Agriculture Reference
In-Depth Information
In the same soil, the nitrogen concentrations of roots decrease with depth (Abbadie, 1983).
As decomposition proceeds, the chemical composition of the roots changes. In the early
stages, the quality of this material (as assessed by the growth rates of invertebrates feed-
ing on it) increases and this is probably related to the loss of soluble toxic compounds.
3.2.3.2
Availability in time
As a first approximation, roots may be considered a constantly, although irregularly-
renewed resource. Most studies of seasonal changes in root biomass in grasslands or
forests show no clear patterns. In humid African savannas, there appear to be phases of
intense growth or mortality directly related to immediate conditions (Cesar, 1971;
Abbadie, 1983). In climates with more marked seasonality, root growth may have clear
patterns with pulses of fine root production in the growing season (van Praag
et al.,
1988).
However, roots comprise at least two distinct resources, living and dead (see
Chapter III.3). Living roots may be classified as constantly-renewed resources
whereas dead roots may vary from constantly-renewed to a pulsed pattern of production,
following sudden or unpredictable mortality. The scales at which decomposers operate
substantially affect the ways in which such changes are perceived.
Coarse perennial roots are a constantly renewed resource whereas fine roots are
ephemeral, living for only a few weeks to months. While fine roots are constantly
present, pulses of growth and mortality occur regularly. At certain scales of time and
space, they may thus be classified as rapidly decreasing resources (or increasing, if dead
fine roots are considered) (see,
e.g.,
Fairley and Alexander, 1982; Kalela, 1957; Persson,
1978 and 1980; Vogt
et al.,
1980 and 1981).
3.2.3.3
Spatial distributions
Root distributions differ greatly from those of litter materials in that roots may be
dispersed throughout the entire upper soil volume. Also, they are normally in intimate
contact with the soil matrix over their entire surfaces. Roots may occur at any depth
although they are particularly concentrated in the upper part of the profile and diminish
sharply with depth. This pattern is clearer in grassland than in forest, in situations where
the plant-nutrient status of the soil is low or where the water table frequently comes
close to the soil surface (Chapter III.3.1). Tree root systems are more deeply and evenly
distributed with depth than those of grasses. Tree roots are also clearly differentiated into
coarse perennial supporting structures and finer, short-lived absorbing roots. In contrast,
grasses have fasciculated, or bundled systems of functionally-similar roots.
A further factor in the differing “rooting strategies” that occur between plants groups
is the variation that occurs in their patterns of horizontal distribution.
3.2.3.4
Annual inputs and standing crop
Root biomass may vary from a few hundreds of kilograms to more than
depending on the dominance of grasses or tree components and soil characteristics.
As developed further in Chapter III.l, forest ecosystems normally have a higher root
