INORGANIC NITROGEN DETERMINED BY LABORATORY AND FIELD EXTRACTIONS OF 2 FOREST SOILS

To assess the effect of a delay in soil processing on inorganic N leve ls in N-rich soils, field and laboratory extractions were compared at two forested sites with high N mineralization and nitrification potent ial. At eight sampling dates in 1989 and 1990, five mineral soil cores per site were taken between 0- and 10-cm depth and transported on ice to the laboratory for KCl extraction and NH4-N and NO3-N analysis. At three sampling dates in 1990, soil extractions were performed in the field immediately following sampling, and inorganic N concentrations w ere compared between extractions. Nitrate-N increased four- to sevenfo ld (net release of 2-7 mg NO3-N/kg dry soil) due to the transport and relatively short delay (<24 h) in the processing of the soil samples, either coinciding with increased net N mineralization or due to transf ormation of NH4-N into NO3-N. This study indicates that if possible, s oil samples should be extracted in the field, especially at N-rich sit es. The concerns raised by this study may not necessarily apply to N-p oor soils characterized by slow N transformation rates.