Info

Concentration of Roots Near the Soil Surface

As shown above, the concentration of roots in the topsoil can be an effective mechanism for nutrient cycling and conservation, especially in nutrient-poor ecosystems. In general, root tips, essential for nutrient uptake, tend to be congregated in the upper soil horizons of tropical forests (Odum 1970). However, not all tropical forests are dependent upon a concentration of roots on or near the surface to ensure adequate recycling of nutrients. Only those forests on Oxisols, Spodosols, and occasionally Ultisols regularly show concentrations on or near the surface. The relationship between soils and natural vegetation in various tropical regions is shown in Table 2.7. Along a transect from wet tropical forests to seasonal tropical forests, the intensity of weathering and leaching is less because of less moisture in the soil, and so the quantity of roots on or near the surface tends to be less. For example, the Dipter-ocarp forest in Malaysia and the moist forest in Panama, both of which receive 2 m of precipitation per year, have fewer roots on or near the surface compared with the San Carlos de Rio Negro forests that receive 3.5 m of precipitation per year (see Table 3.1, Chap. 3).

Kingsbury and Kellman (1997) found that presence of root mats on top of the soil in southeastern Venezuela was indicative of high concentrations of aluminum in the soil rather than of low levels of soil nutrients. The growth of roots on top of the soil, forming a dense root mat, was apparently a negative reaction of the plants to the high concentrations of aluminum, which was presumably toxic to the plants. However, as discussed above, high concentrations of aluminum cause phosphorus to be unavailable to plants: therefore high concentrations of aluminum and low availability of phosphorus are strongly related to each other.

Nepstad et al. (1995) showed that some secondary successional trees in the lowland tropics have tap roots as deep as 8 m. These roots may have an important function in water absorption. They could also be an adaptation to nutrients that have leached deeply into the soil following clearing. Their finding does not negate the idea that surficial roots in the wet tropics are important for nutrient transfer.

Fig. 2.9. Some of the possible flows of nutrients through the below-ground portion of an undisturbed forest. Dashed lines indicate return of organisms to dead organic matter pool. Soluble nutrients may be excreted by many of the organisms, or may be released during trophic transfers. Nutrients in soluble form may not follow trophic pathways. Soluble nutrients may be taken up by algae, or other plants, or they may be leached into the mineral soil where they can be absorbed onto clay, taken up by roots, or leached through the subsoil or to drainage streams. Gaseous fluxes such as nitrogen fixation and denitrification are not indicated. Deforestation greatly simplifies the cycles, and increases the proportion of nutrients in soluble form. (Adapted from Jordan 1985, with permission of John Wiley and Sons Ltd., publisher)

Fig. 2.9. Some of the possible flows of nutrients through the below-ground portion of an undisturbed forest. Dashed lines indicate return of organisms to dead organic matter pool. Soluble nutrients may be excreted by many of the organisms, or may be released during trophic transfers. Nutrients in soluble form may not follow trophic pathways. Soluble nutrients may be taken up by algae, or other plants, or they may be leached into the mineral soil where they can be absorbed onto clay, taken up by roots, or leached through the subsoil or to drainage streams. Gaseous fluxes such as nitrogen fixation and denitrification are not indicated. Deforestation greatly simplifies the cycles, and increases the proportion of nutrients in soluble form. (Adapted from Jordan 1985, with permission of John Wiley and Sons Ltd., publisher)

Thick root mats also are common in high-elevation cloud forests. Here the concentration of roots on the surface occurs because of low nitrogen availability (Tanner et al. 1998). Nutrients are tied up in a thick organic mat that has formed because the anaerobic conditions of the saturated environment have slowed decomposition (Bruijnzeel and Veneklaas 1998).

Surface concentrations of litter and humus permeated with fine roots are not confined to the tropics. In old-growth stands of beech (Fagus grandifolia) on Ultisols in southeastern USA, there is often a thick mat where roots are in direct contact with decaying leaves (Jordan, pers. observ.). Beech is a climax species and casts heavy shade. Decomposition apparently is slow beneath the canopy and nutrient availability in the decomposing litter may be higher than in the mineral soil below.

Project Earth Conservation

Project Earth Conservation

Get All The Support And Guidance You Need To Be A Success At Helping Save The Earth. This Book Is One Of The Most Valuable Resources In The World When It Comes To How To Recycle to Create a Better Future for Our Children.

Get My Free Ebook


Post a comment