Agriculture Reference
In-Depth Information
Because of variation in the above, substantially different amounts of organic matter
accumulate in soils. This may be considered at the following different scales:
Large biomes;
Climatic zones (as defined by Holdridge, 1947); and
Regional and locational differences due to changes in the vegetation cover.
(i)
(ii)
(iii)
Recent estimations of the stocks in the world's soil and other carbon pools were pre-
sented in Table I.17. Soil carbon stocks differ widely between soil taxa and with climate.
They generally rise with increasing annual average rainfall and, for a given average
rainfall, increase as temperatures fall (Post
et al.,
1982) (Figure I.49). Carbon stocks also
increase with soil clay content and are influenced by litter quality (Schimel
et al.,
1994).
The lowest carbon stocks recorded (Batjes, 1996) occur in certain warm temperate
deserts and subtropical dry forests (2.6) while, apart from the peats, the highest
values in mineral soils are found in rain tundra (36.6), boreal rainforest (25.6), wet tundra
(22.2) and in tropical wet forest (21.0). Further, large amounts of soil organic carbon are
stored below 100 cm in a number of deep tropical soils (Batjes, 1996). Thus, the belief
that tropical rainforests have much lower organic reserves than temperate forests (see,
e.g.,
MacNeil, 1964; Bohn, 1976) is seen to be incorrect (Greenland
et al.,
1992).
Sánchez
et al.
(1982) have shown that no significant differences in organic matter
reserves exist between soils of temperate and tropical areas, provided that they belong to
the same order of Soil Taxonomy (Soil Survey Staff, 1999). In tropical areas, the oxisols
