Agriculture Reference
In-Depth Information
The carbon in the world's vegetation biomass (Post
et al
.‚ 1992) is estimated to be
about 1.2 % of the total dynamic pool‚ approximately 37 % of the soil reservoir and
slightly less than that of the atmosphere. Organic carbon in the top 100 cm of the world's
soils appears to be close to 1500 Pg (Eswaran
et al
.‚ 1993; Batjes‚ 1996) while an esti-
mated further 900 Pg may occur between 100 and 200 cm (Batjes‚ 1996). Estimates for
the carbonate carbon of the top 100 cm of the soil range from 720-930 Pg (Batjes‚ 1996).
Total soil carbon stocks therefore represent approximately 6.9 % of the carbon contained
in all except the largely-inert geological pool.
Figure I.30 presents the major compartments and flows occurring at an ecosystem
scale. The major carbon stores are the above-ground plant structures‚ the litter layer and
the soil store‚ including plant roots. Within the soil‚ the biomass comprises the living
roots‚ the microbial biomass and the meso- and macro-fauna. The soil organic matter
component consists of dead materials of a range of ages and degrees of decomposition
varying from recognisable plant‚ microbial and animal remains to the highly-stable
humic compounds so important for soil stability and functioning. Carbon is transferred
onto the soil surface and into the mineral soil horizons through litter fall‚ root death and
decomposition and in leachates and exudates of various types. Litter fall includes dead
leaves‚ bark‚ stems and reproductive structures originating from the higher plants; it also
includes faecal materials‚ secretory products and the dead bodies of the above-ground
fauna. Carbon losses include the carbon dioxide respired by all sections of the biota and
by abiotic oxidative processes (Warneke
et al
.‚ 1999)‚ that in dissolved and particulate
organic carbon exported off-site in deep drainage‚ overland or subsurface lateral flows.
