dcyphr | Soil carbon loss by experimental warming in a tropical forest


The soil in tropical forests contain about ⅓ of the world’s carbon in soils. Scientists predict that global warming will cause soil instability, leading to the release of carbon dioxide (CO2). The extra CO2 release will worsen climate change effects. The researchers test this theory by experimentally warming the soil in Barro Colorado Island, Panama. They found massive amounts of CO2 emission after two years. Meanwhile, other factors that may affect the soil carbon, such as microbial or enzymatic activity, had not changed. Thus, tropical soil carbon is extremely sensitive to increasing temperature.


The authors aim to examine the effect of warming on the carbon emission from tropical soils.


Tropical forests are essential for the global carbon cycle. The carbon cycle is the exchange of carbon through the atmosphere, soil, and ecosystem. Tropical forests have a lot of biomass. 30-50% of the world’s carbon comes from transpiration from the tropical soils. The large amount of carbon in the soil primarily comes from decomposition. Thus, if the carbon release from soils in the tropics changes slightly, there will be drastic effects in the world.


Scientists are concerned that global warming will result in more CO2 released from soils. Past studies in temperate and arctic forests support this. Some researchers hypothesized that theoretically, tropical forests should not be as affected. Kinetic theory proposes that the soil will not be as sensitive to warming since it is at a higher temperature (and lower latitude). However, this theoretical sensitivity may not translate to actual sensitivity. 


Several factors may affect how sensitive the soil is to temperature. Soil warming often results in soil drying. This will affect respiration of the soil. Furthermore, soil warming may affect microbial communities and the decomposition process.


The researchers call the experiment in this paper SWELTR (Soil Warming in Lowland Tropical Rainforest). They warmed approximately 120 m3 of soil and had control plots. They heated the soil for two years to mimic the predicted 4℃ in temperature at tropical latitudes by 2100.


The soil that was heated emitted 55% more carbon than the control after two years (Figure 1). The carbon generally came from decomposition of heterotrophs, leaf litter, and other organic matter. Soil respiration from heterotrophs increased for plots that were heated. Respiration derived from autotrophs did not change much (Figures 2 and 3).


The heated soils experienced more soil drying and a CO2 efflux increase. However, the effect of soil moisture on CO2 release was not significant. The researchers also did not see any signs of adaptation by the ecosystem. The levels of nitrogen and phosphorus stayed the same rather than decrease as expected. Most enzymes still functioned in the same way. Microbial carbon-use efficiency (CUE) did not change significantly. However, there was an annual increase in the amount of microbes (microbial biomass carbon).


The findings that there is more CO2 release at lower altitudes contradict the theoretical expectations. Tropical forests that were warmed released more CO2 than temperate forests that were warmed. However, the data from this study is consistent with satellite and atmospheric data. This further demonstrates the large role of tropical forests on the global carbon cycle.


Various ecosystem properties likely play a role in the higher sensitivity to temperatures in tropical forests. The sensitivity is not just dependent on kinetic theory. The researchers cannot provide a mechanism at this point. But, the unexpected constant in various factors support this. They find that more organic matter is degraded.


The unexpected findings that CO2 would increase with warming suggest a positive feedback. The increased CO2 would increase warming which would further increase CO2 release. Thus, previous studies underestimate the amount of CO2 that would be released as global temperatures increase. The researchers expect that this CO2 release would decrease in the long run due to substrate limitation. However, researchers do not know how long this will take and how this will affect the global feedback mechanism.


The researchers built warm and control plots. They had thermostats to control and measure the temperature every hour. The temperature of the warm plot was 4℃ more than the control plot. They measured CO2 levels every two weeks using an infrared gas analyzer. They measured temperature and soil moisture with various probes.