17
Deposition of Nutrients and Pollutants
to Ecosystems
Lars O. Hedin
Role of Atmospheric Deposition
Our view of how ecosystems depend upon atmospheric deposition has changed considerably over
the past four decades. Early work primarily focused
on the role of atmospheric nutrient sources in ecosystems that directly depend on precipitation as the
dominant water source, such as ombrotrophic wetlands and surface waters that drain such wetlands
(Tamm 1958; Gorham 1961). However this view
changed dramatically in the 1970s, in response to
suggestions that atmospheric deposition of strong
mineral acids, or "acid rain," can affect nutrient cycles in a variety of ecosystems, including forests,
wetlands, streams, rivers, and lakes (Oden et al.
1968; Likens and Bormann 1974; Gorham 1976;
National Research Council 1986). These concerns
about effects of acid rain (more appropriately
termed "acid deposition") on European and North
American ecosystems spurred an unprecedented effort during the 1970s and 1980s, aimed at improving our understanding of how nutrients, pollutants,
and chemical elements enter ecosystems from the
atmosphere. In fact, most modem methods and theories for studying atmospheric deposition derive
from this period.
A picture gradually emerged, which emphasized
the role that the atmosphere plays in delivering a
host of different pollutants to ecosystems in industrial regions of the world (i.e., hydrogen ions [H+],
sulfate [SO~-], nitrate [NO;], ammonium [NHt],
ammonia [NH3], sulfur dioxide [S02]' nitrogen dioxide [N0 2 ], hydrochloric acid [HCl], heavy metals, and ozone [0 3 ]), We now also know that atmospheric deposition causes, or at least influences,
some of our most challenging environmental concerns, including freshwater and soil acidification
(Schindler 1988; Gorham 1989), soil base cation
loss (Hedin et al. 1994; Hedin and Likens 1996),
forest decline (Schultze 1989; Schulze et al. 1989;
Gawal et al. 1996), ecosystem N saturation (Agren
and Bosatta 1988; Aber et al. 1998), estuarine eutrophication (Howarth et al. 1996), heavy metal accumulation (Gawal et al. 1996), and direct phytotoxic effects of 0 3 and S02 (Lovett 1994).
There is also growing evidence that atmospheric
deposition is important for maintaining ecosystem
nutrient cycles in relatively unpolluted regions of
the world. This means that, even in its unadulterated form, the unpolluted atmosphere can be an important source of nutrients to ecosystems. For example, studies in southern Chile point to the
atmosphere as the source of nitrogen (N) and base
cations for maintaining nutrient cycles in unpolluted temperate rain forests (Hedin et al. 1995; Hedin and Likens 1996; Perez et al. 1998). Others
have proposed that atmospheric inputs of N, phosphorus (P), and base cations are important for tropical forests that grow on old soils (e.g., Prospero et
al. 1981; Lewis 1981; Jordan 1985; Swap 1992;
Lesack and Me1ack 1996; Kennedy et al. 1998;
Chadwick et al. 1999). A recent study in the Hawaiian Islands (Chadwick et al. 1999) indicated that
montane tropical rain forests on highly weathered
soils depend on P inputs from the long-distance
transport of P-containing dust aerosols, originating
as far away as dry regions of China. Such longdistance transport of dust aerosols has also been
implicated as a source of iron and other trace elements to low-productivity areas of the world's
265
Deposition of Nutrients and Pollutants
to Ecosystems
Lars O. Hedin
Role of Atmospheric Deposition
Our view of how ecosystems depend upon atmospheric deposition has changed considerably over
the past four decades. Early work primarily focused
on the role of atmospheric nutrient sources in ecosystems that directly depend on precipitation as the
dominant water source, such as ombrotrophic wetlands and surface waters that drain such wetlands
(Tamm 1958; Gorham 1961). However this view
changed dramatically in the 1970s, in response to
suggestions that atmospheric deposition of strong
mineral acids, or "acid rain," can affect nutrient cycles in a variety of ecosystems, including forests,
wetlands, streams, rivers, and lakes (Oden et al.
1968; Likens and Bormann 1974; Gorham 1976;
National Research Council 1986). These concerns
about effects of acid rain (more appropriately
termed "acid deposition") on European and North
American ecosystems spurred an unprecedented effort during the 1970s and 1980s, aimed at improving our understanding of how nutrients, pollutants,
and chemical elements enter ecosystems from the
atmosphere. In fact, most modem methods and theories for studying atmospheric deposition derive
from this period.
A picture gradually emerged, which emphasized
the role that the atmosphere plays in delivering a
host of different pollutants to ecosystems in industrial regions of the world (i.e., hydrogen ions [H+],
sulfate [SO~-], nitrate [NO;], ammonium [NHt],
ammonia [NH3], sulfur dioxide [S02]' nitrogen dioxide [N0 2 ], hydrochloric acid [HCl], heavy metals, and ozone [0 3 ]), We now also know that atmospheric deposition causes, or at least influences,
some of our most challenging environmental concerns, including freshwater and soil acidification
(Schindler 1988; Gorham 1989), soil base cation
loss (Hedin et al. 1994; Hedin and Likens 1996),
forest decline (Schultze 1989; Schulze et al. 1989;
Gawal et al. 1996), ecosystem N saturation (Agren
and Bosatta 1988; Aber et al. 1998), estuarine eutrophication (Howarth et al. 1996), heavy metal accumulation (Gawal et al. 1996), and direct phytotoxic effects of 0 3 and S02 (Lovett 1994).
There is also growing evidence that atmospheric
deposition is important for maintaining ecosystem
nutrient cycles in relatively unpolluted regions of
the world. This means that, even in its unadulterated form, the unpolluted atmosphere can be an important source of nutrients to ecosystems. For example, studies in southern Chile point to the
atmosphere as the source of nitrogen (N) and base
cations for maintaining nutrient cycles in unpolluted temperate rain forests (Hedin et al. 1995; Hedin and Likens 1996; Perez et al. 1998). Others
have proposed that atmospheric inputs of N, phosphorus (P), and base cations are important for tropical forests that grow on old soils (e.g., Prospero et
al. 1981; Lewis 1981; Jordan 1985; Swap 1992;
Lesack and Me1ack 1996; Kennedy et al. 1998;
Chadwick et al. 1999). A recent study in the Hawaiian Islands (Chadwick et al. 1999) indicated that
montane tropical rain forests on highly weathered
soils depend on P inputs from the long-distance
transport of P-containing dust aerosols, originating
as far away as dry regions of China. Such longdistance transport of dust aerosols has also been
implicated as a source of iron and other trace elements to low-productivity areas of the world's
265