The nitrogen paradox: pollution control can reduce soil carbon sequestration

A study highlights a possible side effect of combating air pollution: the reduction of carbon storage in the soil

by Matteo Cavallito

 

Air pollution is a global problem and initiatives to counter the phenomenon are obviously necessary. In some circumstances, however, such interventions can generate paradoxical consequences, thus complicating climate mitigation. This is the case with actions taken over decades to improve air quality which, a study published in the journal Ecology now claims, may have contributed to a reduction in soil carbon accumulation in an American forest area.

The research, conducted by scientists from the University of Michigan and the University of Wisconsin, analysed the rates of nitrogen deposition in the soil, which had decreased due to international agreements to curb pollution.

The fight against pollution

“In the Northern Hemisphere, anthropogenic nitrogen (N) deposition contributed to the enhancement of the global terrestrial carbon (C) sink, partially offsetting CO2 emissions,” the study explains. This effect, “was determined to be driven, in part, by the suppression of microbial activity associated with the breakdown of soil organic matter.”

Since the start of pollution control efforts in the 1970s, however, “atmospheric N deposition has declined globally, and the consequences of this decline are unknown.”

The authors assessed the impact on soil carbon accumulation and associated microbial activities through a 24-year study based on experimental nitrogen deposition. The investigation, started in 1994, was followed by years of further observations that showed the progressive loss of accumulated carbon after the end of the deposition treatment as a result of mechanical changes in microbial activity.

The experiment

The study covered an area in Michigan that stretches 500 km and includes deciduous forests dominated by maple trees. Four experimental sites were located here, each consisting of six 30-by-30-metre plots. Three of these received 30 kilos of nitrogen in the form of nitrates each year for 24 years until 2017. The deposition “substantially increased soil organic matter (SOM) accumulation in the organic horizon (i.e., forest floor) (+51%) and the mineral soil (+18%).”

This increase “is attributable to mechanistic changes in microbial function, namely the suppression of microbial activity associated with the breakdown soil organic carbon”.

Five years after the end of the administrations, the researchers collected 15 forest soil and 10 mineral soil samples at each site to produce homogeneous collections of the two soil types for each plot. The results of the analyses were very clear: the extra carbon stored in the two different soil types had disappeared.

Impact on climate

According to the scientists, explains a statement, the reduction of nitrogen pollution from the atmosphere therefore helps to accelerate decomposition. Thus reducing the amount of carbon stored. According to the authors, this reduction in the ability of soils to act as carbon sinks could undermine efforts to mitigate global warming. Especially if similar patterns were to be confirmed in other forest systems on the planet.

“Our observations reveal that the global terrestrial C sink in the Northern Hemisphere, which offsets 15%–30% of anthropogenic CO2 emissions in Earth’s atmosphere, could decline due to the reduction in atmospheric N deposition, potentially enhancing climate warming,” the study explains. “These findings will be critical in informing Earth System Models of how to parametrize projected forest C stocks in response to reduced anthropogenic N deposition.”