How Stable Are Soils for the Determination of Available N?

Abstract

When potassium chloride (KCl) is used to extract “available” mineral nitrogen (N) species from field-moist soils, samples are usually transferred to the laboratory in a cool box prior to extraction and stored in a fridge until being prepared for extraction. This study shows that significant changes, especially in nitrate concentration, occur under refrigerated conditions for 16 h, especially in grassland subsoils, and these are reflected in total mineral N levels. Surprisingly, storage at room temperature for the same time caused no significant additional net nitrate production. Extractable ammonium concentrations often fell significantly during storage, especially in woodland soil samples, probably via microbial immobilization as well as nitrification. In the present study, 12 pairs of students were used to perform all extractions within 30 min of sampling in the field, which would not be generally practical. It is suggested that volumetric subsampling in the field and immediate addition to a known volume of KCl solution may be more reliable than methodology currently employed by many soil scientists.

Keywords:

• Ammonium
• available N
• environment
• mineralization
• nitrate
• sample stability

Acknowledgments

The authors are indebted to the Higher Education Commission, Pakistan, for financial support, to Rebecca Sutton for help in some analyses, and to Clair Suddaby for permission to sample at the Hob Moor Nature Reserve.