| In order to assess the components and influence mechanism of soil carbon flux, andunderstand the carbon input and output of forest communities in southern Taihang Mountains,China, it is necessary to analyse the carbon budget of studied forest communities. In this study,the automatic observation system of soil respiration combined with trenched and stableisotopic method were employed to distinguish the components of soil respiration. Four landuse types, including of cropland (CL), abandoned cropland (AL),10-year-old (R10) and43-year-old Robinia pseudoacacia L. plantations (R43) were measured in southern TaihangMountains, China. The dynamic characteristics of soil respiration rate (Rs) and itssubcomponents, and the effects of environmental factors on the soil respiration wereinvestigated. The main conclusions are as following:(1) In order to obtain influences of different land use types on soil quality, soil qualityindex was employed to compare differences of soil quality among various ages of R.pseudoacacia plantations. The results showed that the nutrient of topsoil increased significantlywith the increase of tree ages. Soil properties were improved with the tree age increasing,especially at the soil layer of0-5cm. In the two R. pseudoacacia plantations (R10and R43), soilphysical and chemical properties were improved and the soil microbial biomass C and N wereincreased significantly compared with the abandoned cropland and the cropland. The soilquality index of43-year-old R. pseudoacacia plantation was0.542, larger than the10-year-oldR. pseudoacacia plantation (0.536), the abandoned cropland (0.499) and the cropland (0.498).The results indicated that the soil quality was improved during the conversion from cropland toforestland.(2) Based on the measured data by LI-8150, the proper measuring times to represent dailyRsof43-year-old R. pseudoacacia plantation were analysed in southern Taihang Mountains.The results showed that the diurnal and seasonal variation of soil respiration was almost consistent with that of soil temperature at5cm depth (Ts) in R. pseudoacacia plantation. Ingeneral, soil temperature reached its lowest value at6:00-8:00and achieved the highest valueat14:00-16:00. Except for the minimum soil volumetric water content (18.32%) at0-10cmdepth (W) in January, the values of W were larger than20%in other months. During themeasurement period, monthly averages of soil respiration reached its peak in July anddecreased to the minimum in January gradually. The relative deviations between daily averagevalues and values measured at8:00-10:00from July to November or at9:00-11:00fromDecember to May were1.27%and4.91%, respectively. Comparing with the daily averagevalues, the relative errors of measured soil respiration rate during representative times werewithin±10%. In this study, the representative times for measuring soil respiration of R.pseudoacacia plantation during the measurement period were8:00-10:00from July toNovember and9:00-11:00from December to May.(3) The measurement of the13C value of soil CO2were used to separate total soilrespiration into subcomponents of RAand RHin R10and R43, and the calculated RC wasemployed to compared to the trenched approach. The results demonstrated that the13C value ofsoil CO2decreased with soil depths, while the δ13C of root and soil organic matter did notchange significantly with soil depths. A preliminary test was performed to compare the δ13C ofsoil CO2extracted from buried steel tubes and from GVP (Gas Vapor Probe Kits) at50cm soildepth. It is concluded that the isotopic fractionation parameter of the δ13C of soil CO2at50cmsoil depth in plantation was about2.03‰. Data analysis indicated that the contribution ofautotrophic respiration on total soil respiration (fRA) increased gradually with soil depths, whilethe contribution of heterotrophic respiration on total soil respiration (fRH) decreasing with soildepths. Analysis on contribution of autotrophic respiration on total soil respiration (fRA) withtrenched method and stable isotopic method showed that the difference between fRAestimatedby trenched-plot method (23.50%) and isotopic method (21.03%) was not significantly.(4) Seasonal variations and influence mechanisms of soil respiration rate (Rs) under43-year-old and10-year-old R. pseudoacacia plantations, abandoned land, and cropland were studied at southern Taihang Mountains. The results showed that sequestration capacity of soilorganic carbon was improved in two plantations significantly. At0-5cm depth, for example,soil organic matter of43-year-old and10-year-old R. pseudoacacia plantations were3.9and1.6times of cropland, respectively. For43-year-old and10-year-old R. pseudoacaciaplantations, abandoned land, and cropland, measured annual soil respiration rates were2.33,1.21,2.40, and2.04μmol·m-2·s-1, respectively. The abandoned land had the largest annual Rs,and the annual Rsof43-year-old R. pseudoacacia plantation was larger than that of10-year-oldR. pseudoacacia plantation. Statistical analysis indicated that soil temperature at5cm depthwas the dominant environmental factor affecting the seasonal variation of Rs. In this study,calculated temperature sensitivity of soil respiration (Q10) was2.47,2.53,2.06, and1.56for43-year-old and10-year-old R. pseudoacacia plantations, abandoned land, and cropland,respectively. In contrast to cropland, two plantations presented a larger temperature sensitivityof soil respiration. Moreover, significant seasonal variations of Q10under four land use typeswere observed, and soil temperature was the key factor affecting the temperature sensitivity ofsoil respiration.(5) The trenched method was used to distinguish the autotrophic respiration (RA) andheterotrophic respiration (RH) of10-year-old and43-year-old R. pseudoacacia plantations.Results showed that soil respiration and its components had obvious seasonal dynamics, andthe maximum rate occurred in the summer whereas the minimum occurring in winter. Monthlymean total soil respiration rate (Rs) of R10and R43was significantly higher than autotrophicrespiration rate. No significant difference was found between total soil respiration andheterotrophic respiration. There were significant exponential relationships between soilrespiration and its components in two R. pseudoacacia plantations and soil temperature at5cmdepth (P<0.01). Compared with autotrophic respiration, heterotrophic respiration was affectedmore with changes of soil temperature at5cm depths, which explained77.82%and77.87%ofRHand38.60%and43.43%of RAin R10and R43, respectively. Soil volumetric water content at0-10cm depth demonstrated a highly significant correlation with soil respiration and its components. The W could explain25.54%and31.72%of RHand10.6%and11.55%of RAinR10and R43. In addition, a significant linear relationship between Rsor RHin R10and soil porewater conductivity at0-10cm (ECp) was found. There were similar correlations between RsorRAand ECpin R43. Due to the lower determination coefficient (R2) between ECpand soilrespiration or its components, ECpwas not the major control factors influencing soil respiration.The seasonal trends of temperature sensitivities of soil respiration (Q10) and its components inR10and R43were opposite with that of soil temperature. Comparing with Rsor RA, RHwas moreaffected by soil temperature change during growing season (from May to July). Thetemperature sensitivity variations of Rsor RAin R10were greater than that in R43in growingseason with higher temperature, while the opposite trend was found at non-growing seasonwith lower temperature. The monthly root respiration contribution to total soil respiration (RC)in R10and R43also presented obvious seasonal variations. There was no significant differenceon monthly RC between R10and R43. The estimated contributions of annual autotrophic andheterotrophic CO2fluxes on annual total soil respiration were25.14%and90.69%on R10, and30.84%and70.26%on R43, respectively. In this study, calculated results showed that annualheterotrophic annual CO2flux generally accounted for a high proportion of total soilrespiration in studied region. |
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