Greenhouse Gas Emissions in Managed Pine Forest Ecosystems.

Forests play a critical role in the global carbon (C) cycle because of their large capacity for C storage in biomass and soils. Although these potential C sinks in forests are large, the role of forests in offsetting climate change is less familiar. While there are a large number of studies that have documented the effects of soil fertility management on forest productivity, fewer studies are available evaluating the impacts of fertilization on greenhouse gas (GHG) emissions. To address this critical knowledge gap, field studies were established in managed loblolly pine ecosystems in the Lower Coastal Plain of North Carolina in the summer of 2011. The objective of the study is to quantify the impacts of N fertilization on soil GHG fluxes in managed pine ecosystems across different drainage classes. Three landscape positions: terrace (poorly drained), flat (moderately drained), and upland (well drained) were selected. The treatments in each landscape position included control (no fertilizer), urea, and Arborite^® CUF(phosphate-coated urea fertilizer with boron), applied in the summer, fall, and spring. Within each plot, chambers were installed in both bed and interbed locations. The treatments were laid out in a randomized complete block design with four replications within each drainage class. N-fertilizers were applied at a rate of 168 kg N ha^-1. GHG sampling started immediately after fertilizer application and continued every six weeks for one year. GHG fluxes during the observation period (July 2011 to Feb 2012) ranged from 0.74 to 8.01 g CO₂-C m^-2-d^-1, -0.03 to 2.96 mg N₂O-N m^-2 d^-1, and -1.62 to 0.62 mg CH₄-C m^-2 d^-1. CO₂ emissions were highest during summer and decreased in fall and winter. The highest peaks of N₂O (2.96 mg N₂O-N m^-2 d^-1) and CO₂ (8.01 g CO₂-C m^-2-d^-1) were observed during the September sampling, especially for well drained, urea fertilized, interbed soils. Irrespective of drainage class and microsite (i.e., bed and interbed), N₂O flux rates (average of June 2011 to Feb 2012 samplings) were 0.055 mg m^-2 d^-1 for no-fertilizer, 0.167 mg m^-2 d^-1 for urea, and 0.189 mg m^-2 d^-1 for Arborite treatments. The N₂O fluxes were consistently higher in the interbed relative to the bed. In general, CH₄ uptake occurred over most of the sampling events. However, CH₄ production was occasionally observed in moderately and poorly drained soils.