Inputs to terrestrial communities

Weathering of parent bedrock and soil is generally the dominant source of nutrients such as calcium, iron, magnesium, phosphorus and potassium, which may then be taken up via the roots of plants. Mechanical weathering is caused by processes such as freezing of water and the growth of roots in crevices. However, much more important to the release of plant nutrients are chemical weathering processes. Of particular significance is carbonation, in which carbonic acid (H2CO3) reacts with minerals to release ions, such as calcium and potassium. Simple dissolution of minerals in water also makes nutrients available from rock and soil, and so do hydrolytic reactions involving organic acids released by the ectomycorrhizal fungi (see Section 13.8.1) associated with plant roots (Figure 18.3).

Atmospheric CO2 is the source of the carbon content of terrestrial communities. Similarly, gaseous nitrogen from the atmosphere provides most of the nitrogen content of communities. Several types of bacteria and blue-green algae possess inputs sometimes balance outputs... but not always nutrient inputs . . . ... from the weathering of rock and soil,...

Figure 18.3 Ectomycorrhizal fungi associated with tree roots can mobilize phosphorus, potassium, calcium and magnesium from solid mineral substrates through organic acid secretion, and these nutrients then become available to the host plant via the fungal mycelium. (After Landeweert et al., 2001.)

the enzyme nitrogenase and convert atmospheric nitrogen to soluble ammonium (NH4) ions, which can then be taken up through the roots and used by plants. All terrestrial ecosystems receive some available nitrogen through the activity of free-living bacteria, but communities containing plants such as legumes and alder trees (Alnus spp.), with their root nodules containing symbiotic nitrogen-fixing bacteria (see Section 13.10), may receive a very substantial proportion of their nitrogen in this way. More than 80 kg ha-1 year-1 of nitrogen was supplied to a stand of alder by biological nitrogen fixation, for example, compared with 1-2 kg ha-1 year-1 from rainfall (Bormann & Gordon, 1984); and nitrogen fixation by legumes can be even more dramatic: values in the range 100-300 kg ha-1 year-1 are not unusual.

Other nutrients from the atmosphere become available to communities as wetfall (in rain, snow and fog) or dryfall (settling of particles during periods without rain, and gaseous uptake). Rain is not pure water but contains chemicals derived from a number of sources: (i) trace gases, such as oxides of sulfur and nitrogen; (ii) aerosols produced when tiny water droplets from the oceans evaporate in the atmosphere and leave behind particles rich in sodium, magnesium, chloride and sulfate; and (iii) dust particles from fires, volcanoes and windstorms, often rich in calcium, potassium and sulfate. The constituents of rainfall that serve as nuclei for raindrop formation make up the rainout component, whereas other constituents, both par-ticulate and gaseous, are cleansed from the atmosphere as the rain falls - these are the washout component (Waring & Schlesinger,

1985). The nutrient concentrations in rain are highest early in a rainstorm, but fall subsequently as the atmosphere is progressively cleansed. Snow scavenges chemicals from the atmosphere less effectively than rain, but tiny fog droplets have particularly high ionic concentrations. Nutrients dissolved in precipitation mostly become available to plants when the water reaches the soil and can be taken up by the plant roots. However, some are absorbed by leaves directly.

Dryfall can be a particularly important process in communities with a long dry season. In four Spanish oak forests (Quercus pyrenaica) situated along a rainfall gradient, for example, dryfall sometimes accounted for more than half of the atmospheric input to the tree canopy of magnesium, manganese, iron, phosphorus, potassium, zinc and copper (Figure 18.4). For most elements, the importance of dryfall was more marked in forests in drier environments. However, dryfall was not insignificant for forests in wetter locations. Figure 18.4 also plots for each nutrient the annual forest demand (annual increase in above-ground biomass multiplied by the mineral concentration in the biomass). Annual deposition of many elements in wetfall and dryfall was much greater than needed to satisfy demand (e.g. Cl, S, Na, Zn). But for other elements, and especially for forests in dryer environments, annual atmospheric inputs more or less matched demand (e.g. P, K, Mn, Mg) or were inadequate (N, Ca). Of course element deficits would be greater if root productivity had been taken into account, and other sources of nutrient input must be particularly significant for a number of these elements.

While we may conceive of wetfall and dryfall inputs arriving vertically, part of the pattern of nutrient income to a forest depends on its ability to intercept horizontally driven air-borne nutrients. This was demonstrated for mixed deciduous forests in New York State when the aptly named Weathers et al. (2001) showed that inputs of sulfur, nitrogen and calcium at the forest edge were 17-56% greater than in its interior. The widespread tendency for forests to become fragmented as a result of human activities is likely to have had unexpected consequences for their nutrient budgets because more fragmented forests have a greater proportion of edge habitat.

Streamwater plays a major role in the output of nutrients from terrestrial ecosystems (see Section 18.3). However, in a few cases, stream flow can provide a significant input to terrestrial communities when, after flooding, material is deposited in floodplains.

Last, and by no means least, human activities contribute significant inputs of nutrients to many communities. For example, the amounts of CO2 and oxides of nitrogen and sulfur in the atmosphere have been increased by the burning of fossil fuels, and the concentrations of nitrate and phosphate in stream-water have been raised by agricultural practices and sewage disposal. These changes have far-reaching consequences, which will be discussed later.

Figure 18.3 Ectomycorrhizal fungi associated with tree roots can mobilize phosphorus, potassium, calcium and magnesium from solid mineral substrates through organic acid secretion, and these nutrients then become available to the host plant via the fungal mycelium. (After Landeweert et al., 2001.)

... from hydrological inputs...

... and from human activities

Figure 18.4 Annual atmospheric deposition as wetfaU (WF) and dryfaU (DF) compared to annual nutrient demand (ND; to account for above-ground tree growth) for four oak forests along a rainfall gradient (S1 wettest, S4 driest) in Spain. (After Marcos & Lancho, 2002.)

Figure 18.4 Annual atmospheric deposition as wetfaU (WF) and dryfaU (DF) compared to annual nutrient demand (ND; to account for above-ground tree growth) for four oak forests along a rainfall gradient (S1 wettest, S4 driest) in Spain. (After Marcos & Lancho, 2002.)

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