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Carbon content of soil fractions varies with season, rainfall, and soil fertility across a lowland tropical moist forest gradient

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Abstract

Tropical forests contain some of the largest soil carbon (C) stocks on Earth, making them broadly relevant to terrestrial-climate feedbacks, yet our understanding of how their soil organic C (SOC) fractions vary over space and time is limited. We studied effects of season, fertility, and mean annual precipitation (MAP) on the C contents of soil fractions across 14 lowland forests in Panama. We measured free-debris, occluded-debris, and mineral-associated SOC fractions, as well as soluble C associated with each fraction. We hypothesized that mineral-associated SOC would be greatest in infertile, strongly weathered soils with large reactive mineral contents. We also hypothesized that the debris SOC fractions would accumulate during the dry season, reflecting seasonal increases in litterfall. To address this, we compared soil fractions in wet and dry seasons from fertile and infertile soils across a range of 1809–2864 mm MAP. The C content (mg C/g soil) of all soil fractions varied with fertility and MAP: specifically, free-debris SOC was greatest in wet, high-fertility soils, and occluded-debris SOC was greater in high-fertility than low-fertility soils. The mineral-associated SOC fraction, which contained the majority of bulk soil C, showed increasing C content with greater MAP in infertile sites, presumably driving similar spatial patterns in the bulk soil. Only the free-debris SOC fraction showed strong seasonal variation, increasing in mass during the dry season. Nitrogen behaved similarly to C. In summary, soil C contents increased with MAP in infertile sites but not fertile sites, driven by the mineral-associated SOC fraction. The dry season had greater free-debris SOC, but this seasonal trend was not apparent in bulk soil C, likely because of the small size of the free-debris SOC fraction. Thus, changes in the quantity and seasonality of precipitation, which are projected for tropical forests, might shift spatial and temporal patterns of soil C storage, which would in turn influence forest-climate feedbacks for this C-rich biome.

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Data availability

The raw data from this project is being submitted as Online Resource 1.

Code availability

The R script used in analyzing the data is being submitted as Online Resource 2.

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Acknowledgements

Funding was provided by NSF GSS Grant #BCS-1437591 and DOE Grant DE-SC0015898 to D. F. Cusack. We thank B. L. Turner for help selecting and characterizing sites and for providing constructive comments on the manuscript, and we thank Lucas Lu, Emma Friedl, Yun Choi, Amanda Lai, Shirley Lin, Dayana Agudo, and Aleksandra Bielnicka for laboratory support.

Funding

Funding was provided by NSF GSS Grant #BCS-1437591 and DOE Grant DE-SC0015898 to D. F. Cusack.

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The study was conceived and samples were collected by DFC. Laboratory analyses were performed by LHD, JK, AN, and MC. LHD analyzed the data and wrote the paper. All authors contributed to reviewing and editing the manuscript.

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Correspondence to Lee H. Dietterich.

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The authors declare no conflict of interest.

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Responsible Editor: Jonathan Sanderman.

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Dietterich, L.H., Karpman, J., Neupane, A. et al. Carbon content of soil fractions varies with season, rainfall, and soil fertility across a lowland tropical moist forest gradient. Biogeochemistry 155, 431–452 (2021). https://doi.org/10.1007/s10533-021-00836-1

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  • DOI: https://doi.org/10.1007/s10533-021-00836-1

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