Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 98

Consequences of Residue Quality Attributes for
Quantification of C Sequestration

Differences in the 13 C isotopic signatures between C 3 and C 4 species have
been used to quantify the contribution of a newly introduced species to soil
organic matter, e.g. tropical pastures following rainforest clearing, using the
relationship (Cadisch et al ., 1996):

13 13

δ δ
δ δ
where f g(G) is the proportion of SOM derived from the C 4 species,

C −

−

C

(G) (RF)

f g(G) =

13

C

13

C

(g) (RF)

δ (G) the

δ

13 C value of the soil under the pure grass pasture (G),

δ (RF) the δ

13 C value

of the soil under C 3 rainforest (RF) and

13 C value of SOM derived
from the C 4 species plant material (g). The 13 C signature of SOM entirely
derived from the C 4 species under investigation is, however, often not
known and the isotopic signature of bulk plant material is used instead. As
the 13 C signature of SOM under a particular vegetation is often close to the
value of the

δ g the δ

13 C of its vegetation, this assumption appears to hold (Tieszen
and Boutton, 1989). However, plant materials are not uniformly labelled in
13 C (Benner et al ., 1987). Recalcitrant plant fractions such as lignin are
more depleted in 13 C compared with the bulk plant material (Schweizer
et al ., 1999; Table 3.3). These materials, due to their slow degradation, may
accumulate during decomposition, and hence the 13 C signature of the soil
organic matter formed may have a different 13 C signature from that of
the original bulk plant material. For the correct use of the 13 C isotopic
method for decomposition and SOM studies, it is therefore necessary to
know the extent of possible isotopic fractionation which occurs during

δ

Table 3.3.

13 C signatures of bulk plant and lignin material (adapted from Schweizer et al., 1999)
and their potential implications for soil organic matter origin estimations.

δ

Total plant
δ

Lignin C
δ

Potential error in
grass-derived C (%)

13 C (‰) 13 C (‰)

Brachiariahumidicola
Leaves − 11.4 − 16.1 +20 a

Roots − 11.9 − 16.6 +22 a

Desmodium ovalifolium
Leaves − 27.3 − 29.8 − 7 b

− 6 b Roots − 25.8 − 27.8

− 1 0.1 − 1 0.1 SEM

a On rainforest soil ( δ 13 C = 28‰) (Cadisch et al., 1996).

b On a C 4 -derived soil ( δ 13 C = 12‰).