Abstract. In this paper, we present a novel approach, enabling the measurement of nitrate concentrations in natural soil porewater containing natural soil dissolved organic carbon (DOC). The method is based on UV absorbance spectroscopy, combined with fluorescence spectroscopy, for simultaneous analysis of DOC and nitrate concentrations. The analytical procedure involves deduction of the absorption caused by the DOC from the total absorbance in the UV range that is attributed to both DOC and nitrate in the water solution. The analytical concept has been successfully tested in soil water samples obtained from five agricultural sites, as well as in water samples obtained from a commercial hummus soil mixture. We believe that the new analytical concept can provide a scientific foundation for developing a sensor for real-time nitrate concentration measurements in agricultural soils. As such, it can play a significant role in reducing nitrate pollution in water resources, optimizing input application in agriculture, and decreasing food production costs.

Figure S1-The Vadose Zone Monitoring System as it is carried prior to its installation (top), and a general layout of the VMS systems when placed in the soil (bottom).

The open crop field and citrus orchard sites are located next to the village of Nir Galim in Israel (34°41`13`` E long; 31°49`42`` N Lat). Over the past 10 years, the crop field has been used for rainfed winter cereal and summer watermelon cultivation, fertilized mainly with dairy slurry manure. In 2015, the field was converted to irrigated jojoba cultivation. The citrus orchard is irrigated during the summer, in addition to the rainfall during the winter. Further information on these sites may be found at Turkeltaub et al., (2014). The additional study sites were two greenhouses used for rotating vegetable crops. One site practiced an organic regime that is based on the application of organic compost as the main fertilizer, while the other applied conventional fertigation methods. A detailed description of the VMS at each site was previously presented in Dahan et al., (2014);Turkeltaub et al., (2014 Turkeltaub et al., ( , 2015aTurkeltaub et al., ( , 2016. Additional samples were taken from an open field located next to Kibbutz Afek (32°50'14.7"N 35°07'28.6"E).