Soil carbon and nitrogen content and stabilization in mid-rotation, intensively managed sweetgum and loblolly pine stands

• We examined soil C and N content in loblolly pine and sweetgum experiments.
• Irrigation increased total soil C in sweetgum.
• Soil C and N was sequentially fractionated in solutions of increasing density.
• Silviculture treatments generally did not affect soil C and N fractionation.

Intensive forestry has resulted in considerable increases in aboveground stand productivity including foliar and belowground biomass which are the primary sources of soil organic matter. Soil organic matter is important for the maintenance of soil physical, chemical and biological quality. Additionally, sequestering carbon (C) in soils may provide a means of mitigating increasing atmospheric carbon dioxide concentrations. In this study, we examined soil C and nitrogen (N) contents and stabilization in 12-year-old, intensively managed sweetgum (Liquidambar styraciflua L.) and loblolly pine (Pinus taeda L.) stands. The treatments examined include: (1) complete weed control; (2) weed control plus drip irrigation; (3) weed control plus drip irrigation and fertigation; and (4) (pine only) weed control plus irrigation, fertigation, and pest control. C and N stabilization was analyzed sequentially by fractionating the soil samples into six fractions using solutions of increasing density. These fractions represented increasingly stable organic matter pools. There was a trend towards increasing C and N contents with increasing management intensity that increase stand productivity; however, these differences were only significant for soil C content in sweetgum. Across all the sweetgum plots, soil C content generally increased with basal area (BA); no such relationship was found in loblolly pine although its BA was equal or higher than that of sweetgum. Generally, across all depths most C was found in the two lightest and in the heaviest fractions. These results suggest that changes to soil C due to increased forest management intensity which increases forest productivity, when they did occur, mostly did not change the percentages of C among soil density fractions over the 12 years of the experiment suggesting minimal inputs of recalcitrant C into the soil; however, even these transient changes may be still be important if intensive management is maintained over subsequent rotations.