Qualitative and quantitative changes in plant nitrogen acquisition induced by anthropogenic nitrogen deposition

Abstract

Because nitrogen is the mineral nutrient needed in largest amounts by plants, it is usually also the limiting factor for plant growth in terrestrial ecosystems (Vitousek & Howarth, 1991). Consequently, the deposition of oxidized and reduced N compounds will almost invariably have large effects in these systems, and because N availability not only regulates plant growth but also that of organisms at other trophic levels, disturbances of several ecosystem processes might occur. The alternations introduced by deposition of atmospheric N compounds are both of a quantitative and of a qualitative nature. Moreover, N deposition can have phytotoxic as well as growth-stimulating effects.This short commentary gives a personal view of some of the possible consequences of N deposition on plants. It refers particularly to the oral presentations given by Professor Heinz Rennenberg and Dr Marta Pere! z-Soba, and to the discussions held after their talks where appropriate. Separate attention is given to four different consequence of anthropogenic N-deposition : N-availability ; N-form, N-uptake by the shoot, and the period of N-uptake. Finally, I have tried to adopt an ecosystem perspective and discuss briefly the concept of critical loads of N.  The talk given by Professor Rennenberg focused on consequences of high loads of N for spruce and beech trees. It was shown that root uptake of NO $ − in particular, but also of NH % + , was low in a stand subjected to high loads of N and that this phenomenon could also be induced by N application to a forest with relatively low N-inputs. Preferential use of NH % + , and inhibition of uptake of NO $ − by NH % + had earlier been shown for conifers (Marschner, Ha$ ussling & George, 1991 ;Flaig & Mohr, 1992 ;Gezelius & Na$ sholm, 1993 ;Kronzucker, Siddiqi & Glass, 1997). It seems clear that trees can decrease N-uptake rates and thereby, at least partly, counteract effects of high loads of N. The mechanism behind such down-regulation of N uptake is not clear. Professor Rennenberg showed that N uptake rates of roots were inversely proportional to concentrations of some amino acids in phloem tissue of roots. Best fit was achieved for Glu and Gln whereas Arg, the other major nitrogenous constituent of the phloem, did not correlate with uptake rates of N. These observations suggest that Glu and\or Gln might have a role in regulating N uptake, although a correlation should not, of course, be interpreted as a causal relationship without further evidence, and more detailed studies on the regulation of N uptake rates would be helpful.High loads of N did not seem to affect trees negatively ; on the contrary, the trees were found to be growing vigorously. Information was given that the stand under investigation was not suffering from mineral nutrient deficiencies, indicating that soil supply of nutrients such as phosphorus and potassium could match the high rates of N supply. So one may ask -is there a problem with high loads of N ? Data presented showing high levels o...

Dynamic niche sharing in dry acidic grasslands -a 15N-labeling experiment

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Needle cytokinin content as a sensitive bioindicator of N pollution in Sitka spruce

“…Industrial and agricultural activity over the last century has increased the aerial deposition of both oxidized and reduced forms of nitrogen (Fowler et al . 1998), promoting the foliar uptake of nitrogen, over which trees do not appear to have effective feedback control (Näsholm 1998). The resulting increase in the N content of needles appears to induce the increase in cytokinin content.…”

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