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Conservative N cycling despite high atmospheric deposition in early successional African tropical lowland forests

Abstract

Background

Across the tropics, the share of secondary versus primary forests is strongly increasing. The high rate of biomass accumulation during this secondary succession relies on the availability of essential nutrients, such as nitrogen (N). Nitrogen primarily limits many young secondary forests in the tropics. However, recent studies have shown that forests of the Congo basin are subject to high inputs of atmospheric N deposition, potentially alleviating this N limitation in early succession.

Methods

To address this hypothesis, we assessed the N status along a successional gradient of secondary forests in the Congo basin. In a set-up of 18 plots implemented along six successional stages, we quantified year-round N deposition, N leaching, N2O emission and the N flux of litterfall and fine root assimilation. Additionally, we determined the N content and C:N stoichiometry for canopy leaves, fine roots, and litter, as well as δ15N of canopy leaves.

Results

We confirmed that these forests receive high amounts of atmospheric N deposition, with an increasing deposition as forest succession proceeds. Additionally, we noted lower C:N ratios, and higher N leaching losses, N2O emission, and foliar δ15N in older secondary forest (60 years). In contrast, higher foliar, litter and root C:N ratios, and lower foliar δ15N, N leaching, and N2O emission in young (< 20 years) secondary forest were observed.

Conclusions

Altogether, we show that despite high N deposition, this early forest succession still shows conservative N cycling characteristics, which are likely indicating N limitation early on in secondary forest succession. As secondary succession advances, the N cycle gradually becomes more open.

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Acknowledgements

The Belgian Development Cooperation has supported this research by funding the VLIR-UOS team project, forest ecosystem monitoring in the Congo (FORMONCO II). VLIR-UOS supports partnerships between universities and university colleges in Flanders (Belgium) and universities in Africa, latin America and Southeast Asia, looking for innovative responses to global and local challenges. FORMONCO II was a collaboration between researcher from Ghent University, Université de Lubumbashi, Université de Kisangani, Université Catholique de Bukavu. We specially thank the Professor Oswald Van Cleemput for accepting to conduct the ‘internal review’ of our manuscript.

Funding

The Belgian Development Cooperation has supported this research by funding the VLIR-UOS team project, forest ecosystem monitoring in the Congo (FORMONCO II).

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Authors and Affiliations

Authors

Contributions

MAI, MB, KV and PB designed the experiment; MAI and MB conducted the experiment and analyzed the samples; MAI and MB conducted the statistical analyses; MAI, with contributions from MB, PB and KV, wrote the paper; all co-authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Isaac Ahanamungu Makelele.

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Competing Interests

Author declare no competing interests.

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Makelele, I.A., Bauters, M., Verheyen, K. et al. Conservative N cycling despite high atmospheric deposition in early successional African tropical lowland forests. Plant Soil (2022). https://doi.org/10.1007/s11104-022-05473-7

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  • DOI: https://doi.org/10.1007/s11104-022-05473-7

Keywords

  • Congo basin
  • Nitrogen cycle
  • Secondary succession
  • Tropical forests biogeochemistry