Feast not famine: Nitrogen pools recover rapidly in 25‐yr‐old postfire lodgepole pine

Abstract

The extent of young postfire conifer forests is growing throughout western North America as the frequency and size of high‐severity fires increase, making it important to understand ecosystem structure and function in early seral forests. Understanding nitrogen (N) dynamics during postfire stand development is especially important because northern conifers are often N limited. We resampled lodgepole pine (Pinus contorta var. latifolia) stands that regenerated naturally after the 1988 fires in Yellowstone National Park (Wyoming, USA) to ask (1) How have N pools and fluxes changed over a decade (15 to 25 yr postfire) of very rapid forest growth? (2) At postfire year 25, how do N pools and fluxes vary with lodgepole pine density and productivity? Lodgepole pine foliage, litter (annual litterfall, forest‐floor litter), and mineral soils were sampled in 14 plots (0.25 ha) that varied in postfire lodgepole pine density (1,500 to 344,000 stems/ha) and aboveground net primary production (ANPP; 1.4 to 16.1 Mg·ha−1·yr−1). Counter to expectation, foliar N concentrations in lodgepole pine current‐year and composite needles (1.33 and 1.11% N, respectively) had not changed over time. Further, all measured ecosystem N pools increased substantially: foliar N increased to 89 kg N/ha (+93%), O‐horizon N increased to 39 kg N/ha (+38%), and mineral soil percent total N (0–15 cm) increased to 0.08% (+33%). Inorganic N availability also increased to 0.69 μg N·[g resin]−1·d−1 (+165%). Thus, soil N did not decline as live biomass N pools increased. Among stands, biomass N pools at postfire year 25 remained strongly influenced by early postfire tree density: foliar and litterfall N concentrations declined with lodgepole pine density and ANPP, but the foliar N pool increased. Lodgepole pine ANPP correlated negatively with annual resin‐sorbed N, and we found no indication of widespread N limitation. The large increases in N pools cannot be explained by atmospheric N deposition or presence of known N fixers. These results suggest an unmeasured N source and are consistent with recent reports of N fixation in young lodgepole pine.

A roadmap for sampling and scaling biological nitrogen fixation in terrestrial ecosystems

“…Certain ecosystem N fluxes are known to be highly episodic (especially gaseous and hydrologic losses; Taylor et al, 2015) and obtaining accurate estimates can require frequent or continuous sampling (Barton et al, 2015). It can also be difficult to measure small changes in large N pools, such as bulk soil, even with large sample sizes (Chalk, 2020;Turner et al, 2019). The impact of unmeasured fluxes (such as N 2 loss) on N balance can be explored using sensitivity analyses (e.g.…”

“…Soper et al, 2015Soper et al, , 2017. Redistribution of N from unmeasured pools (such as deep soil or rock weathering) into measured soil and biomass is often unaccounted for (Turner et al, 2019) but could be similarly constrained. Appropriate error propagation is important for mass balance, where uncertainty can be high for multiple parameters.…”

“…Together, these advances can be used to address some of the significant remaining unknowns in ecological BNF research, such as the drivers of interspecific variation in BNF potential, the persistence of ecosystem N limitation, 'missing' ecosystem N inputs (Turner et al, 2019), the distribution of BNF strategies (Menge et al, 2015), and the importance of N-fixer diversity for BNF rates (Taylor et al, 2020).…”

The impact of wildfire on biogeochemical fluxes and water quality on boreal catchments

“…via N-fixation). In fact, it is unknown how plants can acquire large amounts of N post-fire and how the N pool builds up quicker than estimated N-fixation rates (Turner et al, 2019). Our N losses (ca.…”

“…Despite these large N losses, there is little evidence that either direct or fluvial N losses are relevant for post-fire productivity at a catchment scale. Recently, a study by Turner et al (2019) showed a remarkably rapid post-fire build-up of soil N, and little evidence that the N loss had an long-term impact on productivity. It is hypothesised that post-fire plant communities, if quickly established, can retain N before it is lost hydrologically (Smithwick et al, 2009).…”