Deep‐freezing pretreatment and time of extraction of soil samples for inorganic nitrogen determination

Abstract

Abstract Soil samples for inorganic nitrogen (N) determination are usually deep‐frozen to prevent microbial transformations of N between sampling and analysis. For analysis, frozen soils are thawed, which may also lead to transformations of N. A specially manufactured mill for grinding frozen soil was tested to minimize these transformations. Whether the time of extraction of the samples could be extended to 20 hr to better accomondate routine work and to make the clay aggregates to disperse better during extraction was also investigated. Freezing of the samples did not produce different results to fresh soils from ammonium nitrogen (NH4 +‐N) or nitrate nitrogen (NO3 ‐‐N) determination. Thawing of the samples increased the concentration of NO3 ‐‐N in the extracts and grinding increased that of NH4 +‐N. When either thawing or grinding was applied, the total inorganic nitrogen concentration was about the same. Thawing of the ground samples increased concentrations of NO3’‐N and NH4 +‐N in the extracts. Extending the time of extraction from 0.5 or 1 hr to 20 hr increased the concentration of NH4 +‐N in the extracts, while NO3 ‐‐N content was also increased slightly. It was concluded that sample pretreatment may cause serious errors in the determination of inorganic N even by methods which have proven most successful to prevent microbial transformations of nitrogen, unless the soils are extracted immediately after sampling. The period of extraction should not exceed two hours.