Effects of digestate application, winter crop species and development on dissolved organic matter composition along the soil profile

• Chemical composition of DOM after slurry or digestate application under winter crop. • Crop development influenced topsoil DOC quantity and DOM composition. • Source of topsoil DOM under mustard: persistent molecules from digestate. • Sources of topsoil DOM under wheat: molecules from digestate and root exudation. • Decrease in plant-derived and increase in microbial-derived molecules with depth. Applying organic waste products (OWPs) and sowing cover crops are agronomic practices to improve soil health. OWPs can be used in anaerobic digestion. Because microorganisms consume some of the labile molecules, persistent molecules accumulate in digestate. Few studies have investigated the transfer of dissolved organic matter (DOM) in soil that received digestate. Previously, effects of digestate application on dissolved organic carbon (DOC) were compared to those of the original pig slurry under wheat and a mustard catch crop for nine years at a lysimeter experimental site. DOC concentrations after digestate application were higher in the topsoil every year, due to crop development, but did not differ between treatments in the subsoil. The objectives of this study were to determine whether the observed differences in DOC concentrations caused DOM composition to differ, to identify sources (e.g., digestate, root exudation) that may have contributed to the DOM pool and to assess the DOM composition in the subsoil. The DOM composition of lysimeter samples and water extracts from the OWPs applied were analyzed by thermochemolysis coupled with gas chromatography and mass spectrometry, which identified plant- and microbial-derived biomarkers. Under mustard, the DOM pool seemed to contain mainly persistent molecules from digestate that were desorbed due to the increase in pH caused by nitrate uptake. Under wheat, the DOC pool seemed supplied by both digestate and root exudation. After applying digestate, plant-derived molecules decreased, while microbial-derived molecules increased, as depth increased, and molecules may have been sorbed from the topsoil to subsoil.