Nitrogen mineralization in short-rotation tree plantations along a soil nitrogen gradient

Abstract

We measured soil nitrogen (N) mineralization along an N fertilization gradient (control; irrigation only (I + 0 N); irrigation with 56 (I + 56 N), 112 (I + 112 N), and 224 (I + 224 N) kg N·ha–1·year–1, respectively) in 7-year-old cottonwood (Populus deltoides Marsh.), cherrybark oak (Quercus falcata Michx. var. pagodifolia Ell.), American sycamore (Platanus occidentalis L.), and loblolly pine (Pinus taeda L.) plantations established on a well-drained Redbay sandy loam (a fine loamy, siliceous, thermic Rhodic Paleudult), in Florida, USA. Nitrogen mineralization was measured monthly for 1 year, beginning in April 2001, with the buried bag incubation technique. Irrigation alone or fertigation (irrigation + N) affected annual net N mineralization rates under hardwood species, but no effect was found under loblolly pine. Overall, the rates were higher under cherrybark oak (108 kg N·ha–1·year–1) and cottonwood (101 kg N·ha–1·year–1) than under sycamore (82 kg N·ha–1·year–1) and loblolly pine (75 kg N·ha–1·year–1). Significant correlations were observed between N mineralization and stem volume in all species but loblolly pine. These results suggest that N mineralization response to irrigation or fertigation (irrigation + N) is heavily dependent on species-specific feedback mechanisms. Our results also support the hypothesis that the N mineralization versus productivity relationship is a fundamental feature of forests, resulting from the impact of N availability on productivity and the long-term feedback effects of vegetation on N availability.