Agriculture Reference
In-Depth Information
root mat layer. In a two-year study conducted in the same region, Grimaldi and Chauvel
(unpublished) used lysimeters to assess nutrient losses through leaching. They confirmed
the efficient recycling of most ions since only small amounts of Ca, K, Mg, and Si were
found at 70 cm depth, and only traces reached the river. Nonetheless, some ions were
occasionally more concentrated in leachates sampled during the dry
or wet
seasons
In deforested and cultivated areas, the newly-established juvenile ecosystems typically
lose substantial amounts of nutrients (Odum, 1983). A great challenge is to restore
synchrony through manipulation of the major variables that control decomposition and
mineralisation, notably microclimate, litter quality and decomposer communities. In an
experimental plot at San Carlos de Rio Negro, Jordan (1989) established a balance of
the main nutrients in a traditional slash-and-burn agriculture system (Figure IV.31).
He observed: (i) an increase in all soil nutrient stocks after burning that was maintained
throughout cultivation; (ii) low nutrient stocks in the crop and successional vegetation
in comparison with those contained in the biomass of the original forest; (iii) greater
losses from the entire ecosystem than from the soil alone resulting from the replacement
of nutrients leached from the soil by nutrients leached down from decomposing remains
of the primary forest; (iv) depletion of nutrient stocks from the original forest biomass,
including the root mat.
Developing synchrony in agricultural systems
Agricultural systems lack several attributes that are believed to promote synchrony
i.e.,
a large diversity of decomposing resources with different chemical and physical
structures; a diverse and abundant community of decomposers, especially invertebrates
of the different functional groups (micropredators, litter transformers and ecosystem
engineers); an extended and perennial root system able to capture nutrients released
within the soil volume. Manipulation of any of these ecosystem components may
potentially contribute to the restoration of synchrony.
Mixing organic resources of different qualities.
Decomposition rates are determined by
the chemical quality of the litter material (especially their concentrations of polyphenols,
lignin and nitrogen) and such physical properties as size of the individual litter units and
physical strength (see Section IV. 1.3.3). It is hypothesised that mixing litters of 'high'
and 'low' quality may lead to release patterns that are intermediate between the rapidly
and slowly decomposing components. Adjusting the proportions of each material may
allow accurate calibration of nutrient release patterns to plant needs, under given
conditions of climate, soil and macro-organism activities. Some but not all experiments
support this view
(e.g.,
Bandara and Anderson unpublished data in Myers
et al.,
1994;
Becker and Ladha, 1997). In an experiment in which the decomposition of 21 mixtures
of litters of different types was monitored, interactions between litter types occurred
in 17 cases (McTiernan
et al.,
1997). In 12 cases, decomposition of the mixture was
slower than predicted from a simple additive model. Much still remains to be understood
before reliable predictions are possible.
