Fate of the metal-binding soluble organic matter throughout a soil profile

Crop residues are a major source of soluble organic matter (SOM) in agricultural soils. The fate of this complex mixture of organic compounds is mainly controlled by the biodegradation by soil microbes, sorption to the different soil surfaces, and transfer through the soil profile along with the water flow. During this transfer, the soluble organic matter can bind micropollutants such as metals and co-transport them down the soil profile to the groundwater. However, monitoring usually consists in measuring the organic content of the aqueous sample (dissolved or water-soluble organic matter), rarely its reactivity. The objective of this study was to monitor both the SOM content and its reactivity toward metal cations during its transport through the soil. Therefore, samples were collected from a field experiment performed on a 2-m-deep vadose zone over a 7-month period. The metal-binding capacity of 84 of these samples was quantified for Cu, Fe, and Mn, using the quenching of SOM fluorescence that is observed when increasing concentrations of metal cations are added to the SOM sample. A simple Langmuir-type model was used to quantify the three binding constants for each SOM sample and the amount of metal that it can bind. This method gives binding constants for SOM with each metal cation that respect the general observations made for natural organic matter (log K = 5.31, 4.79, and 4.77 for Cu, Fe and Mn, respectively, for a surface soil sample). Although the SOM content decreased from 10 to 2 mg L over the 2-m-deep vadose zone, the profile of the overall complexation constant was homogeneous (log K = 5.36 ± 0.05 for Cu), indicating that only a small portion of the SOM was responsible for metal binding. The influence of environmental parameters such as residue incorporation, soil temperature, and rainfall events on the amount of metals that SOM can bind was assessed. Strong rainfall events were able to transport a limited amount of metal-binding SOM (up to 1 μM Cu per sample), even in the case where very low organic matter content was found. The freeze-thaw cycle liberated much more metal-binding SOM (4.8 μM Cu per sample) in the aqueous phase at the soil surface. This functional monitoring appears useful to quantify the potential environmental effects of the diverse, complex, and evolving SOM.