Nitrogen supply and demand in short-rotation sweetgum plantations

Intensive management is crucial for optimizing hardwood plantation success, and nitrogen (N) nutrition management is one of the most important practices in intensive management. Because management of short-rotation woody crop plantations is a mixture of row-crop agriculture and plantation forestry, we tested the usefulness of an agronomic budget modified for deciduous perennial trees for estimating N fertilizer recommendations. We studied the foliar N demand, resorption, and uptake response of two sweetgum (Liquidambar styraciflua L.) plantations on a converted agricultural field and a pine cutover site to biannual applications of three nitrogen (N) fertilizer rates: 0, 56 and 112 kg N ha^-1. We also estimated soil N supply, foliar N uptake efficiency, and apparent fertilizer N uptake efficiency. Fertilization increased foliar demand (defined as total foliar N content), resorption, and uptake at both sites, but to a greater degree on the cutover site, which had lower soil N supply. Resorption efficiency did not increase with fertilization, but resorption proficiency, the N concentration of senesced leaves, was reduced (N concentrations higher) at the drier agricultural field site and in the fertilized trees. Based on our budget analysis, we recommend fertilizing sweetgum plantations with 50 kg N ha^-1 for each 2000 kg ha^-1 of foliage biomass, assuming they have 45–65 kg ha^-1 of soil N supply.