Ecologists find nitrogen pollution boosts plant growth in tropics by 20 percent - relevance for bioenergy
These findings could be significant to the bioenergy community, because they would imply that biochar based carbon-negative bioenergy systems ("slash-and-char") could be even more viable than estimated: their capacity to store atmospheric carbon dioxide into soils, via biochar obtained from regrown forests and energy crops, would be enhanced because of the increased plant growth resulting from atmospheric nitrogen fertilization.
Faster plant growth means the tropics will take in more carbon dioxide than previously thought, though long-term climate effects are unclear, the researchers say. Over the next century, nitrogen pollution is expected to steadily rise, with the most dramatic increases in rapidly developing tropical regions such as India, South America, Africa and Southeast Asia.
Major nitrogen sources are N fertilizer, applied to farmland to improve crop yield, which affects ecosystems downwind by seeping into runoff water and evaporating into the atmosphere. Industrial burning and forest clearing also pumps nitrogen into the air.
Using data from more than 100 previously published studies, David LeBauer, graduate researcher of Earth system science at UCI and lead author of the study, and Kathleen Treseder, associate professor of ecology and evolutionary biology at UCI, analyzed global trends in nitrogen’s effect on growth rates in ecosystems ranging from tropical forests and grasslands to wetlands and tundra. Nitrogen, they found, increased plant growth in all ecosystems except for deserts (figure, click to enlarge).
Surprisingly, tropical forests that were seasonally dry, located in mountainous regions or had regrown from slash-and-burn agriculture also responded to added nitrogen. Although these are not the tropical forests that typically come to mind, they collectively account for more than half of the world’s tropical forests.
Tropical grasslands responded too, but here the picture is more complex, as tropical grasslands are also limited by precipitation. The proportional increase in grassland productivity was constant across a rainfall gradient, but total growth increase was greatest at high levels of precipitation:
energy :: sustainability ::biomass :: bioenergy :: biofuels :: tropics :: forests :: slash-and-burn :: slash-and-char :: biochar :: plant growth :: nitrogen :: climate change ::
Scientists believed added nitrogen would have little effect in the tropics because plants there typically have ample nitrogen and are constrained by low levels of phosphorus. If one necessary plant nutrient is in short supply – in this case phosphorus – plant growth will be poor, even if other nutrients such as nitrogen are abundant. Experiments in the study added nitrogen at the high end of ambient nitrogen pollution to test the maximum potential response.
It is difficult to predict the long-term effects of nitrogen on global climate change. One factor will be the degree to which humans change natural ecosystems, for example by cutting down or burning the tropical forests. Further, climate change may determine whether these areas grow back as forests or if they are replaced by grasslands or deserts. It also is unknown how nitrogen will affect the fate of carbon once plants die and begin to decompose.
What is clear is that we need to consider how nitrogen pollution interacts with carbon dioxide pollution. Our study is a step toward understanding the far-reaching effects of nitrogen pollution and how it may change our climate. - David LeBauerThe scientists' work was supported by the National Science Foundation, the Department of Energy and a fellowship from the Kearney Foundation for Soil Science.
LeBauer, David S., and Kathleen K. Treseder. 2008. "Nitrogen limitation of net primary productivity in terrestrial ecosystems is globally distributed", Ecology 89:371–379 (in press).
LeBauer, David: Nitrogen Limitation of Net Primary Productivity - research page at the UCI.
UC Irvine: Nitrogen pollution boosts plant growth in tropics by 20 percent - February 6, 2008.
Biopact: Biochar and carbon-negative bioenergy: boosts crop yields, fights climate change and reduces deforestation - January 28, 2008