Recovery of individual soil nitrogen forms after sieving and extraction

Plant biomass production and species composition is largely regulated by the availability of soil nitrogen (N). Detailed knowledge about the concentrations and composition of soil N pools are crucial for better understanding plant N nutrition. One commonly applied method to characterize soil N pools is the extraction of soil with water or salt solutions. The apparent problem with this sampling technique is the disruption of the soil matrix and the natural equilibrium of soil N pools during sampling and sample handling. Sieving and homogenizing soils as well as the extraction procedure itself are known to alter soil N composition through transformations, losses and contamination. Until now, however, information on the impact of soil extraction on individual N forms is scarce. This study therefore aimed at estimating the effect of sieving and extraction with water or KCl on NH4+, NO3− and individual amino acids. Nine different soils including boreal forest, agricultural and grassland soils were used for a series of experiments. In detail, after initial estimation of inorganic N and amino acid pools in extracts, in two separate experiments a small amount of standard solution containing NH4+, NO3− and amino acids was added either directly to the extractant or to the soils before sieving and extraction and, subsequently, the recovery of standard added was determined. I found that a significant proportion of amino acids were not recovered in any of the treatments and, conversely, there was a significant increase of inorganic N. Sieving and extracting generally led to a lower recovery of amino acids and a stronger increase of inorganic N than extraction only. The recovery of individual N forms strongly depended on soil type, extractant and N form, indicating that a comparison of results from soil extractions between different soils should be done with care. Still, soil extraction can provide valuable information on total plant-available N, as the sum of N added could largely be recovered in all soils and treatments. Future studies investigating the availability of individual N forms in soil for plant uptake should be aware of possible errors introduced during sample handling to avoid misinterpretation.