Estimation of nitrate leaching using suction samplers with time domain reflectometry: A study of nitrate leaching under corn following alfalfa

A common method for determining nitrate leaching under crops is by using soil solution samples collected by suction samplers. The use of suction samplers either on a continuous or on a discrete time basis works under the general principle of either common averaging or linear interpolation, both of which tend to suppress peak nitrate concentrations. Peak events are important in the estimation of nitrate leaching and inability to detect them can result in over or underestimation of cumulative nitrate leaching. The primary objective of this research was to design a system for continuous monitoring of nitrate concentration by using suction samplers with time domain reflectometry, TDR, and to determine (1) whether the two devices can be used together and, (2) if using suction samplers and TDR together has a benefit over using suction samplers alone. The underlying principle was to use suction samplers data to calibrate TDR and to estimate nitrate concentration from bulk soil electrical conductivities, EC_b, measured by TDR. The method was also used to convert discrete suction sampler data to a continuous form. The study was conducted partly in the lab, both to test existing conceptual models and to develop new ones in order to relate EC_b to soil solution electrical conductivity, EC_w. To validate the lab work, field studies were conducted to estimate nitrate leaching under first year corn following alfalfa. Results show that (1) conceptual models developed under laboratory environment using soil column experiments can estimate soil solution EC fairly accurately in the field and (2) TDR can be used with suction samplers to obtain a continuous data of nitrate concentration. The use of TDR with suction samplers showed a promising potential in detecting peak events of nitrate leaching. However, because of lack of sufficient data due to instrument malfunction, no conclusive results could be made in this study regarding the benefit of using TDR with suction samplers as opposed to using suction samplers alone.