Decomposition of organic matter is a process, which includes mostly physical breakdown and biochemical transformation of complex organic molecules into simpler organic and inorganic molecules. The decomposition of organic matter is an important contributor to ecosystem respiration, which together with photosynthesis controls the net carbon emission from ecosystems. Subterranean ecosystems are extended all over the subsurface of our planet, and lack of light and consequently of photosynthetic activity. Understanding the drivers that control the dynamics of the decomposition processes in the deep subterranean spaces is important because they might differ from those at surface, due to factors as low species diversity and abundance, low microbial biomass, nutrient poor conditions, less pronounced variation of temperature, and higher humidity inside cave. Here, we review the existing studies of organic matter decomposition in caves. Decomposition rates are known from only nine caves representing four biogeographic regions, including Europe, North and South America. Most of the studies were performed in the aquatic compartment of caves. The decay of nine different organic substrates have been followed and the incubation time varied from 36 to 439 days. From a cave located in Australia the mass loss of leaf material from three plant species was investigated after 9 days incubation in the terrestrial compartment of the cave. Based on these observations, litter quality seems to be an important driver of decomposition in caves, and invertebrates have a stimulating effect on the decomposition within individual cave zones. The degree of connection to the surface also influences decomposition rate inside the cave. The lack of standard data among the studies is currently the major impediment to evaluate how differently the process proceeds in the underground compared to the surface, and to disentangle the main drivers of decomposition in caves across biomes. Improving our understanding of organic matter decomposition dynamics in caves will require the standardization of protocols and evaluation of the process over space and time, and a better comprehension on how decomposition changes over latitudinal, altitudinal and depth gradients.
Reference: Ravn N., Michelsen M. & Reboleira A.S.P.S. (2020). Decomposition of Organic Matter in Caves. Frontiers in Ecology and Evolution, 8: 554651. http://doi.org/10.3389/fevo.2020.554651