Does the extra nitrogen input from anthropogenic sources mean that more carbon from the atmosphere is being locked up in boreal and temperate forests? 'Yes' is the answer to emerge from the latest analysis.
Since the Industrial Revolution kicked into gear, at around the beginning of the nineteenth century, the atmospheric concentration of carbon dioxide has increased from 280 to 380 parts per million1. Starting a century later, there has been an even more dramatic increase in the industrial fixation of atmospheric nitrogen into agricultural fertilizers, and in the production of nitrogen oxides from combustion processes in vehicles and industry. Together, these anthropogenic nitrogen sources now exceed natural biological nitrogen fixation2.
The fraction of this nitrogen that ultimately becomes deposited on temperate and boreal forests might increase tree growth, and the sequestration of atmospheric CO2 into wood3. Or perhaps the capacity of these forests to use extra nitrogen for growth is marginal or non-existent4. On page 848 of this issue, Magnani and colleagues5 report that nitrogen deposition can, indeed, drive carbon sequestration in most temperate and boreal forests.
The carbon and nitrogen cycles in forests interact in many and complex ways6,7. But increased input of nitrogen to forests with a poor natural supply evidently increases foliar biomass and the concentration of the photosynthesizing enzyme RUBISCO (Fig. 1a, overleaf). These changes lead to greater capture of energy in sunlight and greater photosynthesis per unit area of forest. They also cause a shift in the allocation of plant carbon from roots and their symbiotic mycorrhizal fungi towards above-ground structures such as woody tree trunks (Fig. 1b). As carbon in wood has a much longer residence time than the carbon allocated to short-lived roots and the associated mycorrhizal fungi, nitrogen deposition will increase carbon sequestration by forest ecosystems. The retarding effect of nitrogen on the decomposition of plant litter and soil organic matter has the same effect8.
Hundreds of nitrogen-fertilizer trials have documented growth increases in northern temperate and boreal forests6, especially in areas away from major sources of pollution. Some researchers have thus surmised that nitrogen deposition will also increase carbon sequestration in forest ecosystems (see ref. 3, for example). But others infer that the unnaturally high levels of nitrogen that leach to groundwater and stream water in areas of high nitrogen deposition indicate that extra nitrogen does not stimulate carbon sequestration by trees9, and that relatively little of the nitrogen added to forests becomes immobilized in wood4.
To address this controversy, Magnani et al.5 analysed the carbon balance across a network of forest sites encompassing the levels of nitrogen deposition experienced by most of western Europe and the conterminous United States. The carbon balance, or net ecosystem production (NEP), is the balance between ecosystem carbon fixation through photosynthesis and its release back to the atmosphere through plant and soil respiration. This balance was chiefly determined by the so-called eddy-covariance method — the simultaneous measurement of the upwards or downwards movements of bubbles of air (created by turbulence over rough surfaces such as forest canopies), and of the concentration of CO2 in the bubbles moving up or down. Very high temporal resolution can be obtained with this technique, although diurnal, seasonal and interannual variations in NEP are typically of most interest. Another advantage of this approach is that the equipment, which is placed several metres above the forest canopy, sees and integrates the 'footprint' of a relatively large area (often 1 km2 or more). Problems are that the carbon balances of both natural and managed forests vary considerably during the lifetime of a forest, and that the method is so new that it has not yet been used through the life cycle of individual forests.
To distinguish the true effect of nitrogen deposition, Magnani et al.5 first had to remove the major sources of variability in NEP, which are related to natural disturbances (such as fire) or anthropogenic disturbances (such as forest harvesting). They did this by modelling the mean lifetime NEP using eddy-covariance data from temporal sequences accounting for different stages in forest lifetime. Both ecosystem production and respiration correlated positively with mean annual temperature, but NEP did not because its two components responded similarly to temperature.
Magnani et al.5 could then demonstrate a strong positive relation between mean lifetime NEP — that is, carbon balance — and the deposition of nitrogen. Their analysis captures annual deposition levels of up to 15 kg nitrogen per hectare (10,000 m2), and is thus representative of about 90% of western Europe and the conterminous United States. It does not refute the view that some forests are nitrogen-saturated4,9. But it does show that, in most forests of the Northern Hemisphere at higher latitude, forest capacity to respond to extra nitrogen with increased carbon sequestration is not near saturation.
However, a note of caution is that whereas the carbon balance clearly responds positively to additions of nitrogen, this response is expected to be an order of magnitude smaller10 than the regression presented by Magnani et al.5 suggests (see Fig. 3d on page 850). A hectare of nitrogen-limited forest is supposed to sequester roughly 30 kg carbon in the trees and an additional 10 kg carbon in the soil per kilogram of nitrogen added to the ecosystem10; this response becomes drastically reduced at rates of nitrogen addition above 50 kg nitrogen per hectare per year10,11. The exact relation between nitrogen addition and the response of the carbon balance thus needs clarification.
Such clarification is essential because of the serious practical questions that arise in this area of research. Should forests be fertilized with nitrogen to sequester more atmospheric CO2? And should strategies to reduce levels of CO2 emissions include forest fertilization to produce more wood and wood products to replace fossil fuels, or to replace concrete as a building material (large amounts of CO2 are generated during concrete production)? Positive answers require the demonstration that such artificially fertilized forests can return reasonably quickly to their natural nitrogen-limited condition, and that the environmental impact, including effects on biodiversity, is minimal within and downstream of the treated area.
On the other hand, some forests in densely populated areas in particular are certainly nitrogen-saturated, and they suffer from the adverse impact of excess nitrogen deposition from the atmosphere9. These effects include excessive leaching of nitrate and changes in the species composition of the flora9. Safeguarding the health of these forests in the long run will require reductions of uncontrolled nitrogen emissions.