Determination of nitrogen release from slow and controlled-release fertilizers by Dumas process using TOC/TN analyzer

Abstract: The Kjeldahl distillation-titration method (KDTM) is a standard method used by most laboratories to measure total nitrogen (N) in the Kjeldahl digests of fertilizer samples. However, the KDTM procedure generally is labor intensive and time-consuming for sample digestion process, in which concentrated acids and poisonous gas have a negative impact on the environment and health of laboratory operators. The purpose of this article was to investigate whether the methods of Dumas process (total organic carbon/total nitrogen analyzer, TOC/TN analyzer) can be used to determine N contents in the release solutions from slow/controlled release nitrogen fertilizers (S/CRNFs). Two kinds of S/CRFs with 43.2% and 37.3% of total N content were selected. The solutions containing 10% of S/CRNFs were collected after incubating at 25℃ for 1, 3, 7, 14 and 28 days, respectively. In the TOC/TN analyzer, the solution samples diluted 400 times were oxidized in the combustion tube in the presence of oxygen at high temperature (800℃) using Pt as catalyst. We compared the N values measured by Dumas method and KDTM procedure. The result showed that a linear relationship can best describe the measured N concentration generated by both methods: y (Dumas-N)=0.905 x(Kjeldahl-N)+0.040. The Pearson's correlation coefficient was 0.991 with a significance level (n=30, P<0.01). Paired samples t-test indicated that there was no significant (P > 0.05) difference of the N release contents of the S/CRFs measured by TOC/TN analyzer and KDTM method. Cumulative N release rates of two fertilizers after incubating for 14 and 28 days determined by TOC/TN analyzer were higher than that by KDTM method. However, a positive linear correlation with r=0.997 (n=30, P<0.01) was verified between the two methods. The regression equation was y (TOC/TN analyzer)=0.908x (KDTM method) +0.818. Thus the results suggested that measured N release of S/CRNFs by TOC/TN analyzer agreed very well with that measured by KDTM method. The N release period is an important parameter for assessing the quality of the S/CRNFs. The relative percentage difference of the N release periods of the S/CRNFs was less than 20% between the two N determination methods, which was the acceptance criteria for evaluating the N release period with the national standards "GB/T 23348 -2009 slow-release fertilizer" and the chemical industry standard "HG/T 4215-2011 controlled-release fertilizers". It is suggested that the TOC/TN analyzer according to Dumas principle can be an alternative to the labor-intensive and time-consuming KDTM procedure for measuring N release of S/CRNFs. The TOC/TN analyzer is easy to handle and no preparation of sample digestion for N analysis is necessary.