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Can we see the nitrate from the trees? Long-term linkages between tropical forest productivity and stream nitrogen concentrations

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A Publisher Correction to this article was published on 06 December 2023

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Abstract

High abundance of trees capable of biological N-fixation (henceforth “N-fixers”) in tropical forests has been hypothesized to drive higher stream nitrate (NO3) concentrations compared to temperate counterparts. However, to date there have been no empirical linkages of stream NO3 concentrations with the productivity of tropical forests. Here, we combined three unique long-term datasets from La Selva Biological Station, Costa Rica: 21 years of (1) mean annual stream NO3-N concentrations in six stream sites within the same watershed, (2) annual growth of trees, and (3) annual leaf litterfall. We hypothesized that years of greater growth of N-fixer tree species and of greater leaf litterfall would be correlated with higher stream water NO3-N concentrations. We also hypothesized that landscape position mediates these relationships, with growth of N-fixer trees on adjacent slopes being more strongly correlated to stream NO3-N than the growth of such trees on upland plateau sites. We found that mean annual stream NO3-N concentrations were consistently high (160–260 µg L−1). There was substantial interannual variation in leaf litterfall (inter-year range: 5.4 to 8.1 Mg ha−1 year−1), growth of N-fixers (inter-year range: 1.2 to 2.2 Mg ha−1 year−1), and growth of all other tree species (inter-year range: 2.1 to 3.2 Mg ha−1 year−1). To assess stream NO3-N relationships with forest productivity, we used water conductivity to account for dilution resulting from variable discharge. We found that NO3-N concentrations were positively related to the annual growth of the N-fixers on nearby slopes, and were negatively correlated with annual leaf litterfall. Stream NO3-N concentrations were not related to the growth of N-fixers or other tree species in the more removed plateau areas. Using a mass balance, we estimated that symbiotic N fixation can account for 7–29% of NO3 export. Both the growth of adjacent N-fixers and landscape-wide leaf litterfall are important drivers of the inter-annual variability of stream NO3-N concentrations. Our results suggest that predicted changes in precipitation extremes due to climate change will alter N dynamics in tropical forests both directly, by altering discharge and export, and indirectly, by altering N-fixer tree productivity.

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Data Availability

All data used in this study are publicly available: STREAMS data: Pringle, C.M. 2021. STREAMS Project: Emergent landscape patterns in stream ecosystem processes resulting from groundwater/surface water interactions ver 537849. Environmental Data Initiative. https://doi.org/10.6073/pasta/6cb25ffd722e79690090e387efd2cfe7. CARBONO data: Clark, D. A. 2021. Biweekly fine litterfall in the 18 CARBONO Project plots, La Selva Biological Station, October 1997-October 2018. Dryad. https://doi.org/10.5061/dryad.dfn2z351r. Clark, D. A., and D. B. Clark. 2021. Two decades of annual landscape-scale tree growth and dynamics in old-growth tropical rainforest in the CARBONO Project, La Selva Biological Station, 1997–2018. Dryad. https://doi.org/10.5061/dryad.51c59zw8n

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Acknowledgements

We thank the Organization for Tropical Studies for logistical support and for the local climatic monitoring; L. Campos, W. Miranda, and M. Hidalgo for their expert field measurements; and the Government of Costa Rica for annual research permits. Funding was provided by the U.S. Department of Energy, the Andrew W. Mellon Foundation, Conservation International's TEAM Initiative, the U.S. National Science Foundation long-term research in environmental biology (LTREB), and personal funds.

Funding

STREAMS project NSF LTREB awards: DEB 9528434, DEB 0075339, DEB 055463, DEB 1122389, DEB 165869, DEB-2154228. CARBONO project: US Department of Energy, the Andrew W. Mellon Foundation, Conservation International’s TEAM Initiative, the U.S. National Science Foundation LTREB program, and personal funds.

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CMP started the long-term data collection of the STREAMS project. AR, MA and NSM also helped with long-term data collection of the STREAMS project. DAC started and oversaw the long-term data collection of CARBONO project. MA conceived and conducted the data analyses. All authors contributed to the writing of the manuscript.

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Correspondence to Marcelo Ardón.

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Responsible Editor: Amy Marcarelli.

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Ardón, M., Clark, D.A., Marzolf, N.S. et al. Can we see the nitrate from the trees? Long-term linkages between tropical forest productivity and stream nitrogen concentrations. Biogeochemistry 163, 201–218 (2023). https://doi.org/10.1007/s10533-023-01030-1

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  • DOI: https://doi.org/10.1007/s10533-023-01030-1

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