate at the alkali-spot land showed no significantseasonal variation, with the CO2fluxes ranged from a minimum (-0.1μmol m-2s-1) atSeptember to a maximum (0.95μmol m-2s-1) at July.(2) Air temperature near soil surface and the soil temperature at10cm depth couldexplain32%-85%seasonal variations of Rs and were the two main controlling factorsin regulating the seasonal variations of Rs at the six sites. The seasonal patterns of Q10values derived from soil temperature at10cm depth were consistent at the Cv, Pd, Lcand Pa stie, and showed large seasonal variations during the growing season. The Q10values of Rs at the four sites were moderate in late spring (2.47-4.03in May),decreased to the annual minimum in summer (1.81-2.90μmol m-2s-1in August), andthen increased to a peak value in autumn (3.45-4.54in October). Soil temperaturemay be the main factor affecting Q10values of Rs. The spatial coefficient of variation(CV) of Q10values among the six sites was42.60%. The As, Sg, and Cv site have thelarger Q10values than Pd, Lc and Pa site.(3) Due to the large variations of precipitation in2011and2012, the soil watercontent (Ws) had different effects on Rs for the two measured years. Rs increased inassociation with increasing Ws in the year of low rainfall (2011). In the year of richrainfall (2012), however, when the Ws exceeds the optimum water content for Rs,high soil water content could impede O2diffusion through pore spaces, therebyreducing rates of decomposition and microbial production of CO2. The optimum Wsfor Rs at the six sites was22%-30%and near the field capacity of the study area. TheWs and Ts have an interactive effect on the seasonal variations of Rs at all sites(p<0.01). However, models including Ts and Ws did not significantly improvemodeling fitting (R2=0.34-0.81) compared with those fitted using Ts (0.32-0.82) foreach site.(4) The overall mean value of cumulative CO2emission was255.97±74.85g C m-2(Mean±SE) across all sites in the two growing seasons, with the means of16.65±0.61、74.7±9.8、265.68±11.5、272.3±2.3、432.1±11.18and498.4±10.87g C m-2at the sites of As, Sg, Cv, Pd, Lc and Pa, respectively. There was no significantdifference in cumulative CO2emission between two years at the same site. The spatialCV of cumulative CO2emission among the six sites was73.02%. Among the six sites,cumulative CO2emission during the growing season was positively correlated withmean soil water content (Ws), above plant biomass and soil organic carbon (SOC).Oppositely, negative correlation occurred between cumulative CO2emission andmean soil temperature (Ts), pH, electrical conductivity (EC), exchangeable sodiumpercentage (ESP) and bulk density (BD). Spatial differences in soil CO2emissioncould be accounted for by the changes in soil microclimate, plant biomass and soilproperties. The empirical equation of plant biomass was best to estimate the regionalamounts of CO2emission from this saline and alkaline land during the growingseason.(5) After sterilization, CO2fluxes from soils of As, Cv, Pd, and Lc site to theatmosphere was negative, which suggested that the process of CO2absorption was aninorganic, non biological process. High EC and pH of the soil positively affected theinorganic CO2fluxes from alkaline-saline soils, while high temperature and soil watercontent had negative effects on the CO2absorbing magnitude of these soils. Aninorganic CO2diffusion and dissolution process, derived from the change in the sizeof the reservoir of dissolved inorganic carbon in the soil solution, may explainnegative CO2fluxes in saline-alkaline soils.The results could not only supply fundamental data of soil CO2flux in saline andalkaline land in the western Songnen Plain to the global carbon cycle, but alsoprovide important evidences and basis for figuring out the “missing sink” of carbon. |
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