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ABSTRACT Dry combustion analysis of carbon (C) and nitrogen (N) in materials of biological or mineral origin is safer and less labor-intensive than traditional acid digestion methods, and can give better recoveries of both elements. Dry combustion macro-analyzers have recently been developed, which allow a sample size of up to several grams to be analyzed for C and N, thereby further increasing the precision and accuracy of these determinations. One of these macro-analyzers, the Leco CN-2000 is a dedicated C and N analyzer, and hence, unlike its companion instrument the Leco CNS-2000, can be finely controlled to maximize the recoveries of organic and inorganic C and N. As yet, though, the combustion temperatures and oxygen flow-rates necessary for optimizing the recoveries of C and N from soil have not been determined for this instrument. Two separate combustion conditions (profiles) were therefore sought for the Leco CN-2000, one of which would minimize the decomposition of carbonate C while maximizing the recovery of organic C (Profile 1), and another one which would maximize the recoveries of total C and total N (Profile 2). Both of these profiles were successfully defined for the instrument. Under lower temperature (1040°C) conditions (Profile 1), the decomposition loss of carbonate from samples of pure CaCO3 was minimized to only one third of the minimum value previously found using the Leco CNS-2000 (1000°C). The measured values for organic C obtained under these conditions were in good agreement with the certified or ‘true’ values for a wide range of reference soils. Nitrogen recovery, however, was sub-optimal under these lower temperature conditions (Profile 1), and indeed for many soils was less than 70% of its potential total. Under the higher temperature (1300°C) conditions (Profile 2), up to 98% of carbonate C was recovered from samples of pure CaCO3, and good agreement was obtained between measured and certified values for total C and total N in a wide range of reference soils. It was concluded that a combustion temperature of 1300°C was essential for making accurate determinations of total C and total N in soil, and that measurements of organic C made using the Leco CN-2000 are, inherently, more accurate and precise than those made using the Leco CNS-2000.
Wright et al. (Mon,) studied this question.