Soil water deep uptake

The significant investment made in superficial roots by trees of Amazon forests is a clear indication of the importance of nutrient recycling from organic pools at the soil surface. However, research from the central and eastern Amazon has shown that trees in seasonally dry forests also have roots extending to at least 18 m depth (Nepstad et al. 1994). While the main function of these roots appears to be the uptake of deep soil water and groundwater, there is also potential for these roots to access deeper nutrient pools in the soil column. Nepstad et al. (this volume) elaborate on this issue by demonstrating that secondary forests growing in the eastern Amazon have P and K nutrient needs that cannot be satisfied by available stocks in the... 

Further support of rapid recovery of hydrological functions in secondary forest comes from the Zona Bragantina, east of Belem. Here, Holscher et al. (1997) used a Bowen ratio approach to measure evapotranspiration in a 2.5-3.5-year-old secondary forest on abandoned crop land, and found an average daily rate (3 9 mm d ) very close to that of mature forests receiving similar amounts of radiation. During the dry season, this secondary forest was absorbing water from below 3 m depth, and therefore had recovered a portion of its deep soil water uptake capacity. 

The recovery of deep soil water uptake in the Paragominas secondary forest was possible because of the re-establishment of deep root systems following pasture abandonment (Fig. 9.2). The root systems of secondary forest trees, vines, and palms rapidly penetrate to at least 8 m depth during the first 15 years of regrowth. We identified one third as many morphos-pecies of roots to 8 m depth in the secondary forest as in the neighboring mature forest, with a prevalence of vine and palm roots in the secondary forest. The vine Davilla kunthii, for example, penetrates to at least 8 m depth by the time its stem has attained 1 m height (Restom 1998). Vines in... 

Hodnett, M. G., J. Tomasella, A. d. O. Marques Filho, and M. D. Oyama. 1997. Deep soil water uptake by forest and pasture in central Amazonia predictions from long-term daily rainfall data using a simple water balance model. In Amazonian Deforestation and Climate, eds. J. H. C. Gash, C. A. Nobre, J. M. Roberts, and R. L. Victoria Oohn Wiley Sons, New York), pp. 79-100, 611 pages. 

Especially the dependence on the mineralisation dynamics of a low-input of slow-releasing organic stable or green manure makes the nutrient flow in different soils less controllable, and requires varieties adapted to such soil fertility management. Deep, intensive root architecture may contribute to a more efficient capturing of water and nutrients (Lees and Gahoonia, 2004). Experiments have also shown that varieties may differ in efficiency of nutrient uptake and use (Baresel et al., 2005). The ability to interact with beneficial soil micro-organisms can support this efficiency (Bosco et al., 2006). 

The texture and structure of a soil also affect plant response to nitrogen additions through effects on root development and depth of penetration. A restricted root system limits the ability of the plant to assimilate nutrients and also limits its uptake of water. Since the movement of water through soils by capillarity is slow and limited, the plant roots must literally go after it. A deep root system, therefore, assures that a much greater proportion of the supply in the soil is available to the plant. In addition, soil compaction limits the absorption of the rainfall. Such unfavorable conditions do of course lower the efficiency of applied nitrogen. 

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