Unravelling ammonium and nitrate dynamics in soils: effects of soil type, moisture content, and temperature revealed...

Accurate measurement of ammonium (NH₄⁺) and nitrate (NO₃^-) concentrations in soils is essential for understanding factors that determine nitrogen bioavailability and potential losses to the wider environment. Diffusive gradients in thin-films (DGT) can be used to examine NH₄⁺ and NO₃^- dynamics in soils, while overcoming some of the limitations faced by conventionally used extraction methods. This study evaluated the effects of temperature, soil type, and moisture content on NH₄⁺ and NO₃^- dynamics using DGT across two experiments. In the first experiment, NH₄⁺ and NO₃^- were applied to three South Australian soils and incubated at three moisture contents. Single and mixed binding layer DGT probes were deployed for 12, 24, and 48 h to examine short-term changes in N species concentrations. In the second experiment, NH₄⁺ and NO₃^- concentrations and transformation rates were monitored in one soil over 42 days following urea application, at three soil moisture contents and two temperatures (15 and 24 °C). Soil type, moisture content, and DGT deployment time affected measured NH₄⁺ and NO₃^- concentrations in the first experiment. However, the mixed binding layer DGT underestimated concentrations of both species by up to 66%. In the second experiment, DGT-measured NH₄-N increased to up to 3044 ± 379 µg L^-1 within 3 days under the warmest, wettest treatment, with transformation rates of both N species further influenced by temperature, moisture content, and time after urea application. These dynamics were clearly resolved by DGT, thus demonstrating the technique's potential to provide valuable new insight into soil nitrogen cycling.