Disentangling Drought and Nutrient Effects on Soil Carbon Dioxide and Methane Fluxes in a Tropical Forest

Irene Ramirez-Rojas,Clément Stahl,Ivan A. Janssens,Leandro Van Langenhove,Laëtitia Bréchet,Dolores Asensio,Jennifer L. Soong,Erik Verbruggen,Thomas Saint-Germain,Elodie A. Courtois

doi:10.3389/fenvs.2019.00180

Abstract

Tropical soils are a major contributor to the balance of greenhouse gas (GHG) fluxes in the atmosphere. Models of tropical GHG fluxes predict that both the frequency of drought events and changes in atmospheric deposition of nitrogen (N) will significantly affect dynamics of soil carbon dioxide (CO2) and methane (CH4) production and consumption. In this study, we examined the combined effect of a reduction in precipitation and an increase in nutrient availability on soil CO2 and CH4 fluxes in a primary French Guiana tropical forest. Drought conditions were simulated by intercepting precipitation falling through the forest canopy with tarpaulin roofs. Nutrient availability was manipulated through application of granular N and / or phosphorus (P) fertilizer to the soil. Soil water content (SWC) below the roofs decreased rapidly and stayed at continuously low values until roof removal, which as a consequence roughly doubled the duration of the dry season. After roof removal, SWC slowly increased but remained lower than in the control soils even after 2.5 months of wet-season precipitation. We showed that drought-imposed reduction in SWC decreased the CO2 emissions (i.e CO2 efflux), but strongly increased the CH4 emissions. N, P and N × P (i.e. NP) additions all significantly increased CO2 emission but had no effect on CH4 fluxes. In treatments where both fertilization and drought were applied, the positive effect of N, P and NP fertilization on CO2 efflux was reduced. After roof removal, soil CO2 efflux was more resilient in the control plots than in the fertilized plots while there was only a modest effect of roof removal on soil CH4 fluxes. Our results suggest that a combined increase in drought and nutrient availability in soil can locally increase the emissions of both CO2 and CH4 from tropical soils, for a long term.

Highlights

Climate models predict a range of changes in weather patterns in tropical forest regions (Feng et al, 2013)
One month after initiation of the throughfall exclusion treatment, Soil water content (SWC) of dry plots was roughly half that of control plots, and this difference remained over the course of the experiment (Figure 2A)
The throughfall exclusion treatment extended the duration of the dry season by at least a factor 2 maintaining a SWC of ∼10%, whereas control plots followed precipitation events resulted in SWC between 15 and 35% (Figure 2A)

Summary

Introduction

Climate models predict a range of changes in weather patterns in tropical forest regions (Feng et al, 2013). Across Amazonia, a drier and warmer climate is expected for the coming century, which is predicted to result in an increased frequency of drought events, and can put a strain on the Amazon’s ecological functioning (Duffy et al, 2015). Atmospheric deposition of Tropical Forest Soil GHG Fluxes nitrogen (N) can result in a stoichiometric imbalance of carbon (C) and N relative to phosphorus (P) in tropical biomes (Peñuelas et al, 2013). These global changes can have a broad impact on tropical ecosystem functioning and are predicted to lead to a severe disturbance of the Amazonian forest biome (Huntingford et al, 2004). Soil biological processes, which are an important determinant of forest functioning, remain poorly described in models focusing on forest ecosystems

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Discussion

Conclusion