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Summary Pyrogenic savannas with a tree–grassland ‘matrix’ experience frequent fires (i.e. every 1–3 yr). Aboveground responses to frequent fires have been well studied, but responses of fungal litter decomposers, which directly affect fuels, remain poorly known. We hypothesized that each fire reorganizes belowground communities and slows litter decomposition, thereby influencing savanna fuel dynamics.In a pine savanna, we established patches near and away from pines that were either burned or unburned in that year. Within patches, we assessed fungal communities and microbial decomposition of newly deposited litter. Soil variables and plant communities were also assessed as proximate drivers of fungal communities.Fungal communities, but not soil variables or vegetation, differed substantially between burned and unburned patches. Saprotrophic fungi dominated in unburned patches but decreased in richness and relative abundance after fire. Differences in fungal communities with fire were greater in litter than in soils, but unaffected by pine proximity. Litter decomposed more slowly in burned than in unburned patches.Fires drive shifts between fire‐adapted and sensitive fungal taxa in pine savannas. Slower fuel decomposition in accordance with saprotroph declines should enhance fuel accumulation and could impact future fire characteristics. Thus, fire reorganization of fungal communities may enhance persistence of these fire‐adapted ecosystems.
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Plant and microbial feedbacks maintain soil nitrogen legacies in burned and unburned grasslands
Abstract Nitrogen (N) availability is a well‐known driver of ecosystem structure and function, but as air quality regulations continue to reduce atmospheric N deposition, there is a need to understand how managed and unmanaged ecosystems respond to widespread decreases in terrestrial N availability. Historical N eutrophication, from pollution or fertilisation, may continue to constrain contemporary responses to decreases in available N because of altered plant and microbial feedbacks. Thus, while certain management practices like prescribed fire remove N from grassland ecosystems, the role of fire supporting ecosystems recovering from chronic N input is unknown.To address this knowledge gap, we ceased a 30‐year N‐fertilisation treatment at a field experiment in a tallgrass prairie ecosystem crossed with burned and fire‐suppressed (unburned) treatments. We established subplots within each previously fertilised, recovering plot, fertilised at the same historical rate (10 g N m−2 year−1as NH4NO3), to compare plant and soil properties in recovering plots with control (never‐fertilised) and still‐fertilised treatments within different fire regimes.We document different N‐fertilisation legacies among ecosystem properties in burned and unburned prairies recovering from N‐fertilisation. Soil N availability, nitrification and denitrification potentials in recovering plots remained higher than controls for 3–5 years—indicative of positive legacies—in both burned and unburned prairies, but burning did not reduce this legacy. In burned prairies, however, a positive legacy in above‐ground plant production persisted because a more productive grass species (switchgrass) replaced the previously dominant species (big bluestem) even though root C:N, but not soil C:N, increased to return back to control levels. Consequently, the main N loss pathways in burned and unburned prairies (pyrovolatilisation and microbially mediated processes, respectively) led to similar losses of soil total N (20–28 g N m−2) over 5 years.Synthesis: Our results indicate that N eutrophication induces positive legacies of ecosystem functions that can persist for at least half a decade. N‐induced legacies arise because of shifts in soil microbial N‐cycling and plant functional traits. As a result, different management practices may elicit similar trajectories of ecosystem recovery in terms of total and available soil N because of different plant and microbial feedbacks.
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- Award ID(s):
- 2025849
- PAR ID:
- 10547671
- Publisher / Repository:
- British Ecological Society
- Date Published:
- Journal Name:
- Journal of Ecology
- Volume:
- 112
- Issue:
- 9
- ISSN:
- 0022-0477
- Page Range / eLocation ID:
- 2093 to 2106
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Fire activity is changing dramatically across the globe, with uncertain effects on ecosystem processes, especially below‐ground. Fire‐driven losses of soil carbon (C) are often assumed to occur primarily in the upper soil layers because the repeated combustion of above‐ground biomass limits organic matter inputs into surface soil. However, C losses from deeper soil may occur if frequent burning reduces root biomass inputs of C into deep soil layers or stimulates losses of C via leaching and priming.To assess the effects of fire on soil C, we sampled 12 plots in a 51‐year‐long fire frequency manipulation experiment in a temperate oak savanna, where variation in prescribed burning frequency has created a gradient in vegetation structure from closed‐canopy forest in unburned plots to open‐canopy savanna in frequently burned plots.Soil C stocks were nonlinearly related to fire frequency, with soil C peaking in savanna plots burned at an intermediate fire frequency and declining in the most frequently burned plots. Losses from deep soil pools were significant, with the absolute difference between intermediately burned plots versus most frequently burned plots more than doubling when the full 1 m sample was considered rather than the top 0–20 cm alone (losses of 98.5 Mg C/ha [−76%] and 42.3 Mg C/ha [−68%] in the full 1 m and 0–20 cm layers respectively). Compared to unburned forested plots, the most frequently burned plots had 65.8 Mg C/ha (−58%) less C in the full 1 m sample. Root biomass below the top 20 cm also declined by 39% with more frequent burning. Concurrent fire‐driven losses of nitrogen and gains in calcium and phosphorus suggest that burning may increase nitrogen limitation and play a key role in the calcium and phosphorus cycles in temperate savannas.Synthesis. Our results illustrate that fire‐driven losses in soil C and root biomass in deep soil layers may be critical factors regulating the net effect of shifting fire regimes on ecosystem C in forest‐savanna transitions. Projected changes in soil C with shifting fire frequencies in savannas may be 50% too low if they only consider changes in the topsoil.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1111/1365-2745.14386
