Mineral Soil Properties Influencing Nitrogen and Phosphorus Retention In a Georgia Coastal Plain Loblolly Pine and Sweetgum Plantation.

Fertilization of forest plantations is widely practiced in the southeastern U.S. to improve growth on infertile soils. Fertilization of loblolly pine and sweetgum on a southern Georgia Coastal Plain site increased growth by 30% and 300%, respectively. These growth increases led to much larger C, N, and P inputs to the forest floor. The primary question that arises from fertilization of these soils is whether the applied fertilizer benefits only the current trees in the stand or also improves long term site quality. Long term site quality would improve if fertilization increased mineral soil N and P. Previous studies and our results suggest that N fertilization only benefits the trees while P fertilization benefits the trees and the site. On this site in the Georgia Coastal Plain, fertilization increased N in the forest floor indicating it is a sink for N. However, there was no change in mineral soil N. The response to P was an increase in both the forest floor and mineral soil. Rapid decomposition of the forest floor following harvest will quickly release the accumulated N leading to a strong Assart effect. In contrast, the additional P in the mineral soil will probably lead to a longer term increase in P availability. The low CEC of these sandy soils limits NH₄ retention so that N released from the decomposing forest floor may quickly leach through the soil as NO₃. Acidic conditions lead to the retention of P in the mineral soil through complexation with Al.