The University of Queensland
A substantial gap exists between soil based measurements of nitrogen availability, and knowledge of the true accessibility of these forms for plant roots. Soil based estimates of nitrogen availability are typically based on soil extracts, which require soil to be destructively sampled, taken to the laboratory and mixed with a salt or water based solution. It is becoming apparent that this destructive sampling causes artefacts in nitrogen availabilities, damaging roots, hyphae, and altering microbial communities. It also provides only a ‘moment in time’ snapshot of nutrient availability, and does not reflect the dynamic nature of soil nutrient turnover and movement. Microdialysis is an established minimally invasive technique in the field of neuroscience, and has recently been adapted for use in soil. It works by inducing a diffusive flux across a semi-permeable membrane, and is in some respects functionally similar to the action of a plant root. It allows measurement of the nutrient flux (the rate at which nutrients arrive at the membrane surface over time), providing the ability for direct comparison to physiological measures of plant root uptake capacity. We measured fluxes of nitrogen in agricultural fields under a range of managements, and compared these fluxes to root uptake capacity measurements of the present crop species, sugarcane. We found that soil ammonium and nitrate fluxes exceeded root uptake capacity in fertilised fields, in some cases by orders of magnitude. This mis-match in supply and demand has implications for nitrogen use efficiency and nitrogen loss.