Cropping System Effects on Soil Organic C Storage in Surface and Subsoils.

A large proportion of soil organic C (SOC) is in subsoil, yet few studies have evaluated cropping system effects on SOC below the plow layer. We hypothesized that cropping systems with greater belowground C inputs would increase SOC stocks by delivering C to the relatively C-depleted subsoil. Using three long-term field trials in Iowa (study durations of 60, 35, and 12 years), we examined the effects of cropping system (corn-soybean-oat/alfalfa-alfalfa or corn-corn-oat/alfalfa-alfalfa vs. corn-soybean rotation) on SOC and N content at different depths throughout the soil profile. Average annual C inputs were similar for both cropping systems, but the proportion of C delivered belowground was approximately twice as great in the extended rotations. Despite the differences in belowground C inputs, cropping system effects on SOC stocks (0-100 cm) were inconsistent among sites. However, soil organic C accumulation was observed at depth (15-100 cm) but not at the surface (0-15 cm) across all sites and rotations. Our research highlights the importance of the subsoil as a C sink in agricultural systems.