September 25, 2019

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Soil Phosphorus Availability Moderates Soil CO2 Fluxes Along Tropical Rainfall Gradient

Fertile tropical soils have larger CO2 fluxes with seasonal rainfall compared with infertile soils.

The Wet:Dry season CO2 flux ratios were best predicted by a multiple regression including the Wet:Dry soil moisture ratio, soil resin phosphorus (P), and extractable base cations (R2 = 0.79, n = 15). The strongest univariate predictor of Wet:Dry respiration ratios was resin P (R2 = 0.41, a), followed by Wet:Dry soil moisture ratios (R2 = 0.29, b), and extractable base cations (R2 = 0.20, c). Across sites, the Wet:Dry season respiration ratio ranged from 0.81 to 3.6, indicating declines (0.81) and large increases (3.6) in respiration during the wet versus dry season. The extremes were both in sites at the wettest end of the rainfall gradient.

[Reprinted with permission from Cusack, D., et al. "Seasonal Changes in Soil Respiration Linked to Soil Moisture and Phosphorus Availability Along a Tropical Rainfall Gradient." Biogeochemistry 145 235–54 (2019). DOI:10.1007/s10533-019-00602-4.]

The Science

Humid tropical forests contain some of the largest soil carbon (C) stocks on Earth, yet scientists are uncertain about how carbon dioxide (CO2) fluxes will respond to climate change in this biome. This study revealed a strong seasonal shift in soil respiration from the wet to dry season across 15 distinct tropical forest sites in Panama along rainfall and soil fertility gradients. Soil moisture, air temperature, and rainfall together were the best predictors of instantaneous soil respiration. Somewhat surprisingly, soil phosphorus and base cations were the strongest predictors of spatial variation in the magnitude of season change in soil respiration, which did not follow rainfall trends.

The research sites cover a three-fold range in soil C stock, two-fold range in rainfall, five soil orders, over 25 geological formations, 20-fold range in base cations, and >100-fold range in extractable phosphorus. Thus, research results are robust and likely applicable to a much broader geographic area than the study region.

The Impact

Overall, nutrient availability regulated soil respiration responses to increased moisture during the wet season, while low soil moisture uniformly suppressed soil respiration across sites during the dry season. Phosphorus availability might therefore regulate feedbacks to climate change among humid tropical forests.


This study suggests that variation in soil phosphorus (P) and base cation availability are related to the magnitude of soil respiration seasonality across tropical forests. While shifts in soil moisture were an important driver of soil CO2 flux rates, as expected, variation in soil nutrients appeared to override the influence of natural rainfall gradient. Soil respiration was suppressed in the most infertile sites during the wet versus dry season. These results indicate that accurately predicting how drying will affect tropical soil C losses will require incorporation of P and base cation availability into ecosystem models, as well as explicit microbial and root respiration relationships to moisture.

Principal Investigator

Daniela Cusack
Colorado State University & STRI
[email protected]

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]


Funding was provided by the National Science Foundation (NSF) GSS Grant #BCS-1437591 and the U.S. Department of Energy (DOE) Office of Science Early Career Research Program Grant DE-SC0015898.

Related Links


Cusack, D., et al. "Seasonal Changes in Soil Respiration Linked to Soil Moisture and Phosphorus Availability Along a Tropical Rainfall Gradient." Biogeochemistry 145 235–54  (2019).