Monitoring N Availability from Incorporated Cover Crop Residues Using a Soil Electrical Conductivity (EC) Sensor.

Cover crops can be important sources of N fertility, but research and farm decision-making related to N mineralization from organic sources are limited by the time and cost associated with intensive soil sampling and laboratory analysis. The objective of this study was to evaluate the relationship between sensor-based electrical conductivity (EC) and soil inorganic N following the incorporation of cereal-legume residue mixtures of varying quality. In packed columns of loamy sand field soil, hairy vetch (Vicia villosa Roth) residue was incorporated alone and in combination with two additive rates of cereal straw, in addition to an unamended control. One Decagon GS-3 sensor was installed in each column recording volumetric water content, temperature, and EC at hourly intervals over the course of a 100 d incubation. Net N mineralization/immobilization was monitored through weekly column leaching and pore water sampling. While residue type and loading rate influenced the magnitude of EC readings considerably, temporal changes in EC within treatments were correlated with inorganic N dynamics during the incubation. Inorganic N and EC were positively correlated in both drainage and pore water samples, but the slope of the relationship differed for earlier versus later sampling dates. Sensor-based EC measurements were positively correlated with sampled pore water EC at the time of sampling. Furthermore, trends in data-logged EC readings between leaching events appeared to distinguish periods of net N mineralization (increasing EC) and net N immobilization (increasing then decreasing EC). While challenges remain for using EC as a proxy for inorganic N, opportunities for high temporal resolution monitoring of N availability could support accelerated advances in our understanding of residue mineralization and improved on-farm N management.