Organic carbon, total carbon, and total nitrogen determinations in soils of variable calcium carbonate contents using a Leco CN-2000 dry combustion analyzer

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 degreesC) 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 degreesC). 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 degreesC) 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 degreesC 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.