Nitrogen emissions aid carbon absorption
human activities add loads of nitrogen to forests, posing threats. Scientists at the us Oregon State University, however, say this may be helpful; but only to some extent. They found that artificial nitrogen fixation through burning of fossil fuels and industrial activities leads to increased absorption of carbon by temperate and boreal forests. They call it 'carbon sequestration'.
This does not mean the solution to global warming is to add huge volumes of nitrogen to our forests; heres a note of caution. The nitrogen emitted should be between 15 to 50 kg per hectare per year. The response of the forests is drastically reduced if the nitrogen added crosses the limit.
"We found that continuous deposition of relatively small amounts of nitrogen annually increased carbon uptake," says Beverly Law, a professor of forest science at the university and one of the authors of the study published in the June 14 issue of Nature. "This is called human fixation of nitrogen, which now is much greater than natural fixation (from lightning and nitrogen fixing plants)," he says.
"Here we are talking about the reactive nitrogen in the atmosphere, which results from human activities such as combustion and industrial processes. It is not through direct application of fertilisers to the soil," the researchers point out.
Increased nitrogen inputs result in increase of biomass of foliage and also the concentration of photosynthesising enzyme rubisco (rubilose-1, 5-biphosphate carboxylase), the study said.
This results in greater capture of sunlight and greater photosynthesis per unit area of forests. The researchers said that even at low levels of nitrogen addition, the carbon sequestration was significant.
Nitrogen is deposited either dry or wet (in rain) directly on to the foliage, which then assimilate it. Some studies show that about 80-90 per cent of nitrogen deposition is captured by the canopy on top and the remaining goes to the soil.
"If nitrogen were to deposit on the soil, much of it would be used by soil microbes and returned to the atmosphere in soil respiration. The deposition on the canopy bypasses this. The results are not comparable to direct fertiliser application to the soil because we are interested in net carbon sequestration, which is the sum total of photosynthesis and respiration," Law added.
The researchers claim that this is the first attempt to separate the effects of nitrogen deposition on forests from other variables affecting carbon release like forest age, logging, wildfires, disease or insect epidemics. Their analysis traced up to 15 kg nitrogen deposition per hectare and is representative of 90 per cent of western Europe and the us.
This study was inspired by another research by Elizabeth Holland of the Colorado-based National Centre for Atmospheric Research published in 2005, which said that increase of emissions of ammonia oxides of nitrogen over the last 150 years has accelerated nitrogen deposition in forests.
That study had expressed uncertainty on the impacts of nitrogen deposition on the carbon cycle. It had also said it was likely that forests fertilised by such nitrogen may have increased capability of sequestering carbon.