The Impacts of Fertilization and Irrigation on Forest Floor and Mineral Soil C and N.

At the Southeast Tree Research and Education Site (SETRES), a 23-year old loblolly pine (Pinus taeda L.) plantation received annual fertilization and irrigation after age 7 with the following treatment combinations: control (Ct), fertilization (Ft), irrigation (Irr), and fertilization x irrigation (FxI).  Fertilization includes urea N, super phosphate, and macronutrient salts based on foliar analyses.  Irrigation maintains a soil water content >40% field capacity.  We examined treatment effects on forest floor and mineral soil C and N.  Fertilization (Ft and FxI) increased Oi + Oe mass (16.8 to 30.4 Mg ha^-1), total-N concentration (Oi: 4.7 to 6.7 g kg^-1; Oe: 7.9 to 12.0 g kg^-1), total-N content (115.9 to 316.2 kg ha^-1), and C content (8.9 to 16.0 Mg ha^-1), and increased Oa total-N concentration (14.9 to 19.2 g kg^-1).  In the 0-3.75 cm mineral soil depth, C concentration decreased on the order of Ft (23.2 g kg^-1) > Irr (20.2 g kg^-1) > Ct (19.3 g kg^-1) > FxI (17.0 g kg^-1).  In the 7.5-15 cm depth, C concentration decreased on the order of Ft (8.4 g kg^-1) > Irr (6.5 g kg^-1) > Ct (5.9 g kg^-1) > FxI (5.8 g kg^-1).  In the 30-60 cm depth, C concentration decreased on the order of FxI (2.0 g kg^-1) > Ft (1.7 g kg^-1) > Irr (1.4 g kg^-1) > Ct (1.2 g kg^-1) with proportional differences in C content.  Results suggest that fertilization mainly increased forest floor mass and C and N contents.  Although forest floor mass and chemical properties were similar among Ft and FxI plots, these plots exhibited divergent responses in mineral soil C and N of surface soils suggesting potential differences in organic matter decomposition status.  Therefore, an organic matter fractionation will be performed on these surface soils.