Long-Term Effects of Cover Crops on Soil Organic Matter Composition

Abstract

Cover crops (CCs), as a long-term sustainable agricultural practice, play a key role in enhancing soil organic matter (SOM). Cover crops add SOM directly through the decomposition of aboveground residues and root exudates, and indirectly by stimulating microbial activity. Decomposition of different cover crop residues has distinct functional traits and contributes differently to SOM and water-extractable organic matter (WEOM) production and composition. The objective of this study was to quantify how different cover crop species and soil depth affect the composition of SOM and WEOM at two long-term field sites in the southeastern United States-- the Tennessee Valley Research and Extension Center (TVREC) and the Wiregrass Research and Extension Center (WREC), using spectroscopic techniques. Soil samples were collected in November 2024 at depths of 0-10 and 10-30 cm following 7 years of continuous cover crop management. This study included winter fallow, cereal rye (Secale cereale L.), crimson clover (Trifolium incarnatum L.), and radish (Raphanus sativus L.) as monocultures and a mixture of the three species. WEOM was then extracted from the soil samples to characterize dissolved organic carbon (DOC) concentration, total nitrogen (TN), UV-Vis absorbance, and fluorescence properties. FTIR (Fourier Transform Infrared) spectroscopy was used to characterize the SOM composition of the samples. Results showed no significant differences in SOM functional groups or WEOM composition among cover crop treatments, whereas clear compositional separation was observed between sites and soil depths (p< 0.05). Clover increased TN in the 0-10 cm depth at both sites; responses in the 10-30 cm depth were evident only at TVREC. DOC concentrations were higher at the WREC site under legume-based treatments, but this effect was not observed at the TVREC site. Although cover crop treatments at these sites improved soil health, surface SOC levels, and the soluble nitrogen pool, compositional patterns were primarily governed by differences in climate, soil texture, mineralogy, crop rotation history, and tillage management. These findings emphasize the need for long-term studies to disentangle species-level cover crop effects from dominant site- and management-driven controls on SOM and WEOM dynamics.