Though rivers occupy 0.58 per cent of the non-glaciated global land area, they receive 0.72 billion tons of terrestrial carbon per year
iStock Photo for representation.
Agriculture and urbanisation are likely speeding up the process of breakdown of plant litter in rivers and streams globally, according to a new study.
This could contribute to increased greenhouse gas emissions and disrupt the food chain, the study published in Science warned.
Leaves that reach the rivers are decomposed by bacteria and fungi. They are, in turn, consumed by insects, which are then preyed upon by fish. Faster decomposition rates mean the carbon is released into the atmosphere even before the insects get a chance to absorb the carbon from the leaf.
“When human activities change the fundamental ways rivers work, it is concerning. Increases in decomposition rates may be problematic for the global carbon cycle and animals, like insects and fish, that live in streams,” Krista Capps, co-author of the study and associate professor at the University of Georgia, said in a statement.
The plant litter forms the food resources for aquatic lifeforms. The plant matter is likely being lost to the atmosphere as carbon dioxide.
Earth’s terrestrial ecosystems produce over 100 billion tons of plant detritus annually. When the plant matter reaches waterbodies, its fate — whether it is locked into long-term storage, converted to greenhouse gases, or incorporated into the food web — is determined by the rate at which it is decomposed.
Though rivers occupy 0.58 per cent of the non-glaciated land area, they receive 0.72 billion tons of terrestrial carbon per year.
Credit: Oakland University
Capps and team found a gap in our understanding of decompression rates of plant organic matter in the river and their drivers such as climate, geology, vegetation, water quality, and soils. This, according to the team, is particularly relevant for the tropics and lower-income economies as their rivers are understudied compared to the northern temperate zones.
So the team collected field data from 550 rivers around the globe and then turned to predictive modelling and machine-learning algorithms to fill in those gaps.
More than 150 researchers in 40 countries contributed samples using a standardised field assay based on the decomposition of small pieces of cotton fabric.
This assay estimated cellulose decomposition — the most abundant organic polymer on the planet and a main constituent of plant litter.
Their study found that decomposition rates generally increase with decreasing latitude, with accelerated rates observed in tropical regions such as Central America, the Amazon basin, Western Africa, and the Indo-Pacific.
Further, many areas in middle latitudes with known human impacts — central Europe, eastern China, central North America, southeastern South America, and Japan — also saw elevated decomposition rates while boreal forests exhibit slower rates, especially in northern Asia, eastern Scandinavia, and northeastern Canada.
The drivers of increased decomposition rates, according to the team, are higher temperatures and increased nutrient concentrations.
“Both of these factors are impacted by human activities,” David Costello, co-author of the study and an associate professor at Kent State, said in a statement. “Reducing human impacts on decomposition will keep more carbon in rivers, preventing it from entering the atmosphere as carbon dioxide and contributing to climate change,” the expert added.
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