| The overall objectives of this study were to assess the potential of improving biomass yields with N fertilization and cultivars of switchgrass, changes in SOC concentration and pool size, and effects on soil structural properties.;First, the effects of N fertilization on switchgrass biomass production and SOC sequestration were determined in Ohio. This study was conducted at three research stations (Northwest, Jackson, and Western) of the Ohio Agricultural Research and Development Center (OARDC). In 2008 and 2009, four rates of N (0, 50, 100, and 200 kg N ha-1) were applied to switchgrass plots established in 2004. Measurements included aboveground and belowground biomass, SOC concentration, soil bulk density, and C and N concentrations in plant tissues. To consider the effects of land use changes while establishing plots, SOC concentrations and pools were measured from the reference soils which had been maintained as the same land use before switchgrass plots were established at each site.;Second, laboratory incubation studies were conducted on organic matter (OM) decomposition in response to the addition of the root-C substrate and inorganic N. In general, there was no interaction between C and N rates on the parameter measured. The addition of inorganic N resulted in a decrease in decomposition of OM. Several mechanisms for the negative effect of N on decomposition were tested. One possible explanation was an N mining theory, which entailed microbial mineralization of OM to obtain N from soil organic matter (SOM) under the N-limited conditions. However, a higher concentration of available N in soil even in the N0+CH treatment (the most N limiting condition) rejected the N mining hypothesis.;Third, the effects of N fertilization on soil structural changes under switchgrass were assessed in a field experiment. Soil samples were obtained from a field study in Milan, Tennessee in April and October, after four consecutive years of N fertilization (0, 67, and 202 kg N ha-1). Root weight density (RWD), root length density (RLD), SOC concentration, aggregate stability, and soil moisture characteristics curves (SMCC) were measured. The RWD in April and the RLD in October were the lowest under the 202 kg N ha -1 treatment. In contrast, the SOC concentration was the highest under the 202 kg N ha-1 treatment. No consistent trends were observed in SMCC among N treatments.;Fourth, soil structural changes under four switchgrass cultivars (Alamo, GA992, GA993, SL-93-2) were evaluated in Milan, Tennessee during the fourth year after switchgrass establishment. The RWD, C and N concentrations in roots, aggregate stability by wet-sieving, C concentrations in aggregates (4.75-8 mm size fraction) before and after wet-sieving, and total porosity and pore size distribution in aggregates (4.75-8 mm size fraction) were measured. No significant treatment differences were observed either in plants or in aggregate properties among cultivars during the growing season.;Overall, the data from Ohio and Tennessee experiments showed that the aboveground biomass was more strongly influenced by N fertilization than the belowground biomass. Even when the aboveground biomass was harvested and removed, N fertilization led to an increase in SOC pools, both in Ohio and Tennessee. Data from the laboratory incubation study showed that N additions could retard the decomposition of OM, which may contribute to higher SOC pools in N fertilized plots. Although no clear relationship between root growth and N fertilization were found in the short-term (2 years) experiments conducted in Ohio, reduced root biomass and lower root length were observed with high fertilization (202 kg N ha-1) in the 5-year experiments in Tennessee. The results from the Tennessee experiments indicated the important role of roots in stabilizing soil structure. Despite higher SOC concentrations in plots receiving high rates of N fertilization, higher soil structural stability was associated with greater root biomass and longer root length in plots receiving none or low rates of N fertilizer. These data indicate that root growth is a crucial driver of surface soil structure. Moreover, soil structural properties are strongly affected by the antecedent moisture content and other environmental factors at the time of sampling. (Abstract shortened by UMI.)... |
