Agriculture Reference
In-Depth Information
The relative importance of the various determinants of decomposition rates may
change with time. In a latitudinal series of 14 Swedish Scots Pine (
Pinus sylvestris
)
forests, losses in litter mass occurring during the first year of decomposition were
correlated with climatic factors and site productivity indices; during the later stages of
decomposition, such losses were correlated with the litter holocellulose to lignocellulose
ratio (Johansson, 1994). During the early phase of decomposition, high concentrations
of cellulose, hemicellulose and nutrients in litter are other factors that accelerate decom-
position. After two years, 60 % of the litter had decomposed and decomposition rate
became increasingly independent of climatic variables. However, a significant relation-
ship was found between decomposition rate and the combined (holocellulose + lignin)
contents of the remaining material.
Different patterns and processes of nutrient release result in variations in the turnover
of elements and changes in their relative concentrations in plant biomass and soil.
In a tropical lower montane rainforest at El Verde (Puerto Rico), the turnover rates of
different elements varied greatly: the ratio of total stock (S in g ) to annual cycling
rate (C in g released through decomposition) differed considerably between
elements with values of 0.08, 0.14 and 0.16 years for K, Cu and P respectively contrasting
with 13.3, 47.5 and 81 years respectively for N, Ca and Mg (Odum, 1970; Table IV.7).
A ranking of turnover times follows the order K <Cu <S <P <Na <Fe <Mn <C <N <Ca <Mg.
In a range of temperate and tropical forests, the proportions of nutrients stored
within the plant biomass differ substantially between sites and nutrient elements
(Edwards and Grubb, 1982; Lemée; 1978; Bernhard-Reversat
et al.,
1978; Klinge, 1975;
Greenland and Kowal, 1960) (Table IV.8). K is the most labile element and tends to
accumulate in plant biomass, with a minimum value of 40 % of the total stock of the
ecosystem in a temperate-climate Belgian forest, and a maximum of 88 % in a tropical
