Studies On Soil Respiration And Its Relations To Environmental Factors In Different Ecosystems

The soil respiration(R_s) in different ecosystems were measured,respectively,in Taiyuan,China, and in Copenhagen,Denmark,using LI-COR-6400 portable photosynthesis system(LI-COR, Environmental Division,Lincoln,NE,USA) connected to a LI-COR 6400-09 soil chamber with an area of 71.6 cm².The objectives of this study were to:(1) examine the seasonal patterns of soil CO₂ effiux in the two areas to understand whether there is any difference in soil respiration between the two areas;(2) estimate the amount of soil CO₂ effiux from the two areas;(3) characterize the spatial variation of soil CO₂ efflux across the different sites of the two areas,and(4) identify the relationships between soil CO₂ effiux and soil temperature(T_s) as well as soil water content(W_s) in the two areas.The dissertation consists of five chapters.The chapter one is literature review,and the chapter two through chapter five mainly are research results.The chapter two is the study results mainly on natural ecosystems including forest,shrub and grass lands and so on,and the soil respiration in 11 sites ranging in altitude from 829 to 1443 m with different soil properties,vegetation covers and slop orientation etc. in the Tianlong Mountain,one of the national natural reserve areas(N37°44′19″;E112°22′49″),was measured over two years from 2005 to 2006.The chapter three is on agricultural ecosystems(crop), and we measured the soil respiration over 1 year for 4 different land use covers to identify the relationships between soil respiration and environmental factors.The object of research in the chapter four is also on agricultural ecosystems,but the purpose is to understand the spatial variation in soil respiration in two different scales,middle and small scale.The chapter five is the study results we made in Copenhagen area for agricultural ecosystems whenâ… was a visiting scholar in University of Copenhagen in 2003.In this chapter the soil respiration in different vegetation conditions was measured from February to December in 2003,with an aim to make comparison of soil respiration for different covers and the relationships between soil respiration and soil temperature,and to understand the spatial variation in soil respiration in different vegetation covers.The main results are as follows:1) The soil respiration in the Tianlong Mountain area showed a distinct seasonal variation with the low values in winter and spring and the high values in middle summer,and the seasonal variation of soil respiration in 11 sites could be described by a three-parameters Gaussian equation.The soil respiration rate was,3.92,4.66,4.40,3.01,3.70,3.88,4.00,4.72,5.21,4.52,2.57μmol CO₂ m^-2s^-1 in 2005,and 2.33,2.96,1.93,2.35,2.70,2.89,2.79,3.39,3.08,3.23,1.83μmol CO₂ m^-2 s^-1 in 2006.The overall means of soil respiration across 11 sites were 3.92 and 2.68μmol CO₂ m^-2 s^-1,in 2005 and 2006, respectively,and were different between two years. For most of the sites,the correlation between soil respiration(R_s) and soil temperature(T_s) over 10 cm depth was significant,which could be described by linear,exponential and Lloyd & Taylor equations.T_s explained 28-88%of the seasonal changes of R_s using the three kinds of equations.When the soils were in the drought-affected condition,the controlling of T_s over R_s was less.The correlation parameters of R_s to W_s increased when the measured R_s were normalized to 10℃T_s,and the W_s explained 15-71%of seasonal changes of R_s.When the soil temperature was low the controlling of W_s over R_s was less than that of T_s.In most of the time over the season the combined controlling of T_s and W_s over R_s was existed.The effect of both the T_s and W_s on R_s was different for the 11 sites,and there were more effect of W_s on R_s than that of T_s on R_s for these sites at which water holding capacity(WHC) is low.In compared with the soil temperature-based or soil water-based one variable model the twovariable models integrating T_s and W_s into one equation were better to predict their relation between R_s with both the T_s and W_s,the both variables could explain 55-86%of soil respiration variation over the season.The Q₁₀,which is called temperature sensitivity of R_s,and R₁₀,which represents the respiration rate at a soil temperature of 10℃was different for the 11 sites.The Q₁₀ values ranged from 1.80 to 4.94 and the R₁₀ from 1.47 to 4.75μmol CO₂ m^-2 s^-1.The mean values of Q₁₀ and R₁₀ in our studied sites are 3.14 and 3.54μmol CO₂ m^-2 s^-1,respectively,and are in the range of published values by other researchers.The overall annual mean soil CO₂ efflux,which is the daily-weighted monthly mean R_s multiplied by the respective day numbers of the month,was from 654.9 to 1440.5 g C m^-2 among 11 sites in 2005 from April to December,and 581.2 to 1075.3 g C m^-2 in 2006 from March to December. The mean CO₂ efflux across the 11 sites was 1068.6 g C m^-2 in 2005 and 850 g C m^-2 in 2006, respectively.2) The soil respiration of agricultural ecosystems in 4 kinds of land cover in Taiyuan basin showed a clear seasonally changes over the season,with the lower values about 1μmol CO₂ m^-2 s^-1 in January, February,March,as well as in November and December,with the higher values more than 10μmol CO₂ m^-2 s^-1 in July and August.The higher values during the summer months showed a fluctuation because of the influence of environmental factors.The daily-weighted mean soil respiration from January to December was 3.54±2.6,4.43±3.99,3.95±3.58,3.84±2.93μmol CO₂ m^-2 s^-1,respectively,in