Development and Application of the Diffusive Gradients in Thin Films Technique for the Measurement of Nitrate in Soils.

Nitrate (NO3-N), the main plant/microbial nitrogen source, has a fast turnover in soil driven by species transformation (nitrification/denitrification) and phyto/microbiota assimilation. The technique of diffusive gradients in thin films (DGT) is capable of a robust, low disturbance measurement of NO3-N but has not been implemented due to the absence of a binding layer suitable for deployment in soils. In this study, a new styrene divinylbenzene-based absorbent with amine functional groups (SIR-100-HP) was cast into an agarose gel support. The NO3-N ion selectivity of the SIR-100-HP/agarose binding layer was retained in the presence of high multivalent ion concentrations and was used successfully to acquire in situ NO3-N measurements in bulk soil. The kinetics of binding and the maximum binding capacity were determined. The total capacity of the DGT containing the SIR-100-HP/agarose binding phase was 667 µg of NO3-N. The performance of DGT was not affected by varying pH (3-8) or ionic strength (0-0.018 mol L-1), while anion competition effects at concentrations reflecting those in common agricultural soils were found to be negligible. Complete elution (100% efficiency) of NO3-N from the binding phase was achieved using a solution of 5% NaCl. This technique was validated in three contrasting soils. CDGT measurements were in excellent agreement with pore water NO3-N values. Two-dimensional NO3-N mapping of a profile of flooded rice paddy soil demonstrated the potential of this novel methodology for improved characterization of in situ N speciation for further understanding of bioavailability and biogeochemical processes of NO3-N in soils.