Nitrogen is the principle nutrient of plants, and scientists have long considered it crucial to forest growth. Now a study proposes that existing theories of how nitrogen cycles in and out of forests may share a central flaw: By focusing on forests in the Northern Hemisphere, scientists mistakenly came to consider inorganic nitrogen especially critical to forest ecosystems, when in fact it's a byproduct of industrial pollution.
As scientists studied forests throughout North America, they regularly measured high levels of inorganic nitrogen in streams leaving those forests, implying that the forests were taking up the inorganic nitrogen they needed and letting the rest wash away. But, oddly, the growth of temperate forests in these regions seemed limited. Scientists were further intrigued by studies showing that trees given nitrogen fertilizer absorbed the nutrients and benefited accordingly.
Ecologists Steven Perakis and Lars Hedin at Cornell University in Ithaca, New York, wondered whether pollution from nitrogen--particularly inorganic nitrogen--which has doubled in the last century, could be muddying the picture. So they headed south of the equator to pristine forests spread across Argentina and Chile, where they sampled the chemistry of 100 streams in 26 regions.
Perakis and Hedin were surprised to find that 70% of the dissolved nitrogen was organic, suggesting that inorganic nitrogen may be less central to the forest's natural cycle than studies in the Northern Hemisphere suggested. They also discovered that different plant and soil types, forest ages, and climate--all factors expected to impact the amount and type of dissolved nitrogen--only slightly altered river chemistry. In the 24 January issue of Nature, the pair proposes that atmospheric pollution is the main source of inorganic nitrogen in forests, and that this pollution in the north has been masking the reliance of all forests on organic nitrogen.
The new research calls into question going theories about how forests cycle nitrogen, says John Aber, a biogeochemist at the University of New Hampshire in Durham. Aber notes that because the nitrogen cycle is closely linked to the cycling of carbon in and out of forests, the latter may need to be reevaluated as well.