Fine-root biomass

Mobilization of water from the soil is closely related to root depth and root density in each layer of soil. Fine roots of active B. brizantha pastures, established in deeply weathered clayey soils in eastern Amazonia, reach depths of 8 m or more (Nepstad et al. 1994). In abandoned pastures (50% B. humidicola and P. maximum cover and 50% invading shrubs and small trees), fine roots ( < 1 mm in diameter) were found at depths of 12 m (Nepstad et al. 1994). Fine-root biomass in the superficial soil layers of an active pasture in Paragominas, eastern Amazonia, was 3 times higher than that found in an adjacent primary forest area. Fine root biomass in the active pasture decreased by a factor of 100 between the surface and 6 m depth. In an abandoned pasture area, the distribution pattern of fine-root biomass was similar to that observed in the deeper soil layers of the forest ecosystem. This pattern is associated with the fine roots of the existing dicotyledonous invading species. 

The availability of scarce plant nutrients in deep soil is only relevant to our understanding of secondary forest nutrient acquisition if plants are able to absorb these nutrients with deeply penetrating root systems. Enticing evidence for such an extensive approach to nutrient acquisition is supported by the distribution of fine-root biomass (diameter = 0-1 mm) in the secondary forest, as compared to that in the neighboring mature forest and active cattle pasture (Fig. 9.2). Fine root biomass to 6 m depth is virtually identical in mature and secondary forests, and is > 10 times greater at depth than in the active cattle pasture. Hence, after 16 years of recovery, some of the trees, lianas and palms of the secondary forest had re-established root systems to at least 6 m depth. 

But are there sufficient fine roots at depth to absorb significant amounts of soil nutrients In both mature and secondary forests, there is a hundred-fold decline in fine-root biomass from the soil surface to 6 m depth, which means that fine root length density at 6 m is less than 1 cm of root per 100 cm3 of soil. Moreover, the few roots that occur at depth are concentrated in patches of soft soil that comprise approximately 1% of the soil volume, and that show no nutrient enrichment (Carvalheiro and Nepstad 1996). Given the low mobility of phosphorus in the soil, it is unlikely that such a sparse, patchy root system could absorb substantial amounts of this scarce nutrient. 

Jackson, R. B., Mooney, H. A., and Schulze, E.-D. (1997). A global budget for fine root biomass, surface area, and nutrient contents. Proc. Natl. Aaiil. Sci. USA 94, 7362-7366. 

Castellanos, 1. et al., Slash-and-bum effects on fine root biomass and productivity in a tropical dry forest ecosystem in Mexico, Forest Ecol. Manag., 148, 41, 2001. 

An example of the rejection of a hypothesis on the basis of lack of temporality is cited in Roberts et al. (1989). Ulrich et al. (1980) claimed that the fine root biomass of trees in the Soiling area of northeast Germany had decreased in parallel with an increase in the concentration of aluminium in the soil solution. However, a re-analysis of the data by Rehfuess (1981) indicated that the fine roots had declined prior to the rise in aluminium concentrations and the decline could be related to moisture stress. 

Under M. sinensis the surface of the soil is not covered by a layer of organic matter. The major input of organic matter into the soil occurs through the myriads of fine roots typical of herbaceous species. The products of microbial decomposition of the below-ground biomass are water-insoluble humic... 

In this system, herbaceous species produce fine roots that pervade the soil mass and provide a steady input of relatively easily degradable organic matter. The below-ground biomass is rich in protein and... 

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