Why are new technologies needed for soil nutrient analysis?
Estimates of soil nitrogen can be obtained by a number of ways. Typically, soil samples can be combusted to obtain total N (%N), or can be extracted with a salt solution to get estimates of ammonium and nitrate concentrations. While informative, these methods can only give a rough estimation of what might be available to the plant. In reality, much of this nitrogen is bound to soil surfaces, and is not actually plant available. Total %N measurements also include N which is in complex humic compounds, most of which are not available to plants. This means that standard tests often do not accurately reflect what N is actually available to plants. Determining which N sources plants actually access is difficult: however this information could assist fertiliser management. New technologies are required to better estimate availability of N in soil to plants.
What is microdialysis?
Microdialysis is a technology that works by running a small volume of pure flowing water behind a semi-permeable membrane on a small probe. This flow pulls nutrients from the surrounding environment across the membrane by passive diffusion, and then these are taken to a fraction collector and stored for analysis. This technology has been extensively used in neuroscience to sample nutrient concentrations in brain tissue of living mice, but has only recently been applied in soil. There are three key advantages to the technology: it is 1) non –invasive, and allows sampling without causing major disturbance 2) it is very similar in mechanism to real plant roots, and will access similar pools, and 3) it can be used to capture nutrients over time (like a plant root).
What does microdialysis show in sugarcane soils?
1. Microdialysis shows different results compared to soil extracts.
Compared to soil extracts, microdialysis shows a much greater proportion of N arriving at a plant root is in organic N forms such as amino acids than would be expected from soil extracts. Similarly, nitrate is a higher proportion of the nitrogen fluxes arriving at roots than shown in extracts. Ammonium is less mobile, and is a smaller proportion of the N arriving at the microdialysis probe over time.
2. Levels of ammonium and nitrate far exceed sugarcane root uptake capacity in the weeks after fertiliser application.
Management practices that better match soil N fluxes with plant uptake capacity will improve nitrogen efficiency: these may include nitrification inhibitors, split application of fertiliser, slow release fertilisers and legume intercropping. Future studies will examine the rates at which these release N relative to crop uptake demand.