Fluorescent Properties of Water-Extractable Organic Matter in Low-Gradient, Clay Plain Soils Illustrate Efficacy and Scale of Agricultural Management

Soil organic matter (SOM) serves as the critical component of soil fertility, ecosystem health, and is a key store of carbon, - buffering climate change forcings. Ultraviolet-Visible (UV-Vis) and fluorescence spectroscopy offer cost-effective approaches for distinguishing likely organic matter sources and comparatively assessing the impact of different land management practices on SOM. Here we used optical spectroscopy across a gradient of agricultural practices and restoration to evaluate the relationship between land management, soil health, and properties of water-extractable organic matter (WEOM). Focusing on low gradient, Brookston Clay plain soils of the Essex region in Southwestern Ontario, Canada, the quantity of SOC, and aromatic content of WEOM was found to be correlated to land management (agricultural or forested) and depth (0-60cm). Agricultural sites were converted to agriculture post-colonization, and reforestation occurred in forested sites approximately 50 years before the study began. SOC levels in reforested sites were nearly twice as high as all agricultural sites. Significantly higher levels of absorbance at 254nm (Abs 254 ) were found when in forested sites, however, other differences in SOC composition were not found. Yet, neither SOC content nor WEOM composition differed among agricultural practices. Differences between land management types were limited to shallow samples (0-15cm) only. At all sites, the clay-plain soils displayed strong vertical zonation of SOC, poorly drained shallow A horizons with accumulated organic matter and deeper impermeable clay mineral horizons with lower concentrations of WEOM with more protein-like composition. These results suggest that reforestation of post-agricultural clay-plain soils drives increased soil sequestration, but restoration of desired forest SOC composition may be mechanistically more complex and require multi-decadal time scales. Alternate tillage practices had a limited impact on SOC quantity and WEOM composition, indicating that SOC accumulation under these conservation agriculture practices may be slow. Optical spectroscopy of WEOM provides a rapid, cost-effective method to measure soil health beyond SOC accumulation.