Season of burn and nutrient losses in a longleaf pine ecosystem

Abstract

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.

Contributions of microbial activity and ash deposition to post-fire nitrogen availability in a pine savanna

“…After collection, all cores were stored on ice, immediately transported back to the laboratory, and kept at 4 • C until they were analysed for inorganic N, soil moisture, soil organic matter, pH, and δ 15 N. All cores were homogenized by passing through a 2 mm sieve. Frequent fires in this ecosystem consume aboveground vegetation and litter, preventing the development of an O horizon in these soils (Boring et al, 2004). As a result, sieving removed coarse root fragments, rather than partially decomposed organic matter.…”

“…Fire can decrease N availability if N is volatilized and lost from the system in highintensity fires (Lavoie et al, 2010;Certini, 2005). On the other hand, fire can increase N availability if it spurs microbial turnover of organic matter (Wilson et al, 2002;Certini, 2005), returns nutrient-rich ash to the system (Boring et al, 2004), or decreases the vegetation demand for N. Short-term increases in soil N availability may not translate to longerterm ecosystem retention if N is lost through leaching or as gaseous products during turnover.…”

“…Pine savannas in the southeastern US are often managed with prescribed fires in the absence of recurring natural fires to maintain habitat for endangered species (Sorrie et al, 2006), and nutrient losses or pool redistributions from these fires can be substantial (Boring et al, 2004;Wilson et al, 2002Wilson et al, , 1999. In addition to large quantities of carbon (C) released through fuel consumption (Boring et al, 2004), prescribed fires can release up to 50 % of the phosphorus (P) and up to 75 % of the N that was stored in the understory biomass (Carter and Foster, 2004;Wan et al, 2001).…”

“…Despite the ecosystem-level nutrient losses associated with fires, short-term pulses of increased N availability in the soil are also observed following prescribed forest fires across vegetation types (Certini, 2005;Schafer and Mack, 2010;Smithwick et al, 2005;Wan et al, 2001), including in longleaf pine (Pinus palustris Mill.) savannas (Boring et al, 2004). In longleaf pine savannas, because these nutrient pulses occur during a period of rapid post-fire plant regrowth, they may influence successional patterns (Shenoy et al, 2013), plant diversity, and ecosystem productivity.…”

“…In addition to large quantities of carbon (C) released through fuel consumption (Boring et al, 2004), prescribed fires can release up to 50 % of the phosphorus (P) and up to 75 % of the N that was stored in the understory biomass (Carter and Foster, 2004;Wan et al, 2001). These nutrients can be lost through volatilization, or redistributed as ash in low-intensity fires.…”