Fertilization Effects On Long-Term Nutrient Availability in Southeastern US Pine and Hardwood Plantations.

Management of forest plantations in the southeastern United States includes nutrient additions to increase volume production on infertile, sandy soils. Fertilization affects cycling of nitrogen (N) and phosphorus (P) by altering litterfall chemistry resulting in changes in nutrient pools and fluxes in the forest floor and mineral soil. Our objective was to determine the effect of fertilization on long-term nutrient availability. Nitrogen and P were quantified at two sites similar in terms of soils but different in terms of fertilizer regime, stand age, and species. Site one, SETRES, was planted with loblolly pine (Pinus taeda) in 1985 and has been fertilized annually since 1992. Site two, Mt. Pleasant, was planted with loblolly pine and sweetgum (Liquidambar styraciflua) in 1997 and was fertilized annually until 2006. Fertilization increased forest floor N and P at SETRES by 270 and 20 kg ha^-1, respectively. At Mt. Pleasant, fertilization increased loblolly pine forest floor N and P by 140 and 7 kg ha^-1, respectively. Also, sweetgum forest floor N and P were increased by 100 and 7 kg ha^-1, respectively. No fertilization effect was observed for mineral soil total-N at both sites while mineral soil P increased by 50 kg ha^-1 (SETRES), 45 kg ha^-1 (Mt. Pleasant, loblolly), and 18 kg ha^-1 (Mt. Pleasant, sweetgum). Measurements of forest floor cumulative inorganic-N flux indicated a fertilization effect at SETRES (+30 kg ha^-1). This effect was much less at Mt. Pleasant (+0.50 kg ha^-1). Fertilization altered mineral soil cumulative NO₃-N fluxes at SETRES (+12 kg ha^-1) while no effect was observed at Mt. Pleasant. Results suggest that fertilization will increase long-term availability of P but not N by increasing the mineral soil pool. Furthermore, differences in N fluxes among sites suggest fertilization effects on N are short-lived and decline rapidly when nutrient additions cease.