Perennial bioenergy crops are projected to reduce US reliance on fossil fuels, bring down greenhouse gas emissions and enhance rural economies. In the Northeastern United States, seasonally-wet underutilized marginal lands are being considered as a resource base for the renewable bioenergy sector. Studies incorporating soil moisture and other field characteristics are needed for commercial production of bioenergy. The objective of this study is, therefore, to investigate the impact of converting wetness-prone fallow lands to perennial grass cultivation and specifically to examine the effect of moisture controls on soil carbon dynamics. In the first study, quadruplicate treatments were established in a wetness-prone marginal fallow grass site that was converted to perennial grass bioenergy feedstock production, consisting of fallow-control, reed canarygrass with nitrogen (N) fertilizer, switchgrass without and with N fertilizer in a four-year study. After three years, the loss of organic matter (OM) and soil organic carbon (SOC) due to plowing of formerly fallow land had not been repaid. The wettest soils had the greatest OM, SOC, and active carbon, independent of the cropping treatment, while driest soils had the lowest pH. During the last year, the wettest soils had significantly greater SOC and total nitrogen (TN) than drier soils, and fallow soils had significantly greater SOC than soils of switchgrass and switchgrass + N. In the relatively short timeframe of this study, neither crop species photosynthetic pathway (C3 vs. C4) nor moderate rates of N fertilization had a significant impact on soil properties. The second study found that surface volumetric water contents had a stronger correlation with soil properties (OM, SOC, TN, bulk density and δ13C) down to 120 cm depth, than did shallow perched water depth. Correlation between loss-on-ignition (LOI) derived C and combustion elemental analysis measured C contents to soil depths of 60 cm (using New York State soil testing conversions of LOI), was very good. The third study showed that in a 371-day incubation experiment, C mineralizability was significantly less with high field moisture compared to low field moisture, indicating possible stabilization mechanism facilitated (through mineral interactions of SOC) under high water content.