Carbon Stock And Stabilization In Lou Soil

Soil carbon pool as the biggest carbon pool in the terrestrial ecosystem, has played animportant role in the carbon dynamic and climate change, and has a great potential of carbonsequestration. To investigate the stock of carbon pool and the stabilization of carbon pool canprovide the theoretic base for mitigating the greenhouse effect and enhancing the carbonsequestration. Soil carbon pool contains the organic carbon pool and inorganic carbon pool.Organic carbon pool is the main composition in the humid and sub-humid area, whileinorganic carbon pool is the main composition of arid and semiarid area. There were someresearches about the organic carbon stock, but the researchers mostly concentrated on thesurface soil. There was little research on the stock and stabilization of inorganic carbon, andthe role of inorganic carbon in the greenhouse effect was not clear yet. Soil carbon pool in thecalcareous soil contains organic carbon and inorganic carbon pool. During in the closed-jarincubation, some studies suggested CO₂released not only from the mineralization of organiccarbon, but also the inorganic carbon. The percentage of CO₂released from the inorganiccarbon and the factors that might affect the carbonate release process was not clear, either.Lou soil as an ancient soil with a long hiotory of cultivation, a depth of50-100cmanthropogenic layer occurred on the original profile, leading to the different distribution oforngaic carbon and inorganic carbon. Therefore, to estimate the carbon stock of Lou soil, andinvestigate the origin of CO₂and influencing factor during in the closed-jar incubation, mightgive us a brief view about the stabilization of inorganic carbon and its role in the greenhouseeffect.This dissertation studies8Lou soil profiles from different locations in Yangling, ShaanxiProvince, by means of closed-jar incubation and combined with the δ¹³C natural abundancemethod，to investigate the stock stabilization of organic and inorganic carbon. The mainconclusion showed as follows:（1） Regarding Lou soil as the research subject, soil samples from8profiles (depth of0^-200cm) were collected and measured the content of organic carbon, inorganic carbon,labile organic carbon, HCl-resistant and HF-resistant carbon. The results indicated, the stockof soil carbon in0^-200cm layer ranged from266.20-631.59t hm^-2, and the amount of organiccarbon ranged from120.63-177.35t hm^-2, while the amount of inorganic carbon ranged from131.64-504.71t hm^-2, which accounted for20.1-50.8%and49.2-79.9%, respectively. Themain part of carbon pool in0-100cm layer was organic carbon, which was more than60%of soil carbon storage. The main part in100^-200cm layer was inorganic carbon, and the averageamount was64%of soil carbon storage. The amount of labile carbon was highest in the0^-20cm, and decreased as the depth increased. The amounts of HCl-resistant and HF-resistantwere both highest in the top soil and decreased significantly as the depth increased; the ratioof HCl-resistant and HF-resistant to organic carbon was different in the different layer, butdecreased as the depth increased.（2） To evaluate the effect of additional carbonates on CO₂emission from calcareous soilduring closed-jar incubation, three incubation experiments was conducted by adding differenttypes （CaCO₃and MgCO₃） and amounts of carbonate to the soil. The addition of carbonatessignificantly increased CO₂emission from the soil; the increase ranged from12.0%in theCaCO₃amended soil to460%in the MgCO₃amended soil during a100d incubation.Cumulative CO₂production at the end of the incubation was three times greater in the MgCO₃amended soil compared to the CaCO₃amended one. The CO₂emission increased as theamount of CaCO₃added to the soil increased. In contrast, CO₂emission decreased as theamount of MgCO₃added to the soil increased. Our results confirmed that the closed-jarincubation method could lead to an overestimate of organic C mineralization in calcareoussoils. Because of its effect on soil pH and the dissolution of carbonates, HgCl₂should not beused to sterilize calcareous soil if the experiment includes the measurement of soil CO₂production.（3） To determine the effect of soil water content （air dry,30%water-holding capacity（WHC）,70%WHC, or100%WHC）, carbonate type （CaCO₃or MgCO₃）, and carbonateamount （0.0,0.5, or1.0%） on CO₂emission from calcareous soil, a closed–jar incubation wasconduted. Soil CO₂emission increased significantly as the water content increased to70%WHC, regardless of whether or not the soil was amended with carbonate. Soil CO₂emissionremained the same or increased slowly as the soil water content increased from70%WHC to100%WHC, When the water content was≤30%WHC, soil CO₂emission from soil amendedwith0.5%inorganic C was greater than that from unamended soil. In contrast, soil CO₂emission from soil amended with1.0%inorganic C was less than that from the control. Whenthe soil water content was70or100%WHC, CO₂emission from CaCO₃amended soil wasgreater than that from the control. Furthermore, CO₂emission from soil amended with1%CaCO₃was greater than that from soil amended with0.5CaCO₃. Soil CO₂emission washigher in the MgCO₃amended soil than from the unamended soil. Soil CO₂emissiondecreased as the MgCO₃content increased. Cumulative CO₂emission from MgCO₃amendedsoil was3-6times higher than from CaCO₃amended soil. There was significant interactionbetween soil moisture and carbonates on CO₂emission. Soil moisture plays an important role in CO₂emission from calcareous soil because it affects both biotic and abiotic processesduring incubation.