Inter-annual Variation Of Soil Respiration And Its Temperature Sensitivity In An Abandoned Farmland

Soil respiration is one of the most important components of the ecosystem carbon cycle,so it is essential to conduct a further study of the dynamic change of the soil respiration of terrestrial ecosystem and its relations with affecting factors.Up to now,most studies have concentrated on diurnal and seasonal variations of soil respiration,but a few studies have focused on inter-annual variation.For further study on the inter-annual variation of soil respiration,we made measurements in soil respiration and its affecting factors both soil temperature and soil moisture for ten years at an interval of one to three times per month in an abandoned cropland?H?and a bare land?L?.The purpose was to: 1)study the seasonal variations in soil respiration in the two lands,and analyze the relations between soil respiration and the affecting factors;2)understand inter-annual variations in both soil respiration and temperature sensitivity of soil respiration as well as their relations to soil temperature and soil moisture;3)estimate the annual carbon dioxide fluxes;and 4)quantify contributions of root respiration to the total respiration.The main results are as follows:Variations in both soil respiration and temperature for both sampling lands showed obvious seasonal variation,with low values in early spring,and the maximum in middle summer and early autumn and the minimum in winter over a year.A three-parameter Gaussian equation could be used to describe the course of soil respiration and soil temperature against day number of the year for the sampling lands.The course of soil moisture over a year in both lands didn't show a regular variation,depending on the amount of precipitation and its distribution over a year.However,over ten-year data,the monthly average of soil moisture over the year showed an inverted “S”curve,the lowest occurring in early summer and the highest from middle summer through early autumn.Annual mean soil respiration from H and L lands for 10-year measurements was 4.11±3.12 ?mol CO₂ m^-2 s^-1 and 2.74±2.35 ?mol CO₂ m^-2s^-1,respectively,and the lowest value of about 1 ?mol CO₂ m^-2 s^-1 appeared in the winter for both lands.Mean annual soil CO₂ efflux from the land H and L over 10 year measurements was 1208.7 g C m^-2 a^-1 and 805.8 g C m^-2 a^-1respectively,and it was different in different years.The contribution of the root respiration to total respiration in the H land ranged from 21.73 %through 42.13%.The relationship between soil respiration and soil temperature for the H and L could be described by an exponential equation,and the fitted equation for each year data explained from 55% through 92% of the soil respiration for the H land and 40 % through 84% in the L land.This indicated that the dependence to temperature in the H land is higher than that in the L land.The relationships between soil respiration and soil moisture in most of the years were not significant in the two lands,and the effect of soil moisture on soil respiration in the H land was larger than that in the L land.In comparison with the fitted one-variable equation of soil respiration against soil temperature or soil moisture,the two-variable equation taking both soil temperature and soil moisture as independent variables could be well simulated the seasonal variation of soil respiration in the both lands.Between the fitted two-variable equations,the power-exponential functions were better than the fitted product functions as a whole.The Q₁₀ values calculated with a soil temperature at 5,10,15 cm depth from 10-year measurements were 2.32,2.59,2.75 in the H land and 1.92,2.05,2.18 in the L land.When we pooled all ten year measurements and classified them according to the soil moisture to make analysis,the results showed that the highest Q₁₀ values at three depths appeared corresponded soil moisture of above 21% in the land H,and 16%²1% in the L land.The temperature sensitivity(Q₁₀)of soil respiration and the soil respiration at10 ?(R₁₀)at different measuring depths in the both lands was different,showing an increase with increasing depth.The R₁₀ values for each year data ranged from 1.18 ?mol CO₂ m^-2 s^-1 through 2.79 ?mol CO₂ m^-2 s^-1 in the H land,and 0.83 ?mol CO₂ m^-2 s^-1 through 1.50 ?mol CO₂ m^-2 s^-1 in the L land.The annual Q₁₀ values were significantly related with the annual soil moisture in the H land,but not in the L land.