Comparison of classic with novel in situ extraction of soil amino acids from grassland soil

Characterization of organic and inorganic soil nitrogen availability is important for determining ecosystem response to global change, as nitrogen limitation is often a major constraint on ecosystem productivity. Classic methods of soil nitrogen extraction involve field collection of soil samples and disturbance of soil aggregates during processing. A novel method of soil amino acid extraction is described that allows the collection of semi-sterile soil water extracts in situ with minimal disturbance to soils. Comparison of samples collected using this novel method to samples collected in parallel using classic methods developed by Brookes et al. 1985 and Kielland 1994 revealed different detectable amino acid N pools relative to ammonium. Glutamate and arginine comprised the highest amino acid N pools from extracts collected from a semiarid grassland site using this new method of extraction. In contrast, samples collected and extracted using the classic method contained higher relative levels of serine, glycine and glutamate. The amounts of dominant amino acids relative to ammonium were significantly greater using the classic method compared to the new method. These observed higher ratios of amino acids to ammonium are likely the result of additional amino acid inputs by lysis of microorganisms which are not removed when filtering in the classic method. Disturbance associated with classic methods of soil N determination may have led to alterations in the quantity and distribution of ammonium and amino acids in extracts. Minimizing disturbance of soil aggregates when sampling nitrogen pools and selection of an appropriate filter for collecting free amino acids may be important for accurately determining nitrogen availability to plant roots and soil microbes.