shrub plantation,grass land,medicinal herb land and crop land.There was no difference of soil respiration among 4 sites.The correlation of linear,exponential and Lloyd & Taylor equations of R_s to T_s was all significant(P＜0.05),but the R² from different equation was different,the highest one is from Lloyd & Taylor equation and the lowest is from linear equation.When the soil water is in no drought-stress condition(here the definition criteria is that soil water content is more than 1/3 of WHC) the soil temperature explained 40,23,33,29%of the soil respiration variation,respectively,for shrub plantation,grass land,medicinal herb land and crop land,for linear equation;56,45,62,41%for exponential equation and 60,56,69,44%for Lloyd & Taylor equation.Comparison with the results from Tianlong mountain area,soil water content has more influence on soil respiration.The correlation of R_s to W_s for linear equation was better than that for exponential one,but when the soil respiration was normalized to 10℃the R² from both the equation showed less difference,ranging from 37 to 64%for linear one and 39 to 61%for exponential one.Furthermore,the two-variable models including both T_s and W_s variable could be used accurately to explain the R_s variation over the season,with the R² ranging from 52 to 82%,an obvious increment comparing with those values from single-variable models.The 24-h measurement of soil respiration in June,August and October showed a diurnal variation of R_s,with the higher values occurring between 11:00-15:00,and the lower values at about 6 o'clock in the morning.The correlation of the diurnal R_s and Ts was not significant on most of measurement days for 4 kinds of land use.When the mean value of R_s measured between 8 h-12 h was used to represent the 24-h mean value the error is about 10%larger than actual R_s.According to all measurements the annual CO₂ efflux from the soils were,1227,1732,1509 and 1477 g C m^-2 a^-1 for shrub plantation, grass land,medicinal herb land and crop land respectively.The amount of CO₂ effiux from May to October was 996.3,1361.1,1300.7 and 1191.4 g C m^-2,which were larger than the values we measured in Tianlong mountain area.3) Significantly spatial variations of R_s existed at different scales,either between-sites or withinsite. In the northern Taiyuan basin,the measurements of soil respiration of 42 sites(among them including 23 crop fields and the others being grass land,waste land etc.) in July and October showed that spatial variation in R_s in this area existed.The variation of R_s indicated by the coefficent of variation(CV) ranged from 25-50%.The correlation of R_s to T_s at spatial scale was significant only in June when the soil moisture was changed greatly among the all measurement sites.The CVs within-site spatial variation of R_s for two-time measurements in 3 measurement sites were,respectively,21,32, 39%in September and 40,46,58%in November.The CV was higher in November than those in September.The correlation of R_s to T_s,or to W_s was not significant for the measurements.Further studies are needed to understand the relations of soil respiration to soil variables,such as microbial population,root density and so on. 4) The soil respiration of agricultural ecosystem in Copenhagen area of Denmark also showed a seasonal variation with the highest value of 5.77μmol CO₂ m^-2 s^-1 occurring in July and the lowest value of 0.22μmol CO₂ m^-2 s^-1 appearing in February.The trend was in accordance with the seasonal changes in soil respiration in our above research areas.The correlations of seasonal variation in soil respiration to day number of the year in winter wheat could be described by a three-parameter Gaussian equation.The mean CO₂ efflux for 18 measurements between February and December from the soil in winter wheat was 2.21±1.45μmol CO₂ m^-2 s^-1,and 2.39±1.50μmol CO₂ m^-2 s^-1 from April to October. The CO₂ efflux values were less than the values we observed in most of the sites in our study area,but nearly equalled the value in the site of bare land in Tianlong maintain area.When there was no water stress in the soil,the soil temperature explained 68%of soil respiration variation.The annual amount of soil CO₂ efflux from the winter wheat was 655.5 g C m^-2 a^-1.The soil CO₂ efflux between April and December under 4 different kinds of land cover showed that the soil CO₂ efflux were,2.71±1.74μmol CO₂ m^-2 s^-1 for winter wheat land,3.90±2.47μmol CO₂ m^-2 s^-1 for fallow grass land,2.65±1.24μmol CO₂ m^-2 s^-1,for forage grass land and 1.54±0.91μmol CO₂ m^-2 s^-1 for flower land.The correlation of R_s to T_s for 4 separate sites was significant,and the temperature explained 75%,82%,86%and 45%of variation in soil respiration in winter wheat land, fallow grass land,forage grass land and flower land,respectively.The Q₁₀ values calculated from the soil respiration measurements of the 4 sites were 3.16,2.87,2.04 and 2.09 in winter wheat land,fallow grass land,forage grass land and flower land,respectively,and the R₁₀ were 2.32,3.88,2.16 and 1.22μmol CO₂ m^-2 s^-1,respectively.The amount of soil CO₂ efflux between April to December was 655.5, 1129,835.4 and 374.6 g C m^-2,respectively,in winter wheat land,fallow grass land,forage grass land and flower land.Both Q₁₀ and R₁₀ were in a range comparable with those measured in our other research sites.There were a spatial variation of soil respiration in winter wheat and grassland in Copenhagen area of Denmark,and the CVs of soil respiration in Auguest ranged from 30 to 70%,with the lager ones of CV appearing in winter wheat rather than in grassland.