Season of burn and nutrient losses in a longleaf pine ecosystem

Fire regulates the structure and function of longleaf pine ecosystems, including potential nutrient controls on productivity, forest floor and groundcover nutrient pools, and nutrient availability. Little is known about comparative influences of seasonality of fire, litter types, and mass on N and P balance and soil processes in longleaf pine ecosystems. This study primarily addresses the hypothesis that nutrient volatilization during growing season burning, due to combustion of live biomass, exceeds losses from winter burning of standing dead plant litter. Summer and winter burns were conducted experimentally in different groundcover types with ambient, double-ambient and no litter loadings to contrast 2–3 years of litter accumulation with very low and high fuels. As a comparison, the seasonal burns were repeated with fuel and temperature measurements on sites that had actual fuel accumulations ranging from 1 to 3 years following the last fire. Peak fire temperatures and duration of burning were similar, but with high variation across groundcover types and seasons due to variation in fuel moisture content. The highest pine litter loadings produced maximum mineral soil/litter interface temperatures that never exceeded 700°C. Groundcovers without pine litter burned incompletely and with low temperatures. Biomass and N content were greater in summer groundcover than winter groundcover, and were greater in wiregrass than old-field groundcover. More N was lost from growing season burning as biomass had higher N in green foliage at that time. With ambient litter loadings, mass losses were 88–94% of total litter and groundcover. Percentages of N lost were comparable (80–90% across all groundcovers and seasons), but amounts of N lost were below that estimated to be replenished by legume N fixation and regional atmospheric deposition over a dormant season prescribed fire cycle. Net N balances with growing season fire were generally negative only if growing season burning was projected exclusively over the long-term. P content was not significantly different among groundcovers, but summer standing stocks were higher than winter. No P losses were detected with any experimental treatments and, following burning, all P was returned to soil pools, attributable to soil surface temperatures remaining largely below 700°C. We conclude that frequent, dormant season, or even variable season burning should not seriously deplete long-term nitrogen balance of longleaf pine ecosystems.