Could fertilising trees save the climate?

New Scientist (Environment) has a fascinating story by Catherine Brahic: Should we "dope" trees with nitrogen fertiliser to engineer a cooler global climate? New findings suggest the nutrient could be a switch for determining how much solar energy forests in Earth's cooler regions reflect back out into space. Scott Ollinger of the University of New Hampshire, Durham, and colleagues measured the concentrations of nitrogen in forest canopies in 181 plots across the US, sampling trees that were anywhere from 15 to 500 years old.

The team compared this data with satellite data of albedo - a measure of how much solar energy is reflected by various surfaces - and data on the amount of carbon absorbed by the forests. "The principal findings are that the nitrogen concentration of forest canopies is a good predictor of their capacity to absorb carbon dioxide, and that forests with high nitrogen levels reflect more solar radiation than nitrogen-poor forests," says Ollinger.

"A simplistic implication is that fertilising trees with nitrogen, or simply planting trees that have naturally higher nitrogen concentrations, might help offset climate change." While this is theoretically possible, Ollinger warns that more research should be done "before we run around spraying trees with nitrogen".

For starters, the exact relationship between nitrogen and albedo remains a mystery. Nitrogen could act like a switch, changing the structure and cellular properties of leaves so that they become more mirror-like. If this is the case, nitrogen fertilisation might work. But if there is a correlation between the albedo of certain tree types and the amount of nitrogen they hold, then governments would have to plant species that naturally have high albedo in order to take advantage of the effect.

There could also be serious disadvantages to nitrogen doping. "Increasing the amount of nitrogen in the environment can have its own negative consequences, including leaching of nitrate to groundwater and emissions of nitrous oxide - itself a greenhouse gas - from soils." Tree species that hold a lot of nitrogen also tend to need more water, which means doping trees with nitrogen could contribute to drying up streams and groundwater reservoirs. "This would be an undesirable consequence in dry climates," says Ollinger….

The world tree Yggdrasil with the assorted animals that live in it and on it: From the 17th century Icelandic manuscript AM 738 4to, now in the care of the Árni Magnússon Institute in Iceland