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 (delta C-13, natural abundance) in both whole soil and the fine particulate organic matter (POM) fraction (53-500 mu m 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 delta C-13 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.