Automated elemental analysis: A rapid and reliable but expensive measurement of total carbon and nitrogen in plant and soil samples

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Abstract
The performance of a commercial automated CHN elemental analyzer was evaluated by comparison with classical wet methods and with another commercial analyzer. With proper standardization, calibration, and sample preparation, the Perkin‐Elmer 2400 CHN elemental analyzer was shown to give reliable carbon (C) and nitrogen (N) analyses of plant and soil materials. Precision was demonstrated by the consistent reference rice straw C and N results obtained (1.6 to 2.8% CV for N, and 0.3 to 0.7% CV for C) when 11 samples were analyzed consecutively within a day or on other days. A simple linear regression analysis showed generally higher plant N values measured by the CHN analyzer than the Kjeldahl method. Predicted analyzer plant N values were only slightly lower than Kjeldahl N, with plant materials containing less than 1% N. Recovery of different amounts of nitrate‐N (NO3‐N) added to rice straw samples was better with the CHN analyzer than with both the common and the salicylic acid‐modified Kjeldahl method. A very good 1:1 relationship between analyzer soil N values and the permanganate‐reduced iron modified Kjeldahl N values was also shown at the range measured (0.005–0.200% N). However, the soil C values determined by the analyzer were generally lower than the Walkley‐Black C values. Based on precision, analyzer soil C results with 0.4 to 5% CV appear to be more reliable than the Walkley‐Black C results with 0.3 to 18% CV. In spite of its reliability, speed of analysis, and low manpower requirement, studies showed the high cost of analyzing samples (minimum of US$2.38 per plant and US$3.83 per soil sample) with the CHN analyzer and of maintaining such a sensitive equipment.