Agriculture Reference
In-Depth Information
quality assessed by extraction techniques and C:N ratios. In the A1 horizon, for example,
the proportion of total humin decreased whereas inherited humin increased from the
eutrophic mull to the dysmoder. Inherited humin corresponds to the resistant organic
matter accumulated as lignin and as invertebrate faecal pellets; the difference between
total humin and inherited humin comprises insolubilised humin
i.e.,
soluble organic
compounds leached out of the litter system, flocculated and associated with clay
minerals in organo-mineral complexes. This trend was confirmed in a review by Zech
et al.
(1996) who stressed that organic matter accumulates in mor litters as strongly-
humified, decomposition-resistant plant residues, whereas in mull systems, organic
matter is bound to minerals in less humified forms.
2.6.2
NUTRIENT CYCLING
Decomposition and mineralisation processes release nutrient elements largely in soluble
forms that may be taken up by plants and other soil organisms, or leached. The regulation
of nutrient release rates is of paramount importance in nutrient conservation since most
nutrient elements occur in limited stocks. Only small external inputs occur in rainfall, dry
deposition and through N-fixation, which generally accounts for a few kilograms
In tree plantations in the Congo, for example, N-fixation rates have been estimated at
respectively 14-21 kg in stands of
Eucalyptus
sp. (Bernhard-Reversat, 1993) and
120-141 kg in stands of N-fixing
Acacia
sp. (Bernhard-Reversat
et al.,
1993).
Inputs in rainfall and as 'dry' deposition may represent significant amounts on
an annual basis. For example, 2-21 kg of mineral and organic N, 0.2-0.5 kg
P, 4-9 kg K and 3-16 kg recorded annually per hectare from tropical forests in Sénégal, the
Côte d'Ivoire and North Cameroon (Bernhard-Reversat
et al.,
1978; 1987; Harmand, 1997).
In highly-industrialised areas, air pollution from industrial and agricultural sources
may lead to the deposition of much larger amounts of nutrients from aerial sources,
particularly N and S. High N deposition rates (
e.g.,
>18 kg wet deposition over
a ten year period in England, Tamm, 1991) results in N saturation, acidification and
cation leaching in poorly-buffered soils and this, in turn, leads to nutrient inbalances and
changes in plant community composition. N saturation is defined as the situation in
which the physiological N requirements of the primary producers are satisfied and
considerable nitrate leaching takes place (Tamm, 1991).
At a given site, rates of release differ between nutrients: K and Mg are generally lost
rapidly, whereas the release of P, N and Ca has variable kinetics (Figure IV. 30).
Depending on the local availability of nutrients, N, P or Ca may be immobilised
for longer periods than other nutrients. In a French temperate-climate forest (Foljuif),
N is retained in fungal biomass for a longer period than other nutrients (Fig. IV.30a).
During the summer period (months 6 to 9 and 18 to 21), intense fungal growth leads to
the accumulation of N beyond the stocks initially present in litter: this N may have been
translocated from deeper horizons, or input with rainfall and throughfall (Garay
et al.,
1986a) or through N fixation which has been reported to range from not detectable to
relatively small amounts (less than 1 to more than 5 kg ) in different litters (see, for
example, Heath
et al.,
1988). N fixation also occurs in the harvest residues of crops such
as sugarcane (Patriquin, 1982).
