Studying The Effect Of Temperature On Soil Organic Carbon Dynamics Using Radioactive Carbon Isotope And Biomarkers

Soil organic carbon(SOC)is the largest carbon reservoir in the terrsttrial ecosystem,and it is also the most important source and sink of atmosphic CO2,which is rapidly exchanging a large amount of carbon with atmosphere.Given the importance of SOC in global carbon cycle,the accurate prediction of how SOC decomposition and feedback to the ongoing climate warming is becoming an important topic for discussion.Temperature and precipitation are the two most important environmental factors which influence the turnover of SOC.This study explored the influence of temperature on soil organic carbon dynamics in natural environment while escludung that of precipitation using 14C and biomarkers.In this study,the turnover times and temperature sensitivity of four fractions(light fraction,particulate organic matter,acid hydrolizable fraction and recalcitrant fraction)of surface soil(0-5 cm)was investigated for 12 sites along a temperature gradient with 400 mm of mean annual precipitation(MAP)on the Qinghai-Tibet Plateau of China.The results show that the fractionation procedure separated the soil into four fractions with distinctive 14C signatures.The turnover times estimated based on the structures of soil organic carbon turnover models and 14C contents is LF<POM<HF<RF.The relative peak area of functional group calculated from the analysis of fourier transform mid-infrared spectroscopy(FTIR),is not related to turnover times,suggesting that protect mechanisms rather than molecular characteristics dominate the long-term persistence of organic compounds.Mean annual temperature(MAT)and pH have a significant effect on the carbon and nitrogen content of soil bulk and fractions.Regression analysis shows that turnover times of LF and POM are not related to MAT,suggesting that decomposition of the two fractions is not sensitive to temperature;the turnover times of HF and RF decrease exponentially with MAT,and the temperature sensitivity of RF is larger than that of HE These results indicate that stable fractions are more sensitive to temperature than liable fractions,which accords with kinetic theory.We proposed a new conceptual model of SOC decomposition of temperature sensitivity,i.e.apparent sensitivity is primarily determined by de-protected sensitivity,and intrinsic sensitivity has a secondary contribution to apparent sensitivity of fractions.The temperature sensitivity of SMT(summer mean temperature)is larger than that of M/AT.The applications of different connections of carbon pools induce uncertainties existing in estimated turnover times and temperature sensitivity.Compared with turnover time and temperature sensitivity estimated by the carbon model with stepwise series pools,both of turnover time and temperature sensitivity could be underestimated using the model with parallel pools.This study also reveals that,compared with the estimated released carbon flux and carbon stock based on the temperature sensitivity values obtained in this study,the released carbon of the grasslands on the Qinghai-Tibet Plateau is underestimated,whereas the carbon stock is overestimated in the next 100 years based on one-pool model.Therefore,the estimation derived from on one-pool model would exaggerate the negative feedback of soil carbon to global warming.Along the 400 mm isopleth of mean annual precipitation in China,soil carbon and nitrogen contents decrease significantly with temperature;there is a strong positive correlation between soil carbon content and soil nitrogen content,suggesting that the same processes control soil carbon decomposition and soil nitrogen decomposition.The absolute contents of biomarkers(free lipids and lignins)in soil also decrease with MAT,this suggests that these biomarkers are biodegradable.However,the relative amounts of the biomarkers in SOC increase with MAT;the possible reason is that the liable compounds of SOC are quickly consumed,therefore the relative proportion of these biomarkers with slow decomposition in SOC increase with temperaure.Based on the analysis of lignin monomer,we find that the oxidation state of lignin increases with MAT.The soil quality indicator-lignin/N,which indicates the relative change of soil recalcitrant pool and soil liable pool,could accurately reflect the variations in SOC quality with MAT.