Soil Respiration Characteristics And Its Controlling Factors In Chongqing, China

The global climate system has been changing dractically and violently due to the increases of CO₂,CH₄ and N₂O concentrations in the atmosphere which are known as green house effect gases, resulting in a series of severe enviromental problems, such as ascension of sea level, the aberrance of the precipitation distribution, the exasperation of desertation, highly-frequent natural disasters etc. The change of the global climate system will definitely influence land ecosystem and social economic system widely. Since the late of 1970s, a great interest is attached to the research on the global change. Carbon circulation of land ecosystem is on part of the global change and the global carbon circulation, and it is pivotal for predicating the density of CO₂ as well as other greenhouse gases in the atmosphere and for understanding the sentific problems of the interaction between atmosphere and biosphere.CO₂ is one of the main green house effect gases, contributing over 60% of the global green house effect. Soil respiration is a main part of land ecosystem carbon circulation and main emission source of CO₂ to the atmosphere beyond those discharged from combustion of fossil fuels. Agricultural soil accounts for the main part of the total soil area. Consequently, the investigation on the agricultural soil respiration in different area has meaningful importance to the estimation of the amount of CO₂ discharged from land ecosystem to the atmoshphere. Man-made effect on respiration of agricultural soil is greater than that on respiration of forest soil and grass-land, which makes the investigation on the respiration of agricultural soil complex and necessary and increases the difficuly on the estimating the total amount of CO₂ discharged from respiration of agricultural soil.The effect of soil temperature, soil water content, plant and fertilization on the seanonal variation of respiration of two types of soil, calcareous yellow soil and neutral purple soil, under the regular cultivation was investigated by using the method of static dark chamber/infrared CO₂ controller in Chongqing for 16 monthes from march 2005 to June 2006. Simultaneously, the proportion of root respiration and microorganism respiration and the annual discharged amount of CO₂ under different cultivation condition (no-planting and no fertilization, NPNF; planting and no fertilization, PNF; plant crop and chemical fertilizer only, PCF; and plant crop and chemical fertilizer in combination with manure, PCFM) were estimated.The results were presented as follow: Soil respiration rate showed a seasonal variation with higher in summer than in winter. During the whole observation, the soil respiration rate of calcareous yellow soil and purple soil treated by PCFM were 71.7～620.75 mg·m^-2·h^-1 with the average 255.85 mg·m^-2·h^-1 for whole period, 387.5 mg·m^-2·h^-1 in summer (2.94 times as in winter) and 131.8mg·m^-2·h^-1 in winter, and 37.08～685.34 mg·m^-2·h^-1 with the average 289.75 mg·m^-2·h^-1 for whole period, 499.1 mg·m^-2·h^-1 in summer (8.05 times as in winter) and 62mg·m^-2·h^-1 in winter, respectively. The seasonal variation tendency of soil respiration was same as and complete accordance with the trend of temperature when the soil temperature was under 15℃. While the seasonal variation tendency was not consistent with each other when the soil temperature was above 15℃. However, when the land temperature is above 25℃, the seasonal variation of soil respiration was accordance with soil water content.There was significant (p＜0.01) correlation between soil respiration and soil temperature and could be described as y=a×x^b. The R² of the regression equation for calcareous yellow soil were 0.3464～0.6086 (N=38), 0.3306～0.6231 (N=38) and 0.3341～0.6155 (N=38), when x is atmosphere temperature, soil temperture at 5cm depth and at 10cm depth, respectively; and the R2 for purple soil were 0.6272～0.7123, 0.617～0.7419 and 0.6786～0.7736 (N=35), respectively. When soil temperature was above 25℃, the correlation between respiration of calcareous yellow soil and soil water content was significant (p＜0.01) and could be described as y=a.x+b with the R² of the regression equation was 0.198～0.386 (N=16)。The relationship between soil respiration and temperature was usually described with The Q₁₀ the in Q₁₀ model, which represented the sensitivity index of soil respiration to temperaturetemperature,. And the result showed that the Q₁₀ was not a fixed value, but a function of soil temperature, which increased with the decrease of soil temperature and with the raise of soil depth's increasing at which the temperature was measured and the depth of the active soil layer and soil temperature. Soil respiration was more sensitive to soil temperature at 10cm. At same time, the Q₁₀ value varied with the different cultivation condition and decreased with increase of the effect factors on soil respiration. Q₁₀ value by different treatments followed the order of NPNF＞PNF＞PCF＞PCFM. Q₁₀ value varied within 1.69～2.65 at 5 cm depth and 1.76～2.64 at 10 cm depth for calcareous yellow soil and 2.58～4.49 at 5 cm depth and 2.76～4.94 at 10 cm depth for purple soil, respectively.The effect of fertilization on the yellow soil respiration was obvious. Under the same plant, the soil respiration rate increased by 36.9% when applied carbamide and the difference between PNF and PCF was significant (p＜0.05); and the soil respiration rate increase by 56.2% when applied carbamide combination with animal manure, and the difference between PNF and PCFM was significant (p＜0.01). The difference between PCF and PCFM was not significant. However, the effect of fertilization on the purple soil respiration was not significant. Fertilization could not increase the respiration of purple soil obviously.The soil respiration of NPNF was approximate equal to heterotrophic (microbes' and soil fauna) respiration. The proportion of root respiration in the soil respiration by PNF, PCF and PCFM treatment was as follow: R_rootPNF =37.3%, R_rootPCF%=18.2%, R_rootPCFM%=27.9% for yellow soil and R_rootPNF%=21.2%, R_rootPCF%=18.2%, R_rootPCFM%=27.9% for purple soil, respectively。The total amount of CO₂ discharged from yellow soil from 1st march 2005 to 28th February 2006 with the treatment of NPNF, PNF, PCF, PCFM was 0.9139 kg CO₂.m^-2.y^-1 1.2638 kg CO₂.m^-2.y^-1,1.6441 kg CO₂.m^-1.y^-1 and 1.9799 kg CO₂.m^-2.y^-1 respetively, and from purple soil from 1st May 2005 to 30th April 2006 was 1.5941 kg CO₂.m^-2.y^-1 2.5654 kg CO₂.m^-2.y^-1,2.3694 kg CO₂.m^-2.y^-1 and 2.4504 kg CO₂.m^-2.y^-1, respetively. The total amount of CO₂ discharged by purple soil respiration treated with four methods was larger than that of yellow soil. The amount difference of CO₂ discharged from same soil in each treatment was determined by the part of plant above the land and the amount of microbe. The amount difference of two types soil respiration was determined by the amount of organic carbon in soil, the type of plant and the amount of microbe.The difference of plants on two types of soil resulted in the difference of the amount of CO₂ at growth. The amount of CO₂ discharfed by soil with planted crop was larger than that of soil with planted vegetable. Under different cutivation conditions, the total amount of CO₂ discharged from soil covered with cabbage, lettuce and corn was 0.2167～0.6777 kg CO₂.m^-2.y^-1, 0.3921～0.5242kg CO₂.m^-2.y^-1 and 1.0922～1.3144kg CO₂.m^-2.y^-1, respetively.