Seasonal Variation Of Soil Respiration And Carbon Source Analysis Of Diff Erent Forest Structures In The Loess Plateau

CO2 released by soil respiration is an important part of global greenhouse gases and has a profound impact on global climate change.With the advancement of ecological civilization construction in the loess region,the forest structure in the region has changed,and the subsequent changes in water and heat factors,litter coverage,soil physical and chemical properties,etc.indirectly affect the contribution of soil respiration to atmospheric CO2.Therefore,it is of great significance to study soil respiration and its influencing factors in different forest structures,and the carbon source of soil respiration for estimating the impact of ecological construction on the carbon balance of the region.In this study,the soil of three forest structures of Pinus tabulaeformis forest,Quercus acutissima forest and mixed forest in Shuanglong National Ecological Experimental Forest Farm in the loess region was taken as the research object.Combining the methods of field dynamic monitoring and indoor experiments,the change characteristics of soil respiration rate in different seasons and the impact of influencing factors on the CO2 release of soil respiration were analyzed,the carbon release law in the process of soil respiration was revealed,and the soil respiration database of different forest structures in the loess region was extracted,The change characteristics of soil respiration carbon flux in three forest structures were estimated and the influencing factors were analyzed.The main conclusions obtained are as follows:(1)The change rule of soil respiration rate is that the daily change of soil respiration rate of three forest structures presents a single peak change trend,and the low peak and high peak appear at 24:00 and 12:00 respectively.In spring,summer and winter,the daily average soil respiration rate of Quercus acutissima forest was higher than that of Pinus tabulaeformis forest and mixed forest,and the daily average soil respiration rate of Pinus tabulaeformis forest was higher than that of the other two forests in autumn.The seasonal variation of soil respiration rate of the three forest structures is as follows:summer>spring>autumn>winter.The annual average soil is as follows:Quercus acutissima forest>mixed forest>Pinus tabulaeformis forest.(2)The effects of hydrothermal factors and litter removal on soil respiration rate are as follows:the most important hydrothermal factors affect soil respiration rate is surface soil temperature(T5cm)and surface soil moisture(W5cm).Soil respiration rate is positively correlated with T5cm and it is negatively correlated with W5cm(P<0.05).The interpretation degree of soil temperature to soil respiration rate is higher than that of soil moisture to soil respiration rate,and the joint interpretation degree of the two to soil respiration is higher,up to 98.6%.The temperature sensitivity index Q10 approaches 2.4 in spring and summer,and is less than 2.4 in autumn and winter.The two factor model is better than the single factor model when the measured data of surface soil temperature and soil moisture are used to predict the soil respiration rate.The soil respiration rate(RS-L)of three forest structures after litter removal showed a single peak trend,and litter removal significantly reduced the soil respiration rate.The contribution rate of fall and winter litter to soil respiration is higher than that of spring and summer.The contribution rate of litter to soil respiration is higher in four seasons Quercus acutissima forest,followed by mixed forest and Pinus tabulaeformis forest.(3)The dynamic characteristics of stable carbon isotopes of soil respiration and the sources of carbon in soil respiration are as follows:three forest structures there is a significant diurnal variation in the δ13C value,with daytime δ13C value is enriched at night.The seasonal changes show:winter>autumn>spring>summer.Annual average CO2 emission from soil respiration.Theδ13C value is expressed as:Pinus tabulaeformis forest=mixed forest>Quercus acutissima forest.CO2 release from soil respiration of three forest structures δ13C value is positively correlated with surface soil temperature and atmospheric temperature,and negatively correlated with surface soil moisture and atmospheric humidity.Surface soil temperature and soil moisture affect soil respiration and release CO2 the most important hydrothermal factor is the δ13C value.The litter of three forest structures in the four seasons has the largest contribution to the source of soil respiration carbon(46.13%-68.02%),the second source is plant roots(19.96%-29.96%),and the third source is soil microorganisms(11.70%-26.03%).(4)The soil respiration database of coniferous forest,broad-leaved forest and mixed forest in the loess region was extracted.The characteristics of soil respiration carbon flux change and its influencing factors were as follows:between 1995 and 2005,the average annual carbon flux of soil respiration of the three forest structures in the loess region was 500.46-780.98gc·m-2 · yr1,and between 2006 and 2015,the average value was 667.38-875.43gC·m-2·yr-1.The growth rates of coniferous forest,broad-leaved forest and mixed forest were 33.35%12.09%、11.61%。The annual carbon flux of soil respiration from 1995 to 2015 was broad-leaved forest>mixed forest>coniferous forest.The soil respiration carbon flux in growing season was greater than the annual average soil respiration carbon flux.The soil respiration carbon flux of the three forest structures was the largest in summer,followed by spring,autumn and winter.The multi-year average precipitation is the most significant factor affecting the soil respiration carbon flux.Among them,the annual average temperature and multi-year average precipitation have a very significant positive correlation with the soil respiration carbon flux,the soil bulk density has a very significant negative correlation with the soil respiration carbon flux,the soil organic carbon concentration has a significant positive correlation with the soil respiration carbon flux,and the soil carbon nitrogen ratio has a negative correlation with the soil respiration carbon flux.