Soil carbon stocks across tropical forests of Panama regulated by base cation effects on fine roots
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Tropical forests are the most carbon (C)-rich ecosystems on Earth, containing 25–40% of global terrestrial C stocks. While large-scale quantification of aboveground biomass in tropical forests has improved recently, soil C dynamics remain one of the largest sources of uncertainty in Earth system models, which inhibits our ability to predict future climate. Globally, soil texture and climate predict ≤ 30% of the variation in soil C stocks, so ecosystem models often predict soil C using measures of aboveground plant growth. However, this approach can underestimate tropical soil C stocks, and has proven inaccurate when compared with data for soil C in data-rich northern ecosystems. By quantifying soil organic C stocks to 1 m depth for 48 humid tropical forest plots across gradients of rainfall and soil fertility in Panama, we show that soil C does not correlate with common predictors used in models, such as plant biomass or litter production. Instead, a structural equation model including base cations, soil clay content, and rainfall as exogenous factors and root biomass as an endogenous factor predicted nearly 50% of the variation in tropical soil C stocks, indicating a strong indirect effect of base cation availability on tropical soil C storage. Including soil base cations in C cycle models, and thus emphasizing mechanistic links among nutrients, root biomass, and soil C stocks, will improve prediction of climate-soil feedbacks in tropical forests.
KeywordsAboveground biomass Clay Litterfall Phosphorus Precipitation Rainforest
Funding was provided by NSF GSS Grant #BCS-1437591 and DOE Grant DE-SC0015898 to D. F. Cusack, and NERC Grant NE/J011169/1 to O. T. Lewis. We thank Julio Rodriguez, Didimo Urena, David Brassfield, Evert Carlos Green, and Christian Harris for field support, and Dayana Agudo, Aleksandra Bielnicka, Dianne de la Cruz, Tania Romero, Irene Torres, Tyler Schappe and Stanley L. Walet for laboratory support. Assistance with map and graphics was provided by Matt Zebrowski, cartographer, at UCLA. Data supporting the conclusions is available in tables, figures, and SI.
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