Residual Soil Nitrate: A Comparison between Air-Dried and Field-Moist Soil Samples.

In the framework of the European nitrate directive (91/676/EEG), losses of nitrate (NO3)– nitrogen (N) to both surface and groundwater are limited to 50 mg/l. Because the residual NO3-N in the soil profile after harvest is considered the main determinant of nitrate leaching during wintertime, the Flemish government imposed a limit value of 90 kg NO3-N ha−1 up to a soil depth of 90 cm between 1 October and 15 November. This study compared two different soil sample preparation methodologies. When samples were analyzed immediately upon arrival, no differences in NO3-N concentration were observed. However, although field-moist samples are maintained at 4 ◦C, nitrification is not completely stopped, as indicated by the increased NO3-N concentration in field-moist samples 10 days after storage at 4 ◦C . In contrast, nitrification in air– dried samples is stopped during the oven drying when 40 ◦C is reached. Moreover, the reproducibility was significantly greater in air-dried samples as compared to field-moist samples.