Patterns and controls of ecosystem function in longleaf pine - wiregrass savannas. II. Nitrogen dynamics

The productivity of many temperate forests is nitrogen limited. Controls on N availability are particularly important in fire-maintained ecosystems such as longleaf pine - wiregrass (Pinus palustris Mill. - Aristida stricta Michx.) forests of the Coastal Plain of the southeastern United States, where periodic burning can result in substantial N losses. This study quantifies variation in net N mineralization across a complex ecological gradient for longleaf pine - wiregrass forests, from dry sandhills to wet-mesic sites adjacent to wetlands, for the first year after burning. Net N mineralization was consistently higher for the xeric site and declined as soil moisture status increased. Higher N availability was primarily due to higher rates of net nitrification in these plots, suggesting possible substrate-induced influences. Temperature was positively related to net N mineralization, while percent soil moisture was inversely correlated to net N mineralization. The greater N availability on dry sites may reflect greater quality organic matter inputs resulting from a significant hardwood (Quercus laevis Walt. predominately) component, and (or) it may reflect microclimate differences (warmer soil) that accelerate decomposition of detritus in soil.