Agriculture Reference
In-Depth Information
Production is generally calculated by summing the positive increments of root biomass
observed at regular time intervals. Such a method certainly underestimates production
because it does not take into account the roots which may have died and decomposed
between adjacent sampling occasions. Methods taking into account root decomposition
or using direct measurements based on rhizotrons give estimates up to 180 % higher than
simple core harvest techniques (see,
e.g.,
Hansson and Steen, 1984; van Praag
et al.,
1988).
Root production, particularly of fine roots, may be highly seasonal and heterogenous
in space. Root systems seem to develop in an opportunistic way, taking advantage of
favourable conditions to develop new fine roots; when conditions become difficult,
a large part of these rootlets may die. Consequently, root biomass may vary greatly
with season and because of particular climatic episodes. Methodologies which do not
accurately track these variations are likely to give highly-biased estimates of biomass
and production.
Estimates of fine-root production vary from 0.6 to 11 Mg over a range of
temperate climate ecosystems (Fogel, 1985; Aber
et al.,
1985; van Praagh
et al.,
1988).
Turnover of fine roots is rapid and the ratio of fine root production to standing crop may
be as high as two in a young stand of
Pinus sylvestris
in Sweden, and 0.9 in an adjacent
120-year old stand (Persson, 1985). Similarly, the proportion of fine roots dying annually
ranged from 40 to 92 % of total weight in the series of deciduous and coniferous temperate
forests listed by Fogel (1985). In a coniferous forest, estimates of the annual input to the
soil of dead fine roots and associated mycorrhizae ranged from 14.6 to 18.8 Mg
These values are two to five times greater than the contributions of leaf litter and
branches to soil organic matter. Forests have the highest production levels, whether they
are coniferous (with production of fine roots of 1.2 to 11 Mg
or deciduous
(2 to 9 Mg In a tall-grass prairie, production was 5.1 Mg In crops,
estimates range between 0.6 and 4.2 Mg on the same areal basis, depending on the type
of crop and soil conditions.
In tropical grasslands and savannas, root production increases over that of cooler
climates and, within the tropics, increases with rainfall. Root production estimates as
high as
ca.
20 Mg have been measured in moist African savannas at Lamto
(César, 1971). Nonetheless, in many situations, waterlogging or nutrient deficiencies
may limit root growth (see
e.g.,
Cuevas and Medina, 1988). In South American savannas,
Fisher
et al.
(1994) found that conversion of savannas into highly productive pastures might
account for the sequestration of 100-507 Mt carbon per year. Too few measurements of root
production in tropical forests have been made to identify general trends.
Root production is a highly seasonal process. For example, in temperate areas fine
root production by apple (
Malus pumila
) trees showed two marked annual peaks in spring
(April to June) and late summer (September) respectively (Figure III.23). In African
savannas, root production by perennial grasses seems to be more opportunistic since
rapid increases of root biomass may occur at any time of the year, provided that the soil
moisture status is favourable (César, 1971; Fournier, 1991).
