Soil Carbon and Nutrient Contents Following Afforestation and Reforestation of Marginal Cropland in Iowa.

Forest ecosystems offer great potential to improve soil quality of degraded or marginal cropland by reducing soil disturbance and increasing soil organic carbon (SOC) content with commensurate enhanced nutrient cycling, soil biodiversity, and plant available water.  The goals of this study were to identify the spatial distribution of marginally productive cropland in Iowa that could be converted from row crop production to a forestry practice.  A statewide GIS survey was completed to identify the extent and distribution of vulnerable, low-productivity soils that had formed under forest vegetation.  Based on the GIS analysis, four sampling locations were selected to provide a range of soils, land use history (pasture and arable cropping), and trees (conifer and deciduous).  At each location, composite soil samples were collected at randomly selected positions within forested and adjacent cultivated areas within the same soil map unit.  Samples were sectioned into 0-10, 10-20, and 20-30 cm depth increments and analyzed for total (TC), inorganic carbon (SIC), total nitrogen (TN), cation exchange capacity (CEC), pH, and available (Mehlich 3) Ca, Fe, K, and P.  SOC was calculated as the difference between TC and SIC for each depth increment.  The surface 30 cm layer of forested soils had on average 23.7±14.7% greater SOC than the adjacent cropped soils.  Assuming that the SOC in cropped soils had not changed significantly since trees were planted (14 to 35 years before sampling), estimates of SOC increase with time under the trees ranged from 249 to 574 kg ha^-1 yr^-1.  These rates are comparable to literature estimates for conversion from conventional to no-till cropping.  Nutrient data showed inconsistent patterns among sites, which is likely at least partly a result of lime and nutrient applications in the cropped fields  Additional research is needed to characterize ecosystem services associated with the observed increases in SOC and changes in nutrient cycling and to quantify C accumulation in the above- and below-ground biomass in forests in Iowa.