Low-nitrogen forests are more likely to have plants growing slowly with fewer leaves
Nitrogen availability in the forests of Europe and North America has been declining since 1850. This could have slowed down the rate of tree growth in these forests Nitrogen is all pervasive. It is more abundant in the air than oxygen and constitutes 78 per cent of the atmosphere. Nitrogen is also vital to life — it is essential for plant nutrition and thus, sustains all other beings.
But plants cannot use atmospheric nitrogen directly the way they absorb carbon dioxide (CO2) for photosynthesis. For this, they depend on a biogeochemical cycle that, with the help of some bacteria or even lightning, combines the inert gas with other elements to form reactive compounds like ammonia and nitric oxide and "fix" them in the soil.
Scientists have known for quite a while that this cycle is getting disrupted. The levels of reactive nitrogen have increased tenfold since the pre-industrial era due to rampant use of synthetic nitrogen fertiliser and 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 oceans and accelerated biological diversity loss in aquatic and terrestrial ecosystems.
A recent study, however, states that scientists have so far only partly understood the scale of this disruption and where it is unfolding.
“There is both too much nitrogen and too little nitrogen on Earth at the same time,” said Rachel Mason, lead author of the study, published in the journal Science in April 2022. Just like too much of nitrogen, declining availability of nitrogen is also a cause of worry.
Marcos Fernández-Martínez, a postdoctoral researcher at the Centre for Ecological Research and Forestry Applications, Spain, said:
While forests with high nitrogen availability suffer from less diversity as they allow certain species to flourish at the cost of others, low-nitrogen forests are more likely to have plants growing slowly with fewer leaves.
His research on the impact of atmospheric CO2 concentrations on declining nutritional status of European forests was published in the journal Nature in March 2020.
The study by Fernández-Martínez is one of the 100 research papers that Mason and her colleagues from the United States and Europe have analysed to collate data on nitrogen availability across ecosystems between 1750 and 2017. In their review paper, they have found that nitrogen availability is declining in “many non-agricultural terrestrial ecosystems" that do not receive additional reactive nitrogen because of human activities.
The decline is sustained across a range of terrestrial ecosystems, including forests in the US and Europe as well as grasslands in central and north America, dating at least as far back as the early 20th century.
Mason’s study establishes that human activities are also to be blamed for the decline in nitrogen availability and identifies multiple environmental changes, particularly elevated atmospheric CO2 and rising global temperatures, as the drivers.
Atmospheric CO2 concentration has increased by 50 per cent since the 1750s, as per US National Oceanic and Atmospheric Administration. This has quickened the rate of photosynthesis in plants up to a saturation point, leading to higher nitrogen demand by plants. Rising temperatures is also known to stretch the growing seasons, adding to this demand.
The study thus warns that large parts of Australia, sub-Saharan Africa, parts of Asia and South America and vast swathes of boreal forest, that have remained free of additional reactive nitrogen, could lose their natural deposits of nitrogen in a warming world.
The researchers, however, refrain from drawing a general conclusion as they found a high load of reactive nitrogen in non-agricultural ecosystems in China, Panama and the Democratic Republic of the Congo alongside reduced nitrogen availability in Europe and North America.
Besides, not much data is available for other places. For instance, in India, researchers have focused on nitrogen supply but not on availability, which is a more robust metric based on the difference between supply and demand.
Manaswi Raghurama, a research student at the National Centre for Biological Sciences in Bengaluru, says that data gaps exist because determining nitrogen levels in plants is expensive and time-consuming. Fernández-Martínez also says fund crunch is a problem. This is the reason the link between climate change and the nitrogen cycle remains understudied, he says.
But understanding the nitrogen cycle in its entirety is crucial as it is a key component of amino acids that form the building blocks of plant proteins and enzymes.
Reduced levels of nitrogen in plants can have a worrying impact on the health of insects and grazing mammals that depend on leaves for protein. Herbivores may initially respond to the situation by increasing consumption, but it eventually affects their growth, survival, reproduction and population size.
The study by Mason cites a PNAS paper published in 2020, in which researchers have found a link between the decline in nitrogen levels in Konza Prairie, a grassland in Kansas, US, and a 36 per cent drop in grasshopper abundance over the past 30 years. With a low protein concentration in pollen, bees might find it harder to resist pests and survive over winter. Grazing mammals on a low-protein diet have poor growth.
The story was first published in the August 16-31 print issue of Down To Earth magazine.
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