Plant responses to nitrate and ammonium availability in Australian soils as measured by diffusive gradients in thin-films (DGT) and KCl extraction

Determining soil nitrogen (N) availability is essential in agriculture to minimise over-application, maximise growers’ returns and reduce potential environmental consequences. The present study assesses soil mineral N (nitrate-N and ammonium-N) using the diffusive gradients in thin-films (DGT) technique against the conventional potassium chloride (KCl) extraction. The DGT technique has demonstrated reliable predictability for plant-available P, Cu and Zn. However, the use of DGT to quantify soil N bioavailability is underreported and N measurements made with DGT have not been compared to plant growth responses or N uptake. A pot trial using wheat was performed to determine the suitability of the DGT technique to predict N plant uptake and plant biomass. Four contrasting soil types from South Australia were used, and four rates of N were applied to the soil. DGT devices and KCl extraction were used at sowing to measure soil mineral N. These data were then compared with plant relative yield (Y_R) and N uptake after harvesting the plants. Soil mineral N, as measured by both the DGT and KCl extraction techniques, demonstrated a significant positive correlation with Y_R, with an R² value of 0.6; however, DGT-N extracted comparatively more nitrate (NO₃^–, >87 % of C_N) than KCl-N (65 % of E_N). Mineral N and NO₃^– extracted by both DGT and KCl significantly correlated with plant N uptake albeit this correlation was stronger for KCl (R² = 0.8) than DGT (R² = 0.6). The same parameters also positively and significantly correlated with Y_R, however in this case, both correlations were similar and only modest (R² < 0.6 in all cases). These results are explained in terms of the differences between the pools of N accessed by these techniques and limitations related to soil N dynamics. In conclusion, KCl showed similar or better predictive ability for N uptake and yield response (Y_R) in wheat compared to DGT across four Australian soils. Given its low cost and ease of application, KCl presents a competitive advantage in this study.