Loss of low molecular weight dissolved organic carbon (DOC) and nitrogen (DON) in H₂O and 0.5M K₂SO₄ soil extracts

Soil extracts are routinely used to quantify dissolved organic nutrient concentrations in soil. Here we studied the loss and transformation of low molecular weight (LMW) components of DOC (¹⁴C-glucose, 1 and 100μM) and DON (¹⁴C-amino acid mixture, 1 and 100μM) during extraction of soil (0-6h) with either distilled water or 0.5M K₂SO₄. The extractions were performed at 20°C, at 4°C, or in the presence of an inhibitor of microbial activity (HgCl₂ and Na-azide). We showed that both glucose and amino acids became progressively lost from solution with increasing shaking time. The greatest loss was observed in H₂O extracts at 1μM for both substances (>90% loss after 15min). Lower temperature (4°C) and presence of K₂SO₄ both resulted in reduced loss rates. The presence of microbial inhibitors effectively eliminated the loss of glucose and amino acids. We conclude that microbial transformation of LMW-DOC and DON during H₂O or K₂SO₄ extraction of soil may affect the estimation of their concentrations in soil. This finding has significant implications for methods that rely on chemical extractions to estimate LMW-C components of DOC and DON.