Nitrate leaching in cottonwood and loblolly pine biomass plantations along a nitrogen fertilization gradient

Nitrogen (N) fertilization has become a basic component of short-rotation woody biomass plantation management. This study was conducted to examine N leaching losses from 7-year-old cottonwood (Populus deltoids Marsh.) and loblolly pine (Pinus taeda L.) plantations treated with repeated N fertilization for seven years. We also wanted to establish an optimal N fertilization rate, which would yield acceptable tree growth with minimal groundwater contamination. The plantations were located in northwest Florida on a well-drained, Redbay sandy loam (a fine-loamy, siliceous, thermic Rhodic Paleudlt). N fertilizer was applied through a drip irrigation system with rates of 0, 56, 112 and 224 kg N ha−1 per year. We collected soil water samples monthly from June 2001 through May 2002 using tension lysimeters installed at 30 and 120 cm soil depths for analyzing NO3-N, and NH4-N. The concentration data were incorporated with drainage data from LEACHN model simulation for estimating the amount of N leaching losses. Annual mean nitrate-N concentrations at 120 cm depth under cottonwood were 0.5, 7.4, 14.1, and 37.7 mg NO3-N L−1 and under loblolly pine were 1.0, 10.2, 30.3, and 50.7 mg NO3-N L−1 for 0, 56, 112, and 224 kg N ha−1 per year treatments, respectively. Estimated N leaching losses were not different between species, however, increasing N fertilization rate above 56 kg N ha−1 per year increased N leaching without benefits to tree growth in both species.