Highly resolved thermal analysis as a tool for simultaneous quantification of total carbon, organic carbon, inorganic carbon and soil organic carbon fractions in landscapes

Abstract

To quantify and evermore qualify carbon in soils have become highly timely task in landscape ecology. However, separating soil organic carbon (SOC) into different ecologically relevant fractions is difficult. Particularly for landscape approaches needing high numbers of samples, it is necessary to find a simple, economic and suitable method that can quantify total carbon, organic carbon and inorganic carbon as well as separate SOC fractions, preferably with a single method and a single run for each sample. A thermal gradient method was modified and tested for this purpose. The objective of the present work were to: (i) To verify the cost-effective method developed by Chichester and Chaison (1992) with a larger set of samples representing a much wider range of soil properties (carbonate and noncarbonated soils) and with a new generation of the analyzer. (ii) To test the ability to quantify dolomite-derived IC by ThG in dolomite soil samples. (iii) Assessing the suitability of ThG for the quantification of ecological meaningful differences in organic carbon fractions of soil and material samples with different carbon stability. Topsoil samples were collected from cropland, forest, grassland and wetland in temperate, subtropical and tropical regions in order to represent a wide range of soil properties. Standard materials with different thermal stabilities were used for testing as well. The main findings were: (i) All methods (ThG, Calcimeter and acidification methods) were suitable to separate soil carbon into SIC and SOC. However, Calcimeter and acidification methods resulted in lower estimates of SOC contents due to the use of acid. When comparing soil samples with differing carbonate concentrations, the use of the ThG method was more reliable. Moreover, ThG was as suitable as standard methods (CN analyser or dry combustion) to measure total carbon (TC). (ii) Compared to Calcimeter (CALC) and Loss on ignition (LOI) in quantifying OC and IC in dolomite soil samples, the ThG method was the most accurate method in the reference sample set, especially when dolomite contents were high. On the soil sample set, ThG and CALC performed equally, but only when two outliers were eliminated. LOI was not satisfactory in any case on the sample set. Overall, ThG was the most reliable method for measuring IC and OC in dolomite-containing samples over the wide range of concentrations, but the more widespread CALC method is also acceptable. (iii) By using ThG for isolation SOC fractions, four main peaks of organic carbon fractions were recorded at the temperature ranges: 140 - 300°C, 300 - 400°C, 400 - 450°C and 450 - 600°C. By testing with different materials with different thermal stabilities (e.g. grass char, wood char and wheat starch), results showed that the thermal fractions are ecologically relevant. Thermal fractions are various with different materials and relevant to different turnover rate of SOM. Therefore, thermal gradient analysis can be effectively used in routine measurement to determine organic, inorganic and total carbon as well as the stability of organic matter in a single analytical run. Care has to be taken when samples with high C-concentrations are analysed. In any case, first examples like comparing different land use within a region according to their soil organic carbon fraction revealed that much more information with a single analysis (i.e. TC, SOC, SIC, thermal fractions of SOM) can be achieved from the presented thermal gradient method in comparison to regular bulk C measurement (providing only TC value). Overall, ThG can be used to quantify TC, IC and OC as a simple, precise and economic method without complicated pretreatment. Moreover, it can effectively measure the organic, inorganic and total carbon as well as the stability of organic matter in a single analytical run. It can be applied to landscape scale, particularly for larger scale analyses on land use systems and land use change.