nitrogen deposition has been considered to be a potential regulator of the carbon cycle. In the 1990s a number of careful studies 55 evaluated the potential effects of different forms of nitrogen deposition and concluded that global changes to the nitrogen cycle may have a serious impact on the global carbon cycle (see Plate 1). This analysis includes both deleterious effects in the core pollution areas and modest fertilizer effects in the margins of polluted airsheds. 56 It has also been shown, and is likely from first principles, that high N deposition is correlated with high ozone levels (because of the photochemical coupling of NO and O3), leading to a potential multiple-stress situation combining the effects of N loading and oxidant stress.57 This is of particular concern as these stresses, especially in deposition, are also implicated in changes to biodiversity.

Early experiments tend to support a “subsidy-stress” paradigm for N, in which N deposition can lead to increased growth up to a critical level, beyond which deleterious effects dominate. 58 The effects of previous land use may also play a major role in the vulnerability of systems to N stress, with the corresponding prior history of N budget changes and the physiology of current vegetation changing the quantitative relationships between N loading and ecosystem impacts.


The Carbon Cycle

Ecosystem science has played a major role in studies of the carbon cycle. Although there remain substantial uncertainties about the carbon cycle and how it may behave in the future, significant advances have been made. Progress in this area is critical because carbon cycle research forms the basis for setting targets in international negotiations to mitigate climate change.59 Understanding contemporary and possible future fluxes of carbon is the essential underpinning of sound policy to manage radiative forcing of the atmosphere. The development of accurate and reliable measurement techniques for carbon fluxes is a prerequisite for evaluating the success of measures undertaken to comply with the Framework Convention on Climate Change and to monitor international compliance with other treaty measures. Carbon cycle science is thus essential to international decision making.

By the late 1980s the budget of carbon dioxide in the atmosphere was obviously unbalanced (see Table 2.1). Carbon modeling suggested that fossil fuel and cement manufacture sources had essentially been balanced by ocean uptake and atmospheric accumulation.60 However, beginning in the 1970s, ecologists increasingly found that substantial emissions were being made to the atmosphere from land-use change (in the 1980s these sources were largely tropical). The land-use contribution was based on estimates from carbon budgets for land conversion and area converted. Thus, even the definition of the complete set of terms in the carbon cycle (industrial, marine, atmospheric, and terrestrial) required

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