High fire-derived nitrogen deposition on central African forests

Bauters, Marijn; Drake, Travis W.; Verbeeck, Hans; Bodé, Samuel; Hervé‐Fernández, Pedro; Zito, Phoebe; Podgorski, David C.; Boyemba, Faustin; Makelele, Isaac Ahanamungu; Ntaboba, Landry Cizungu; Spencer, Robert G. M.; Boeckx, Pascal

doi:10.1073/pnas.1714597115

Cited by 54 publications

(66 citation statements)

References 51 publications

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“…Interestingly, the proportion of relative abundance from condensed aromatics was higher in the forested area DOM compared to DOM from the deforested catchments (Figure a), potentially indicating the input of combustion byproducts via atmospheric deposition (Stubbins, Silva, Dittmar, & Van Stan, ). Indeed, recent research has shown that mixed lowland forests of the Congo capture high amounts of atmospheric aerosols relative to open field sites due to their large foliar surface areas upon which dry deposition can occur (Bauters et al, ).…”

Section: Discussionmentioning

confidence: 99%

Land‐use controls on carbon biogeochemistry in lowland streams of the Congo Basin

Drake

Podgorski

Dinga³

et al. 2020

Global Change Biology

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The flux and composition of carbon (C) from land to rivers represents a critical component of the global C cycle as well as a powerful integrator of landscape‐level processes. In the Congo Basin, an expansive network of streams and rivers transport and cycle terrigenous C sourced from the largest swathe of pristine tropical forest on Earth. Increasing rates of deforestation and conversion to agriculture in the Basin are altering the current regime of terrestrial‐to‐aquatic biogeochemical cycling of C. To investigate the role of deforestation on dissolved organic and inorganic C (DOC and DIC, respectively) biogeochemistry in the Congo Basin, six lowland streams that drain catchments of varying forest proportion (12%–77%) were sampled monthly for 1 year. Annual mean concentrations of DOC exhibited an asymptotic response to forest loss, while DIC concentrations increased continuously with forest loss. The isotopic signature of DIC became significantly more enriched with deforestation, indicating a shift in source and processes controlling DIC production. The composition of dissolved organic matter (DOM), as revealed by ultra‐high‐resolution mass spectrometry, indicated that deforested catchments export relatively more aliphatic and heteroatomic DOM sourced from microbial biomass in soils. The DOM compositional results imply that DOM from the deforested sites is more biolabile than DOM from the forest, consistent with the corresponding elevated stream CO2 concentrations. In short, forest loss results in significant and comprehensive shifts in the C biogeochemistry of the associated streams. It is apparent that land‐use conversion has the potential to dramatically affect the C cycle in the Congo Basin by reducing the downstream flux of stable, vascular‐plant derived DOC while increasing the transfer of biolabile soil C to the atmosphere.

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Section: Discussionmentioning

confidence: 99%

Land‐use controls on carbon biogeochemistry in lowland streams of the Congo Basin

Drake

Podgorski

Dinga³

et al. 2020

Global Change Biology

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“…The combination of high N deposition rates (Bauters et al. ) and the downregulation of symbiotic BNF (Bauters et al. ) suggests that lowland tropical forests in the Congo Basin are indeed N rich.…”

Section: Discussionmentioning

confidence: 99%

“…A final important assumption of the N paradox is that there is low N deposition in most remote sites. Again here, recent studies have shown that DON deposition can contribute largely to total N deposition although it has been ignored in most studies and that the atmospheric N deposition in central Africa is much higher than expected from simulations (Mace et al 2003, Cape et al 2011, Cornell 2011, Bauters et al 2018. The few studies that include DON deposition in N balances usually come up with N inputs that could outbalance reported N export ranges (Appendix S1: Table S2).…”

Section: Introductionmentioning

confidence: 90%

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Contrasting nitrogen fluxes in African tropical forests of the Congo Basin

Bauters

Verbeeck

Rütting

et al. 2019

Ecological Monographs

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The observation of high losses of bioavailable nitrogen (N) and N richness in tropical forests is paradoxical with an apparent lack of N input. Hence, the current concept asserts that biological nitrogen fixation (BNF) must be a major N input for tropical forests. However, well‐characterized N cycles are rare and geographically biased; organic N compounds are often neglected and soil gross N cycling is not well quantified. We conducted comprehensive N input and output measurements in four tropical forest types of the Congo Basin with contrasting biotic (mycorrhizal association) and abiotic (lowland–highland) environments. In 12 standardized setups, we monitored N deposition, throughfall, litterfall, leaching, and export during one hydrological year and completed this empirical N budget with nitrous oxide (N2O) flux measurement campaigns in both wet and dry season and in situ gross soil N transformations using 15N‐tracing and numerical modeling. We found that all forests showed a very tight soil N cycle, with gross mineralization to immobilization ratios (M/I) close to 1 and relatively low gross nitrification to mineralization ratios (N/M). This was in line with the observation of dissolved organic nitrogen (DON) dominating N losses for the most abundant, arbuscular mycorrhizal associated, lowland forest type, but in contrast with high losses of dissolved inorganic nitrogen (DIN) in all other forest types. Altogether, our observations show that different forest types in central Africa exhibit N fluxes of contrasting magnitudes and N‐species composition. In contrast to many Neotropical forests, our estimated N budgets of central African forests are imbalanced by a higher N input than output, with organic N contributing significantly to the input‐output balance. This suggests that important other losses that are unaccounted for (e.g., NOx and N2 as well as particulate N) might play a major role in the N cycle of mature African tropical forests.

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“…We also acknowledge N deposition may be underestimated due to the Tdep model's limited treatment of dry organic N, which can be particularly high in regions exposed to savanna fires where N‐rich, fire‐derived aerosols can be trapped by the forest canopy and enter the system via throughfall (Bauters et al. ). However, previous research in pine stands at Benning reported a flux of throughfall organic N of only 0.28 kg N·ha −1 ·yr −1 (Bhat et al.…”

Section: Discussionmentioning

confidence: 99%

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

Tierney

Hedin

Wurzburger

2019

Ecology

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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.

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High fire-derived nitrogen deposition on central African forests

Cited by 54 publications

References 51 publications

Land‐use controls on carbon biogeochemistry in lowland streams of the Congo Basin

Land‐use controls on carbon biogeochemistry in lowland streams of the Congo Basin

Contrasting nitrogen fluxes in African tropical forests of the Congo Basin

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

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