Field evaluation of Gee Passive Capillary Lysimeters for monitoring drainage in non-gravelly and gravelly alluvial soils: A useful tool to estimate nitrogen leaching from agriculture

The development of accurate methodologies for monitoring drainage and evaluating nitrogen leaching from agricultural land is an absolute necessity, particularly considering the growing problem of nitrogen pollution of groundwater throughout the world. In this context, the Gee Passive Capillary Lysimeter appears to be an innovative tool that allows direct and continuous measurement of drainage and enables drainage water to be sampled for chemical analysis. The main objective of this study was to evaluate how the Gee Passive Capillary Lysimeter works in alluvial soils. The study was conducted at two agricultural field stations: Site 1 (central Spain), with a non-gravelly soil, and Site 2 (north of Spain), with a gravelly soil. An installation procedure that leaves part of the soil profile undisturbed was selected for the soil without gravel, whereas a procedure that may alter the soil physical properties was used for the gravelly soil. The experiment was carried out over two consecutive crop cycles at both field stations. Soil water balances were obtained through two different methods: a direct method based on direct measurements of drainage using the Gee Passive Capillary Lysimeter, and an indirect method based on the calculation of daily crop evapotranspiration. A statistical comparison of results obtained by the two methods showed no significant differences in estimates of drainage or crop evapotranspiration from both the non-gravelly and the gravelly soil. The efficiency of leachate collection with the Gee Passive Capillary Lysimeter in the non-gravelly soil was 101 ± 1% (mean ± standard deviation), while in the gravelly soil, it was 142 ± 52%. Drainage and nitrogen leaching below the root zone were determined to be primarily triggered by excessive irrigation. This study helps to validate the use of the Gee Passive Capillary Lysimeter in gravelly and non-gravelly alluvial soils under irrigated agriculture.