Abstract

We tested the hypotheses that Pinus taeda L. ecosystems (trees plus forest floor plus soil) can retain nitrogen, phosphorus, potassium, calcium, and magnesium applied as fertilizer. We accomplished this by comparing the ecosystem nutrient content in fertilized plots with the content in nonfertilized plots under ambient rainfall and ambient rainfall plus irrigation conditions. Our data were from a study in which fertilizers were applied annually for 16 years to a P. taeda stand growing on a somewhat excessively drained sandy soil in the Sandhills Region of North Carolina in the United States. Cumulative fertilizer applications were 1,322, 168, 393, 168, and 146 kg ha(-1) of elemental nitrogen, phosphorus, potassium, calcium, and magnesium, respectively, whereas irrigation applications averaged 632 mm year(-1) of water applied in addition to ambient rainfall. We rejected our hypothesis (fertilized stand nutrient content minus nonfertilized stand nutrient content did not equal the amount applied) for phosphorus (77 kg ha(-1), 46%, less) and potassium (281 kg ha(-1), 71%, less) in the ambient rainfall test and for nitrogen (652 kg ha(-1), 49%, less), potassium (294 kg ha(-1), 75%, less), and magnesium (143 kg ha(-1), 99%, less) in the irrigated test. Our ambient rainfall tests support data from the literature that forest fertilizer applications may be made with little risk of offsite nutrient movement for nitrogen, calcium, and magnesium. Our soil phosphorus methodology (extraction versus digestion) may have underestimated soil phosphorus, which influenced the result for this element. Recovered potassium may have been low from leaching losses due to application in conjunction with nitrogen. If irrigation is necessary and feasible at a stand scale, our data indicate that additional considerations are required to ensure that all applied elements are held in the system.