Studies of commercial plantations have provided insights into how trees affect soils and how nutrients are cycled through the soil and trees by comparing the before and after picture following tree planting. In the coastal dunes of Argentina, Jobbagy and Jackson (2004) looked at the vertical distribution of potassium (strongly cycled) and sodium (more weakly cycled) after planting with maritime pine Pinus pinaster, native to Europe. Fifteen years later, potassium was significantly concentrated in the top 50 cm of the soil while sodium was commonest 1-4 m down. This implies that potassium (a nutrient often in short supply) is much more strongly cycled than sodium, such that the trees are efficient at capturing potassium in the soil and delivering it back to the surface in litter. On the whole, deciduous trees are more active in nutrient cycling than conifers. The difference is particularly marked for calcium which is much more strongly cycled by deciduous trees and is probably responsible for the higher pH brown earths under deciduous trees such as birches, which counteract the leaching process by returning nutrients from deeper horizons to the surface.
Temperate and boreal forest soils tend to be uniformly nitrogen-poor due to a lack of nitrogen fixers, significant losses as DON and, especially in the boreal zone, episodic losses associated with fire and other local disturbances such as wind (see Box 8.3 for a detailed example from Hubbard Brook). McGroddy et al. (2004) point out that N/P ratios in foliage vary from < 5/1 in coniferous and temperate deciduous forests to > 30/1 in tropical forests; that is, there is proportionately more nitrogen in the tropics relative to phosphorus (explained
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