Tropical forests play a crucial role in the global carbon cycle, serving as a significant terrestrial carbon sink. However, recent research indicates that climate change poses a serious threat to the carbon balance of these ecosystems. A study conducted by Lawrence Livermore National Laboratory (LLNL), Colorado State University, and the Smithsonian Tropical Research Institute reveals that warming and drying of tropical forest soils may increase soil carbon vulnerability. This research, published in Nature, sheds light on the potential impacts of climate change on tropical ecosystems.
The study conducted in tropical forests in Panama during climate manipulation experiments showed alarming results. In situ heating of soil by 4°C and exclusion of 50% of rainfall led to an increase in carbon-14 in the CO2 released by the soil, indicating a higher average age of the carbon. Interestingly, the mechanisms behind this shift differed between warming and drying. Warming accelerated decomposition of older carbon, depleting newer carbon, while drying suppressed the decomposition of newer carbon inputs, leading to an increase in the contributions of older carbon to CO2 release.
The findings of this study have significant implications for the carbon balance of tropical forests. Climate projections suggest that the tropics will experience both warmer and drier conditions in the future, with increasing drought intensity. This could result in a net loss of global soil carbon to the atmosphere. The shorter mean residence time for carbon in tropical terrestrial ecosystems means that any changes in carbon inputs or outputs could have rapid consequences for the carbon balance and climate feedbacks of these ecosystems.
Previous studies in tropical forests have mostly focused on total CO2 flux rates, which are essential for determining the overall carbon balance. However, understanding the mechanisms behind changes in carbon dynamics requires analyzing carbon-14 values. These values indicate the average age of the carbon sources being metabolized and released as CO2. Young carbon is fixed from the atmosphere in recent years, while older carbon is enriched or depleted in carbon-14 relative to the current atmosphere.
The research conducted by LLNL and its partners used carbon-14 and carbon-13 isotopes of soil-respired CO2 to determine the impact of warming and drying on soil carbon. The results showed that soil warming increased the carbon-14 of respired CO2 during the wet season, leading to a greater release of “bomb” carbon. This indicates a higher decomposition rate of older soil carbon under warmed conditions. In contrast, drying reduced total soil CO2 release but increased the carbon-14 of respired CO2 by limiting the delivery of fresh carbon to decomposers.
The research highlights the vulnerability of tropical forest soils to climate change. Warming and drying have the potential to alter the carbon balance of these ecosystems, leading to increased soil carbon losses and negative impacts on carbon storage. Understanding the mechanisms behind these changes is crucial for developing effective strategies to mitigate the effects of climate change on tropical forests. Further research and monitoring are essential to assess the long-term impacts of climate change on these valuable ecosystems.