Nitrogen fixation does not balance fire‐induced nitrogen losses in longleaf pine savannas

Abstract

Fire is a critical force in structuring ecosystems, but it also removes substantial amounts of nitrogen (N), which can limit plant growth. Biological N fixation (BNF) may alleviate fire‐induced N deficiencies that inhibit ecosystem recovery, yet if and how BNF achieves this under frequent fire is unclear. This problem is further complicated in the context of modern human influences (such as land‐use history and atmospheric N deposition), which may confound the relationship between fire and fixation. Here, we investigate whether BNF supplies the N necessary to replace fire‐induced N losses in restored longleaf pine savannas, and, if so, what factors control fixation. We established 54 1‐ha plots of longleaf pine capturing 227 yr of forest recovery and a broad gradient of fire return interval (1.5–20 yr) at two sites in the southeastern United States. We quantified N fixation from three functional groups (herbaceous legumes, soil crusts, and asymbiotic N fixing bacteria), N losses from individual fire events and ecosystem dynamics of N supply and demand. We found that BNF rates were low but sustained over stand age but were substantially below estimated rates of atmospheric N deposition. While fire temporarily stimulated BNF from herbaceous legumes, neither BNF nor atmospheric N deposition were sufficient to balance N losses from fire and soil N stocks declined over stand age. However, rates of N mineralization were surprisingly high and tree productivity was unrelated to N availability, questioning the importance of N limitation in these temperate savannas. While it is possible that progressive N losses signal a decline in ecosystem resiliency, N enrichment from multiple land‐use transitions and anthropogenic N deposition may suppress rates of BNF or diminish its importance as a long‐term N balancing source in these pyrogenic ecosystems. In this case, fire may be acting as relief mechanism, critical for returning the modern longleaf pine landscape to its historical oligotrophic condition.

Fire as a fundamental ecological process: Research advances and frontiers

“…In tropical grasslands, savannas and forests, repeated burning can also deplete total soil carbon and nutrients, but usually only at high frequencies and over decadal time‐scales (Liu, Chen, Wang, Hughes, & Lewis, 2015; Pellegrini, Hedin, Staver, & Govender, 2015). These declines may or may not have deleterious impacts on ecosystem productivity (Tierney, 2019).…”

Intra-Annual Variation in Soil C, N and Nutrients Pools after Prescribed Fire in a Mississippi Longleaf Pine (Pinus palustris Mill.) Plantation

“…Longleaf pine does respond to fertilization with N, P, and K, but the species' ability to take advantage of nutrient additions and parlaying it into aboveground biomass is limited compared to the responsiveness of loblolly and slash pines [17]. While longleaf pine are well adapted to soils with low nitrogen availability and low N contents [42], particular regions in the lower Coastal Plain and Gulf Coast are noted for severe P deficiencies and very responsive to P additions [40,43]. We observed a temporary spike in soil P at depths of 0-5 cm and a significant decline from pre-burn levels at depths of 5-10 cm after 12 months, though longer term studies of multiple burn cycles show no effect of fire on soil P [6,44].…”

Long-Term Effects of Repeated Prescribed Fire and Fire Surrogate Treatments on Forest Soil Chemistry in the Southern Appalachian Mountains (USA)

“…This would particularly apply to pine forests in the southern United States, an area heavily influenced by prescribed fires as opposed to wildfires. The Green River results and those from other prescribed fire studies (15,25,26) bear consideration in the interpretation of fire-related soil impacts, as well. Soil N may not be a limitation for all managed vegetative systems and structures in all locations globally [16,17,25,26].…”

“…The Green River results and those from other prescribed fire studies (15,25,26) bear consideration in the interpretation of fire-related soil impacts, as well. Soil N may not be a limitation for all managed vegetative systems and structures in all locations globally [16,17,25,26]. Some management objectives, such as wildlife habitat management and hazardous fuel reduction, for example, necessitate different forest structures and species compositions [15,27].…”