Total soluble nitrogen in forest soils as determined by persulfate oxidation and by high temperature catalytic oxidation

Speedy and reliable measurements of soil soluble nitrogen (N) are critical for estimating N fluxes in forest ecosystems. The high temperature catalytic oxidation (HTCO) method was assessed and compared with persulfate oxidation (PO) to measure total soluble N in water, hot water, 2 M KCl, and 0.5 M K2SO4 extracts of 24 forest soils collected from south-east Queensland. All salt extracts were diluted 5-fold before measurement by the HTCO method to minimise the effects of salt precipitation on the surface of the Pt/Al2O3 catalysts that may impair oxidation efficiency. Drifts of sensitivity of signals in diluted KCl (0.4 M), K2SO4 (0.1 M), and water matrixes by the HTCO method were minor, with <2% in KCl matrix and <3% in K2SO4 and water matrices. Nitrogen recoveries from most standard N-containing compounds (5 mg/L) analysed by the HTCO method in all the matrices tested were >94%. The values of total soluble N in all extracts of soils obtained by both the PO and the HTCO methods were highly correlated. However, the HTCO method generally gave greater values than the PO method, particularly with high concentrations of N. We consider the HTCO method to be a simple, automated, rapid, quantitative, and reliable method for determining total soluble N in both water extracts and diluted salt extracts of forest soils.