Nitrogen-depleting forests, grasslands; can affect animal growth

Low nitrogen forests are more likely to have slow growing plants with fewer leaves

Nitrogen availability in forests in Europe and North America has been declining since 1850. This may have slowed the growth rate of trees in these forests.

Nitrogen is everywhere. It is more abundant in the air than oxygen and constitutes 78% of the atmosphere. Nitrogen is also vital for life – it is essential for plant nutrition and therefore sustains all other beings.

But plants cannot use atmospheric nitrogen directly as they absorb carbon dioxide (CO2) for photosynthesis. For this they depend on a biogeochemical cycle which, with the help of certain bacteria or even lightning, combines the inert gas with other elements to form reactive compounds such as ammonia and nitric oxide and the “fix” in the ground.

Scientists have long known that this cycle is disrupted. Reactive nitrogen levels have increased tenfold since pre-industrial times due to the widespread use of synthetic nitrogen fertilizers and the burning of fossil fuels, according to a 2019 study published in Frontiers in Ecology and the Environment. This has caused algal blooms, created dead zones in the oceans and accelerated the loss of biological diversity in aquatic and terrestrial ecosystems.

A recent study, however, indicates that scientists have so far only partially understood the extent of this disruption and where it is taking place.

“There is both too much nitrogen and too little nitrogen on Earth,” said Rachel Mason, lead author of the study, published in the journal Science in April 2022. Just like excess nitrogen, decreasing nitrogen availability is also a cause for concern.

Marcos Fernández-Martínez, postdoctoral researcher at the Center for Ecological Research and Forestry Applications, Spain, said:

While forests with high nitrogen availability suffer from lower diversity as they allow some species to thrive at the expense of others, forests with low nitrogen availability are more likely to have slow-growing plants with less leaves.

His research on the impact of atmospheric CO2 concentrations on the declining nutritional status of European forests has been published in the journal Nature in March 2020.

Fernández-Martínez’s study is one of 100 research papers that Mason and his colleagues from the United States and Europe analyzed to gather data on the availability of nitrogen in ecosystems between 1750 and 2017. In their review article, they found that nitrogen availability is declining in “many non-agricultural terrestrial ecosystems” that do not receive additional reactive nitrogen due to human activities.

The decline is sustained in a range of terrestrial ecosystems, including forests in the United States and Europe as well as grasslands in Central and North America, dating back at least to the early 20th century.

CRISIS APPROACH

Mason’s study establishes that human activities are also to blame for declining nitrogen availability and identifies multiple environmental changes, particularly elevated atmospheric CO2 and rising global temperatures, as drivers.

Atmospheric CO2 concentration has increased by 50% since the 1750s, according to the US National Oceanic and Atmospheric Administration. This accelerated the rate of plant photosynthesis to a saturation point, leading to increased plant demand for nitrogen. Rising temperatures are also known to stretch growing seasons, adding to this demand.

The study therefore warns that large parts of Australia, sub-Saharan Africa, parts of Asia and South America and large swaths of boreal forest, which have remained free of reactive nitrogen additional, could lose their natural nitrogen deposits in a warming world.

The researchers, however, refrain from drawing a general conclusion because they found a high load of reactive nitrogen in non-agricultural ecosystems in China, Panama and the Democratic Republic of Congo as well as a reduced availability of nitrogen in Europe and North America.

Also, few data are available for other locations. For example, in India, researchers focused on nitrogen supply but not on availability, which is a more robust measure based on the difference between supply and demand.

Manaswi Raghurama, a research student at the National Center for Biological Sciences, Bengaluru, says there are data gaps because determining nitrogen levels in plants is expensive and time-consuming. Fernández-Martínez also says the fund crisis is a problem. This is why the link between climate change and the nitrogen cycle remains understudied, he says.

But understanding the nitrogen cycle in its entirety is crucial because it is a key component of amino acids that form the building blocks of plant proteins and enzymes.

Reduced nitrogen levels in plants can have a worrying impact on the health of grazing insects and mammals that depend on leaves for protein. Herbivores may initially react to the situation by increasing their consumption, but this ultimately affects their growth, survival, reproduction, and population size.

Mason’s study cites a PNAS paper published in 2020, in which researchers found a link between declining nitrogen levels in Konza Prairie, a grassland in Kansas, USA, and a 36% drop. abundance of grasshoppers over the past 30 years. . With a low protein concentration in the pollen, bees may have a harder time resisting pests and surviving through the winter. Grazing mammals on a low-protein diet have poor growth.

The story first appeared in the August 16-31 print issue of Down to earth magazine.




Teresa H. Sadler