Carbon Sources and Dynamics in Afforested and Cultivated Corn Belt Soils

Afforestation of degraded cropland can sequester atmospheric C; however, source partitioning and turnover of soil organic C (SOC) in such ecosystems are not well documented. This study assessed SOC dynamics in two 35‐yr‐old, coniferous afforestation sites (i.e., a forest plantation situated in northwestern Iowa on a silty clay loam soil and a shelterbelt situated in eastern Nebraska on a silt loam soil) and the adjacent agricultural fields. Soil samples were collected at both sites to determine SOC and total N concentrations and stable C isotope ratios (δ 13 C, natural abundance) in both whole soil and the fine particulate organic matter (POM) fraction (53–500 μm size). In these fine‐textured soils, afforestation of cropland performed through either shelterbelt or forest plantation caused substantial increases in surface SOC storage compared with conventionally tilled cropping systems (≥57%; P < 0.05); this confirms the direct benefits of tree planting on SOC sequestration. Relative to cropped soils, afforested soils exhibited a more depleted δ 13 C signature (−17 vs. −22‰), indicating a shift in C sources. Source‐partitioning assessment revealed that tree‐derived C contributed roughly half of the SOC found directly beneath the trees. The C‐enriched afforested surface soils exhibited SOC turnover rates of 0.018 to 0.022 yr −1 and mean residence times of 55 to 45 yr. Fine POM in afforested surface soils accounted for a large proportion (21%) of the existing SOC, 79% being derived from tree inputs. This supports the role of POM as a significant sink for recently sequestered SOC in these ecosystems.