Field-based soil extractions capture more amino acids that are lost during short-term storage

Aqueous soil extraction is a commonly used method to extract nitrogen (N) from soil. However, the disturbance of collection, transportation, and storage before extraction can potentially lead to mineralisation of extractable organic N pools, and as such may bias our interpretations of plant-available N towards inorganic N. Although disturbance through soil collection cannot be avoided, we evaluated the impact of short-term soil storage on water-extractable N pools, by extracting soils samples immediately after removal in the field, and again after overnight storage and extraction in the laboratory 24 h later. We chose five boreal forest soil sites within the Svartberget Research Area (northern Sweden). Soils were sampled across three seasonal time-points from June to September. We found that when measurements across all sites and time points were pooled, field-based extractions had significantly greater amino acid concentrations than lab-based extractions, contributing to greater soluble N concentrations (field extractions: 0.77 ± 0.07 µmol N/g soil DW; lab extractions: 0.17 ± 0.03 µmol N/g soil DW). Seasonal and site variation of amino acid concentrations was also much larger when soils were extracted in the field. Within sites, ammonium was often slightly elevated in lab-based extractions, but not to the same magnitude as reductions in amino acid concentrations, which we interpret as an overall N immobilisation effect during storage, likely through a combined effect of microbial utilisation of amino acids, and adsorption to the soil mineral phase. We found that negatively-charged and polar amino acid concentrations were most affected by storage – but the magnitude of loss of most amino acids was generally similar. Hydrolytic enzyme activity was correlated with total protein concentrations across all sites, this association was strongest in June, but was correlated equally with both lab and field extractions. In contrast, enzyme activity was not well associated with amino acids, regardless of extraction type, indicating that hydrolytic enzyme activity does not fully explain our observations of amino acids concentrations. We conclude that field extractions are a cheap and efficient way to capture higher resolution within organic N profiles of boreal soils during sampling, unmasking information that might be lost during storage.