Agriculture Reference
In-Depth Information
Nutrient release during the initial phase may be rapid; during the first 21 days of
decomposition, 25 % of the nitrogen, 38 % of the phosphorus and 46 % of the more
mobile potassium was lost, compared with only 10 % of the biomass. The rate of release
per unit of biomass decomposed is slower during the later stages of decomposition and
its measurement is complicated by other factors, including admixture with soil materials
through earthworm casting into the remaining residues, the activities of other soil animals
and nitrogen fixation (Hill and Patriquin, 1990). Decomposition rates and the dynamics
of nutrient release from decomposing resources are highly variable since they depend on
a suite of hierarchically-organised factors (Chapter IV. 1.2).
3.1.4.3
Ratios of carbon to nitrogen and other elements (see also Chapter IV.1.3.3)
The ratios of carbon to other nutrient elements in decomposing tissues have a controlling
influence on their breakdown and on the recycling of their nutrient elements. In particu-
lar, the ratio of carbon to nitrogen (C:N) (Table I.21) has been widely used as an index
of tissue decomposability and of the capacity of various materials to supply nitrogen to
higher plants and to micro-organisms. However, other tissue properties, including the
lignin content, the ratio of structural tissues to cytoplasm and the presence of secondary
plant compounds also strongly influence the pattern of breakdown.
Heterotrophic micro-organisms decomposing plant tissues in terrestrial environments
normally have to cope with materials with higher C:N ratios than those of their own
tissues. In such environments, micro-organisms in increasing their numbers will absorb
the available inorganic nitrogen to incorporate into their own biomass. Since micro-
organisms preferentially absorb nitrogen as (or as low molecular weight organic
compounds), they may have a competitive advantage in this and immobilise much
nitrogen in their tissues that could have otherwise been available to higher plants.
During decomposition, the C:N ratios of litters and harvest residues vary in similar ways
although the time scales differ widely due to both intrinsic and environmental factors.
In the harvest residues of sugarcane (
Saccharum
spp.), initially high values (
ca.
170)
increase slightly which may result from the loss of the most assailable materials;
this is followed by a lengthy decline and perhaps a plateau at a C:N of
ca
. 70 (Figure I.37)
(Spain and Hodgen, 1994). Description of the changes occurring during the later phases
of decomposition is complicated by N fixation in the decomposing residues (Hill and
Patriquin, 1990) and by their admixture with earthworm casts. In soils, lower C:N ratios
generally pertain in the heavy fraction of the soil organic matter and, in both the light
and the heavy fractions, the C:N ratio generally declines with decreasing particle size
(Baldock
et al
., 1992).
As considered above, nitrogen deficiencies may limit the productivities of both micro-
organisms and plants, depending on the nature of the decomposing materials and their
stage of decomposition. Stevenson (1986) considered that net mineralisation leading to
an increase in inorganic nitrogen will occur below a C:N ratio of 20, an approximate
equilibrium state will pertain between 20 and 30 and that, over 30, net immobilisation
will take place constraining the supply of nitrogen to plants, as described above.
