Effects of Crop Rotation and Depth on Biochemical Composition of Soil Organic Matter.

Previous research has shown that extended crop rotations that include a perennial forage crop can increase soil organic C (SOC) content in surface and subsoils. Greater SOC storage in extended crop rotations may be attributed, in part, to greater plant inputs to the subsoil, where SOC concentrations are relatively low. The purpose of this study was to compare the biochemical composition of soil organic matter between extended rotations (corn-corn-oat/alfalfa-alfalfa or corn-soybean-oat/alfalfa-alfalfa) and simple rotations (corn-soybean) at three long-term research comparisons (study durations of 60, 35, and 12 years) in Iowa, USA. Previous research conducted at these sites demonstrated similar average annual C inputs in both rotations, but a greater proportion of C delivered as root inputs in extended rotations. Therefore, we hypothesized that subsoil organic matter in the extended crop rotations would reflect a greater contribution of plant compounds (e.g., lignin and plant-derived carbohydrates). We collected soil samples at four depths and measured total SOC as well as phenol and carbohydrate chemistry. Soil organic C concentrations decreased with depth and were greater in extended rotations than simple rotations at two of the three sites. We found that the proportion of total SOC as phenolic-C and the ratio of plant- to microbial-derived carbohydrates decreased with depth. The rotation effect on proportion of SOC as phenolic-C was inconsistent among sites and there was no rotation effect on the ratio of plant- to microbial-derived carbohydrates. Our results show that depth has a strong and consistent effect on the biochemical nature of soil organic matter while rotation has a weak and inconsistent effect. Biochemical composition may explain crop rotation effects on SOC in some environments.