Soil CO₂Efflux Of Black Locust Forest In Deep Soil Layers On The Hilly Loess Plateau, China

Deep soil organic carbon is an important part of the soil carbon sequestration.Researching on deep carbon flux characteristics of soil organic carbon transformation issignificant for understanding the stability of deep soil organic carbon and its influence on thedeep soil organic carbon involving the ecosystem carbon cycle.In this paper, Robinia pseudoacacia woodland in the hilly Loess region was selected forthe study, using alkali absorption method and gas wells method to measure the soil respirationand soil carbon fluxes: Four ages of a Robinia pseudoacacia revegetation chronosequenceincluding10(ch10),20(ch20),30(ch30) and40years (ch40) in the hilly Loess Plateau regionwas selected as research object and crop land was taken as a contrast. Setting0-80cm soillayers as a shallow layer and the80-200cm soil layers as a deep layer, soil resporation andcarbon flux through the vertical profiles were monitered each month during a whole researchyear. The purpose of the study was to reveal the deep soil respiration characteristics andinfluencing factors of carbon flux, to understand the seasonal dynamics of carbon flux and itscorrelation with soil environmental factors, and to explore the vegetation restoration, soildepth, soil temperature, humidity and soil characteristics on soil carbon flux.The results wasmeaningful for understanding the contribution of deep soil organic carbon to the ecosystemcarbon cycle. The main conclusions were as follows:1) Deep soil layer respiration in Robinia pseudoacacia woodland in the hilly Loessregion exhibited the following characteristics: The annual soil respiration on the surface,20,80,140,200cm soil layers was7.2g/(m2. d),6.7g/(m2. d),6.1g/(m2. d),5.3g/(m2. d),4.2g/(m2. d) respectively; Deep soil respiration had a significant differences in thenon-growing season between adjacent soil layers(P <005); Deep soil respiration significantlycorrelated with soil organic carbon during the growing season. Coordinated effects of soiltemperature and moisture could explain about50%of the total annual respiration variation indeep soil.2) Five kinds of commonly used diffusion models were borrowed for calibrating soilprofiles carbon flux to get the most suitable model for each soil laters. The optimal diffusionmodels in Robinia pseudoacacia woodland in the hilly Loess region at0-20cm,80-140cm and 200cm soil layers were Moldrup-2000, Moldrup-1997and Millington diffusion models. Inloessal soil at0cm,20cm,80cm,140cm and200cm soil layers, soil gas diffusioncoefficients, which were critical parameters in the models mentioned above, were0.15,0.14,0.20,0.22, and0.27respectively.3) Deep soil carbon flux in Robinia pseudoacacia woodland in the hilly Loess regionwas characterized as: In the2m soil profile, with the soil depth increased, Robiniapseudoacacia revegetation chronosequence including10a,20a,30a and40a average annualoverall soil carbon flux exhibited a downward trend. The carbon fluxes in the surface,20,80,140and200cm soil layers were2.45μmol/(m2.s),0.67μmol/(m2.s),0.32μmol/(m2.s),0.19μmol/(m2.s) and0.13μmol/(m2.s). Shallow soil (0-80cm) carbon flux changed obviouslywith the seasons during the monitering year, however, deep soil carbon flux did not changesignificantly with the seasons, but its carbon fluxes was smaller in non-growing season thanin the growing season (from May to August).4) The soil carbon flux was impacted by soil organic carbon content, soil temperature andmoisture. The correlation between carbon and organic carbon fluxes in deep soil (80-200cm)was stronger than that in shallow soil (0-80cm). Shallow soil carbon flux had a strongercorrelation with the temperature than deep soil; Deep soil carbon flux and soil moisture wasno significantly correlated, indicating that the deep soil organic carbon was more stable thanthe shallow soil organic carbon.5) Carbon fluxe in Robinia pseudoacacia woodland in deep soil (80-200cm) couldaccount for30%of the annual total carbon flux in the2m soil profile. Deep soil organiccarbon involved the ecosystem carbon cycle in some extent, its influence to atmosphericcarbon cycle can not be ignored.