The Global Challenge for Soil Carbon - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 3

Table 1.1. Global soil carbon fact sheet. (From Banwart et al ., 2014.)

Amount of carbon in top 1 m of Earth's soil b
2/ 3 as organic matter
Organic C is around 2× greater C content than Earth's atmosphere

2200 Gt

Fraction of antecedent soil and vegetation carbon characteristically lost from
agricultural land since 19th century c

60%

Fraction of global land area degraded in past 25 years due to soil carbon loss d 25%

Rate of soil loss due to conventional agriculture tillage e ~1 mm year - 1

Rate of soil formation e ~0.01 mm year - 1

a Global mean land denudation ratef f

0.06 mm year - 1

Rate of peatlands loss due to drainage compared to peat accumulation rate g 20× faster

Equivalent fraction of anthropogenic greenhouse gas emissions from peatland loss g 6% annually

Soil greenhouse gas contributions to anthropogenic emissions, in CO 2 equivalents h 25%

a Rate of land lowering due to chemical and physical weathering losses; b Batjes (1996); c Houghton (1995); d Bai et al .
(2008); e Montgomery (2007); f fWilkinson and McElroy (2007); g Joosten (2009); h 2004 data not including CH 4 , IPCC (2007).

indication of the regional and national pres-
sures on soil and the associated trends in the
gain or loss of soil functions. What is note-
worthy is the broad geographical extent of
areas associated with strong degradation.

hyphal networks. These proliferate when
they encounter nutrient resources such as P-
and K-bearing minerals and organic N and P
in decomposing plant debris. Grazing and
predator organisms including protozoa and
soil fauna are sustained by the active micro-
bial biomass. Soil fauna such as worms, ter-
mites, ants and other invertebrates play an
important role in the initial processing of bio-
mass and for physical mixing and transport
through bioturbation, particularly at the sur-
face, but for some organisms throughout the
full depth of the soil profile.
Advanced decomposition of biomass by
soil organisms yields humic material, which
chemically binds to the smallest soil particles
with the greatest surface area per mass: clay
minerals and Fe and Al oxides. This mineral-
adsorbed carbon is chemically more stable
and less bioavailable, and produces a hydro-
phobic coating on the mineral surfaces. The
smallest particles aggregate into micron-sized
fragments, and decomposition of fresh organic
matter by active heterotrophic microorganisms
produces microbial extracellular polymers that
help bind these intermediate aggregates with
rock fragments, decomposing plant debris,
biofilms of living microorganisms and fungal
hyphae and root surfaces (Tisdall and Oades,
1982; Jarvis et al ., 2012, and included refer-
ences). This bound mixture of mineral, dead
and living biomass and pore fluids, forms lar-
ger aggregates, resulting in a system called
soil structure, which produces a pore volume

Soil Carbon in Soil Functions and
Ecosystem Services

The process of adding photosynthate carbon
to rock parent material and the develop-
ment of subsurface biodiversity and the for-
mation of soil aggregates is the foundation
of soil development and the establishment
of soil functions.
Soil forms from parent rock material
that is exposed at Earth's surface, receives
infiltrating precipitation and is colonized by
photosynthesizing organisms (Brantley, 2010):
chiefly plants, but also symbiont algae in
lichens and photosynthetic cyanobacteria.
Organic carbon that is fixed in biomass by
photosynthesis is rooted, deposited and mixed
and transported by soil fauna into the soil
layer, providing carbon and energy for hetero-
trophic decomposer microorganisms. Other
functional groups of microorganisms trans-
form N, P, K and other nutrient elements of
decomposing biomass into forms that are
available to plants for further biological prod-
uctivity. Symbiotic fungi that draw energy
from plant photosynthate carbon that passes
from roots create the pervasive growth of