Microdialysis fluxes of inorganic nitrogen differ from extractable nitrogen by minimising disturbance of mineral-associated sources

Abstract

Measuring soil nitrogen (N) provides important information for ecosystem productivity and improving N use efficiency in agricultural systems. Conventional means of sampling N using soil extractions disturb soil structure and function, and likely distort accurate quantification. In situ microdialysis is a novel sampling method that generates differing N profiles compared to soil extractions. Here we test the hypothesis that differences observed between sampling methods are due to the minimal disturbance and sampling of a mobile N fraction when using microdialysis, with discernible patterns expected across soils with distinct clay and organic matter contents. In a short-term laboratory microcosm experiment with 21 sugarcane cropping soils, we compared salt (potassium chloride; KCl) or aqueous (H2O) extractants and microdialysis. KCl-extractable ammonium (NH4+) was highly correlated with the content of clay, total N and carbon, indicative of bound N being solubilised. In contrast, NH4+ contributed significantly less to microdialysis fluxes and was not correlated with the measured soil properties, which we attribute to minimal disturbance of bound N·H2O extracts sampled proportionally more NH4+ than microdialysis but were significantly correlated with fluxes. This suggests that while microdialysis and H2O extraction sample from a dissolved N pool, H2O extracts sample from an additional pool of loosely-bound NH4+. Nitrate (NO3−) measures were correlated between methods, but shared no relationship with the measured soil properties, indicating that NO3− sampling is less affected by the disturbance introduced by extractions. We conclude that sampling inorganic N is biased by the degree to which soil sampling methods disturb adsorbed N sources with implications for interpreting soil N measurements.