Total and permanganate-oxidizable organic carbon in the corn rooting zone of US Coastal Plain soils as affected by forage radish cover crops and N fertilizer

Forage radish (Raphanus sativus L. var. longipinnatus) is a relatively new winter cover crop becoming widely grown in humid temperate North America. Little is known about how the use of this fall/winter cover crop may influence carbon sequestration and distribution in the soil profile in corn silage production system. The objectives of this study were to determine quantities and distribution in the soil profile of total organic carbon (TOC) and permanganate oxidizable carbon (POXC) as affected by forage radish cover crops and to examine the relationship between TOC and POXC in the profile. While there was no significant difference in TOC between radish (RAD) and no cover crop (NC) treatments for each depth interval at each site, the TOC in RAD (10.3 g C/kg) was higher compared with NC (9.3 g C/kg) in surface soil depth (0-30 cm) when analyzed across all site years. Forage radish impacts on POXC were observed not only for surface horizons (0-15 cm), but also for deep horizons (90-105 cm). Banded nitrogen fertilizer affected the soil C:N ratio deep in the soil profile at both sites (at 90-105 cm in RAD and at 60-75 cm in NC). Where N fertilizer was applied, soil POXC in 0-30 cm was significantly greater following radish (535.7 mg POXC/kg) than following no cover crop (418.2 mg POXC/kg). Additionally, strong positive linear relationships between POXC and TOC were observed (P < 0.05), with a much steeper regression slope (higher POXC/TOC ratio) in the 60-105 cm layer (POXC/TOC ratio = 0.22) was much steeper than for the surface soil (0-30 cm) with POXC/TOC ratio = 0.05. We speculate that the higher POXC levels may have resulted from increased rooting and exudation by both corn and radish where nitrogen fertilizer was placed. Using forage radish cover crops show potential for mitigating against soil C depletion.