Nitrogen Pollution Boost Plant Growth
Overview from Laurens Rademakers on Biopact.com*: A study by UC Irvine ecologists finds that excess nitrogen in tropical forests boosts plant growth, countering the belief that such tropical ecosystems would not respond to nitrogen pollution. Surprisingly, not only pristine rainforests but those regrown from slash-and-burn agriculture - which make up more than half the world's tropical forests - also responded to the added nitrogen. The research results appear in the February issue of the journal Ecology.
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.
Scientists have long known that nitrogen increases plant growth. Indeed, this is the reason that nitrogen fertilizer is applied to farmlands to improve crop yields. However, we know less about what happens when we add nitrogen to natural ecosystems. This is important because pollution from fertilizer use and fossil fuel combustion has already doubled the availability of nitrogen to plants worldwide. In many places, natural ecosystems receive rates of nitrogen from pollution similar to the rates of fertilizer applied by farmers. Over the next century, nitrogen pollution is expected to steadily rise, with the most dramatic increases in rapidly developing tropical regions that include India, South America, Africa, and Southeast Asia.
We conducted a survey of 126 previously published results from nitrogen addition experiments. To better understand the distribution of ecosystem responses to nitrogen, we combined these results using a quantitative statistical approach known as meta-analysis. We found that addition of nitrogen increases plant growth by 30% on average. Increases were found in almost every type of ecosystem around the world.
We were most surprised that tropical forests responded to nitrogen because geochemical theory had previously predicted that the tropics would not respond to nitrogen fertilization. This is because tropical forests are expected to be phosphorus, rather than nitrogen, limited. The reason tropical forests should be phosphorus limited is that in the tropics, soil is old and heavily weathered, so rock-derived nutrients (phosphorus in particular) have been washed (leached) from the soil. By contrast, rocks (and soils) are relatively younger at higher latitudes and elevations because of geological uplift and physical weathering. At the same time, rocks are younger in the temperate regions because glaciations reset soil development. Furthermore, decomposition occurs more rapidly in the tropics because temperature and rainfall are high, so nitrogen is quickly recycled from dead plant material. By contrast, decomposition is slower at higher latitudes.
When tropical forests are cut down for agriculture, much of the nitrogen is lost from the system. This loss of fertility eventually results in declining crop production, and the fields are abandoned. Even though phosphorus remains scarce, nitrogen has become even more limiting. Furthermore, nitrogen takes longer to accumulate in the system through nitrogen fixation and atmospheric deposition.
We did not find enough studies to test if pristine lowland tropical forests respond to nitrogen. These are the ecosystems on which the original hypothesis was based, but we found important exceptions among the dry, disturbed, and montane tropical forests that constitute more than half of the world's tropical forests. The greatest responses overall were observed in in Hawai'ian forests on young volcanic soils where plant growth more than doubled.
The important point for global change is that humans can affect forest response to N by clear cutting and burning forests. This breaks the tight cycling of N and allows it to be lost to the atmosphere or washed away, so the regrowing forest has to build up a store of nitrogen again.
Tropical grasslands also responded, but we found that precipitation also played a key role. Because precipitation controls both nitrogen availability from decomposition and plant demand for nitrogen, the proportional response of grasslands to nitrogen remained constant across large precipitation gradients. This result provides support for coupled carbon-nitrogen biogeochemical models such as CENTURY (Schimel 1996).
We hope that our results will provide information that can be incorporated into models of future global change. Although these results suggest an increase in plant growth as nitrogen pollution rises, it is difficult to assess the long-term effect of nitrogen on carbon sequestration. One important factor will be the degree to which humans change the distribution of ecosystems, for example by the destruction of tropical forests. In addition, we can not tell how nitrogen will affect the fate of carbon once plants die and begin to decompose.
LeBauer, David and Kathleen K. Treseder. "Nitrogen limitation of net primary productivity in terrestrial ecosystems is globally distributed", Ecology 89, February 2008, in press.
Figure 1. Response ratios for overall mean and individual biomes exposed to nitrogen fertilizer. A response ratio of 1.2 indicates a 20% relative growth increase; mean and 95% C.I. responses are shown. The response ratio (R) is calculated as the ratio of plant growth in fertilized to control plots, and statistics are performed on the natural log of this ratio. Thus, when the 95% CI does not intersect the R=1 line, we consider the response significant. Non-overlapping CI are significantly different from one another.
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This paper has potentially important implications. One possible outcome is that increasing nitrogen pollution (perhaps from soot inputs from biomass burning) could lead to higher productivity of tropical forests.
It's not entirely clear if this is happening in lowland forests, but in other forests this might be important, according to this paper.
Such an effect might account for the widespread observations of rising productivity (and increasing carbon sequestration) in nutrient-starved old-growth Amazonian forests, where annual smoke plumes from biomass burning are often huge.
Smithsonian Tropical Research Institute