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.