（4） To determine the origin of CO₂released from calcareous soil during the closed-jarincubation, we incubated the soil which was removed organic carbon by the combustionmethod. We collected the0^-20cm,60-80cm and180^-200cm depth of Lou soil, and burnedthe soil4h at550℃.The results showed, the organic carbon contents in all the depthdecreased by80%compared with the control treatment, but inorganic carbon content onlydecreased significantly in the180^-200cm depth. The cumulative CO₂amount was higher inthe0^-20cm and180^-200cm than in the60-80cm in the control soils. The cumulative CO₂amounts released from the three layers all decreased significantly after burning. And CO₂emission was higher in60-80cm than in the0^-20cm and180^-200cm. CO₂emissionsignificantly increased in the burning soil with solid HgCl₂.（5） To found the effective way to distinguish the resource of CO₂emission bycomparing the effect of different sterilization methods on the characteristics and CO₂production of calcareous soil. In this study, we used0^-20cm and80-100cm layer as the studysubject, which has large variation of soil organic and inorganic content. The sterilizationmethods we used were solid HgCl₂, unbuffered5%（v/v） HgCl₂solution, buffered5%（v/v）HgCl₂solution （pH=7.5） and chloroform fumigation method. We compared the pH andsterilization effect of soil and the amount of CO₂emission during in the incubation. Theresults showed, four sterilization methods all inhibit the activities of soil microorganism, anddecreased the soil pH in varying degrees. Soil pH was lowest in the solid HgCl₂treatment（6.78）. All the sterilization methods except the buffered HgCl₂method significantly increasedthe amount of CO₂emission （P<0.05）, and the effect of same sterilization method on thedifferent soil layers was different. In the0^-20cm layer, the amount of CO₂emission wasordered from large to little as chloroform fumigation method> solid HgCl₂method>unbuffered HgCl₂method, while in the80-100cm layer, the order was solid HgCl₂method>chloroform fumigation method> unbuffered HgCl₂method. The conclusion was CO₂emission increased as the soil pH decreased, and the effect of HgCl₂on the soil pH should beconsidered when used the HgCl₂method to sterilize the calcareous soil to distinguish theresource of CO₂emission.（6） To determine the proper pH of buffered HgCl₂solution to sterilize the calcareous soil,we compared the cumulative CO₂amounts released from the sterilized soil with bufferedHgCl₂at3.5,6,7and8. The results showed, all the treatments significantly decreased soil pHexcept the buffered8treatment. After31days incubation, all the treatments decreased thecumulative CO₂amounts except the buffered8treatment. There was a negative relationship between the CO₂emissions and soil pH. We concluded that soil acidification could increasethe CO₂production, and using the HgCl₂solution to sterilize calcareous soil should adjust tothe proper pH.（7） To quantify the relative contribution of organic and inorganic C sources to CO₂emission from the Ap, Bt, and Bk horizons of a calcareous soil, closed-jar incubations werecarried out over32days to estimate CO₂emission from the soil, with HgCl₂sterilization andδ¹³C signature methods to distinguish whether the CO₂was derived from organic or inorganicsources. Cumulative CO₂emissions ranged from145.80to348.12mg kg-1. The δ¹³Csignature of the CO₂confirmed that CO₂was released from both organic and inorganic Cpools. The contribution of organic C to total CO₂emission differed among the three horizons.The organic C pool accounted for61.0%of the total CO₂emission from the Ap samples,57.4-69.6%of the total CO₂emission from the Bt samples, and15.3-53.9%of the total CO₂emission from the Bk samples. Using buffered HgCl₂solution as a sterilizing reagentsignificantly decreased both the total amount of CO₂production and the amount of CO₂derived from organic C in each soil horizon. The effect of HgCl₂on carbonate-derived CO₂emission varied among the three horizons. We conclude that CO₂emissions from calcareoussoils during closed-jar incubation are derived from both organic and inorganic C sources.Sterilization with buffered HgCl₂solution did not completely inhibit biotic activity in the soil.Furthermore, HgCl₂also affected the abiotic CO₂production. In summary, closed-jarincubation method cannot be used to measure the mineralization of soil organic C in thecalcareous soil.（8） To evaluate the proper method of sterilizing the calcareous soil with buffered HgCl₂solution, incubation was conducted to measure the cumulative CO₂amounts from soilssterilized with HgCl₂solution at different pH and determine the origin of CO₂emission. Theresults showed, cumulative CO₂amount from the unbuffered HgCl₂treatments （solid HgCl₂treatment and unbuffered HgCl₂solution） increased significantly compared with the controltreatment, while cumulative CO₂amount from the buffered HgCl₂treatments （buffered withNaOH or PIPES） decreased significantly. The percentage of organic carbon-derived CO₂inthe solid HgCl₂treatment was higher than in the unbuffered HgCl₂treatment. But there wasno difference between the buffered HgCl₂with NaOH and PIPES treatments in the percentageof organic carbon-derived CO₂. We concluded that, compared between the the effectiveness ofthese four sterilization methods, buffered HgCl₂with NaOH solution is best of all.