Nitrogen availability is an important constraint on NPP (Vitousek et al., 1997),
although phosphorus and calcium may be more important limiting nutrients in
many tropical and sub-tropical regions (Matson, 1999). Reactive nitrogen is
released into the atmosphere in the form of nitrogen oxides (NOx)
during fossil fuel and biomass combustion and ammonia emitted by industrial
regions, animal husbandry and fertiliser use (Chapter 4).
This nitrogen is then deposited fairly near to the source, and can act as a
fertiliser for terrestrial plants. There has been a rapid increase in reactive
nitrogen deposition over the last 150 years (Vitousek et al., 1997; Holland
et al., 1999). Much field evidence on nitrogen fertilisation effects on plants
(e.g., Chapin, 1980; Vitousek and Howarth, 1991; Bergh et al., 1999) supports
the hypothesis that additional nitrogen deposition will result in increased
NPP, including the growth of trees in Europe (Spiecker et al., 1996). There
is also evidence (Fog, 1988; Bryant et al., 1998) that N fertilisation enhances
the formation of modified soil organic matter and thus increases the residence
time of carbon in soils.
Tracer experiments with addition of the stable isotope 15N provide insight into the short-term fate of deposited reactive nitrogen (Gundersen et al., 1998). It is clear from these experiments that most of the added N added to the soil surface is retained in the ecosystem rather than being leached out via water transport or returned to the atmosphere in gaseous form (as N2, NO, N2O or NH3). Studies have also shown that the tracer is found initially in the soil (Nadelhoffer et al., 1999), but that it enters the vegetation after a few years (Clark 1977; Schimel and Chapin, 1996; Delgado et al., 1996; Schulze, 2000).
There is an upper limit to the amount of added N that can fertilise plant growth. This limit is thought to have been reached in highly polluted regions of Europe. With nitrogen saturation, ecosystems are no longer able to process the incoming nitrogen deposition, and may also suffer from deleterious effects of associated pollutants such as ozone (O3), nutrient imbalance, and aluminium toxicity (Schulze et al., 1989; Aber et al., 1998).
Current tropospheric O3 concentrations in Europe and North America cause visible leaf injury on a range of crop and tree species and have been shown to reduce the growth and yield of crops and young trees in experimental studies. The longer-term effects of O3 on forest productivity are less certain, although significant negative associations between ozone exposure and forest growth have been reported in North America (Mclaughlin and Percy, 2000) and in central Europe (Braun et al., 2000). O3 is taken up through stomata, so decreased stomatal conductance at elevated CO2 may reduce the effects of O3 (Semenov et al., 1998, 1999). There is also evidence of significant interactions between O3 and soil water availability in effects on stem growth or NPP from field studies (e.g., Mclaughlin and Downing, 1995) and from modelling studies (e.g., Ollinger et al., 1997). The regional impacts of O3 on NPP elsewhere in the world are uncertain, although significant impacts on forests have been reported close to major cities. Fowler et al. (2000) estimate that the proportion of global forests exposed to potentially damaging ozone concentrations will increase from about 25% in 1990 to about 50% in 2100.
Other possible negative effects of industrially generated pollution on plant growth include effects of soil acidification due to deposition of NO3- and SO42-. Severe forest decline has been observed in regions with high sulphate deposition, for instance in parts of eastern Europe and southern China. The wider effects are less certain and depend on soil sensitivity. Fowler et al. (2000) estimate that 8% of global forest cover received an annual sulphate deposition above an estimated threshold for effects on acid sensitive soils, and that this will increase to 17% in 2050. The most significant long-term effect of continued acid deposition for forest productivity may be through depletion of base cations, with evidence of both increased leaching rates and decreased foliar concentrations (Mclaughlin and Percy, 2000), although the link between these changes in nutrient cycles and NPP needs to be quantified.
It is very likely that there are upper limits to carbon storage in ecosystems due to mechanical and resource constraints on the amount of above ground biomass and physical limits to the amount of organic carbon that can be held in soils (Scholes et al., 1999). It is also generally expected that increased above-ground NPP (production of leaves and stem) will to some extent be counterbalanced by an increased rate of turnover of the biomass as upper limits are approached.
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