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assessment of changes in the phosphorus status of forest ...

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Below, <strong>the</strong>se hypo<strong>the</strong>ses are be<strong>in</strong>g discussed on <strong>the</strong> basis <strong>of</strong> a critical literature review<br />

<strong>in</strong>clud<strong>in</strong>g data from <strong>the</strong> long-term studies. Moreover, <strong>the</strong>re is very little compiled<br />

<strong>in</strong>formation on <strong>the</strong> use <strong>of</strong> o<strong>the</strong>r plant parts (leaf or needle litter, f<strong>in</strong>e roots) to assess<br />

<strong>the</strong> P nutritional <strong>status</strong> <strong>of</strong> trees, which may be used as a better <strong>in</strong>dicator. There is very<br />

little known <strong>in</strong>formation about <strong>the</strong> consequences on plant growth, decomposer<br />

activity and o<strong>the</strong>r ecosystem functions for various P levels <strong>in</strong> <strong>forest</strong> ecosystems. The<br />

review will consider some <strong>of</strong> <strong>the</strong>m by <strong>in</strong>clud<strong>in</strong>g relevant data from <strong>the</strong> literature.<br />

1.1 Changes <strong>in</strong> P supply <strong>in</strong> <strong>forest</strong> soils due to acidify<strong>in</strong>g <strong>in</strong>puts.<br />

Acidify<strong>in</strong>g <strong>in</strong>puts can decrease <strong>the</strong> plant <strong>phosphorus</strong> <strong>status</strong> (normally observed <strong>in</strong> <strong>the</strong><br />

foliage samples), which is related to acidification and Al toxicity caus<strong>in</strong>g a decrease<br />

<strong>in</strong> P supply <strong>in</strong> soils or reduced P uptake capacity. A number <strong>of</strong> soil processes may be<br />

associated to such <strong>changes</strong> <strong>in</strong> P availability and uptake through <strong>changes</strong> <strong>in</strong> vitality <strong>of</strong><br />

<strong>the</strong> mycorrhizal fungi. Several studies conducted <strong>in</strong> temperate <strong>forest</strong>s have presented<br />

evidence <strong>of</strong> <strong>phosphorus</strong> limitation <strong>in</strong> <strong>forest</strong> stands, where <strong>the</strong> availability <strong>of</strong> nitrogen<br />

was high (B<strong>in</strong>kley and Hogberg 1997; Harrison et al. 1999; Corb<strong>in</strong> et al. 2003; Brown<br />

and Court<strong>in</strong> 2003). High N levels may result from natural or man-made atmospheric<br />

<strong>in</strong>puts or from fertilizer additions for experimental or management reasons.<br />

(a) Natural or man-made <strong>in</strong>puts <strong>of</strong> acidify<strong>in</strong>g substances.<br />

Nihlgard (1985) proposed <strong>the</strong> hypo<strong>the</strong>sis that <strong>forest</strong>s will run <strong>in</strong>to nutrient imbalances<br />

because <strong>of</strong> <strong>in</strong>creased N deposition. Commonly N/P ratios <strong>in</strong> <strong>the</strong> foliage samples are<br />

used to assess <strong>the</strong> imbalances <strong>in</strong> P <strong>status</strong>. An <strong>in</strong>crease <strong>in</strong> N/P ratios <strong>in</strong>dicates an<br />

imbalance ow<strong>in</strong>g to low P uptake under high N <strong>in</strong>put conditions. L<strong>in</strong>der (1995)<br />

suggested that <strong>the</strong> ratios between nutrients are much more important for judg<strong>in</strong>g <strong>the</strong><br />

nutrient <strong>status</strong> than <strong>the</strong> concentrations <strong>the</strong>mselves as <strong>the</strong> concentrations are <strong>in</strong>fluenced<br />

by <strong>the</strong> amount <strong>of</strong> non-structural carbohydrates <strong>in</strong> <strong>the</strong> leaves. Deficiencies <strong>of</strong> P <strong>in</strong><br />

<strong>forest</strong> stands receiv<strong>in</strong>g high levels <strong>of</strong> acidify<strong>in</strong>g atmospheric <strong>in</strong>puts have been<br />

reported <strong>in</strong> North America and Europe. For example, Bernier and Brazeau (1988)<br />

reported P deficiencies <strong>in</strong> sugar maple stands <strong>in</strong> Quebec and Mohren et al. (1986) and<br />

Houdijk and Roel<strong>of</strong>s (1993) <strong>in</strong> <strong>forest</strong>s <strong>in</strong> <strong>the</strong> Ne<strong>the</strong>rlands, especially <strong>in</strong> Douglas fir<br />

stands. Harrison et al. (1999) reported that <strong>in</strong> <strong>the</strong> UK <strong>the</strong> poor canopy condition was<br />

related to P stress <strong>in</strong> beech, Scots p<strong>in</strong>e and Sitka spruce stands. Polle et al. (1992)<br />

reported P deficiency <strong>in</strong> needles <strong>of</strong> three Norway spruce stands grow<strong>in</strong>g at three<br />

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