Microdialysis – a new technology for investigating soil nitrogen fluxes in the rhizosphere

Extracts of soil are used to provide estimates of plant-available nitrogen sources such as nitrate, ammonium and amino acids (low molecular weight nitrogen, LMW-N). Soil extracts are a blunt tool; they introduce a number of inaccuracies through soil disturbance, and do not indicate the rapidity of N pool turnover. Microdialysis is used predominantly in neuroscience but was recently introduced in soil research. Small in situ probes cause minimal disturbance, and passive diffusion of solutes across a semi-permeable membrane allows dialysate to be collected over time allowing study of nutrient flux dynamics. This sampling mode is functionally similar to plant roots, and may provide a good estimate of the N forms available to roots. We used microdialysis to quantify induced diffusive fluxes of LMW-N in a subtropical agricultural soil under three fertiliser regimes. Shifts in LMW-N fluxes were detected over time, suggesting the formation of depletion zones around the probe surface similar to those associated with roots. A pronounced difference was observed between results from microdialysis and soil extracts. In dialysate, amino acids contributed up to 70% of LMWN in unfertilised soil and 5-20% in fertilised soils. In contrast, amino acids were a minor constituent in soil extracts, highlighting that soil extracts underestimate amino acid availability in soils. Modelling plant N uptake based on soil N fluxes and root uptake kinetics, we show that use of inorganic N in fertilised soil was constrained by the root’s uptake. In contrast, fluxes of amino acids and the root’s uptake capacity were closely matched.