Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 99

decomposition. We investigated these effects for C 3 and C 4 pasture species
introduced in the Latin American savannas. As expected, the grass
materials ( B. humidicola ) were less depleted in 13 C( δ

13 C

− 11.4 to − 11.9‰)

than those of the legume ( 25.8‰) ( D. ovalifolium ) (Table 3.3;

Schweizer et al ., 1999). Plant lignin C was strongly depleted in 13 C
compared with the bulk material by up to 2.5‰ in the legume and up to
4.7‰ in the grass. If lignin is a major source of material remaining as
'humus', this would lead to potential errors in the estimation of grass- or
legume-derived SOM of 6-22%.
These plant materials subsequently were incubated for 1 year.
Significant depletion in 13 C of the evolved CO 2 was observed during the
initial stages of decomposition (Fig. 3.5) probably as a result of microbial
fractionation as it was not associated with the 13 C signatures of the
measured more-decomposable fractions (non-acid detergent fibre and
cellulose). The cumulative CO 2 - 13 C signatures of both grass and legume
materials tended to converge towards the 13 C signature of the original bulk
material. Analysis of the residual material after 1 year of incubation showed
no significant change in the 13 C signature from that of the original bulk
material (Table 3.4). Hence, during the incubation time of 1 year, the
recalcitrant, 13 C-depleted lignin did not exert a major influence on the
isotopic signature of the remaining material, providing evidence against a
direct lignin pathway to SOM formation. It is thus likely that lignin is at
least partly degraded and that the formation of new SOM is largely the
result of microbial transformations. On the other hand, isotopic effects due
to decomposer growth rate, efficiency and stabilization of the degradation
products increase 13 C concentration in the remaining material (Agren et al .,
1996). Although full evaluation of the potential errors associated with
isotopic fractionation may necessitate several years of incubation due to
the slow decomposition of the more recalcitrant fractions (Fig. 3.1),
the importance of 13 C-depleted lignin as a source of errors in calculating the
origin of soil organic matter may have been overestimated in the past.

− 27.3 to −

Conclusions

Definition of residue quality attributes ('high' or 'low') should vary accord-
ing to the desired management objective (e.g. C sequestration or crop N
supply). Soil C sequestration is favoured by lignin and other high molecular
weight secondary plant metabolites. Plant roots seem to have a particularly
important role in soil C sequestration in productive tropical pastures.
Plant quality attributes also affect isotopic carbon signatures of chemical
fractions, but 13 C-depleted lignin in residues resulted only in minor errors
in identification of sources of SOM within 1 year. Protein-condensed
tannin complexes appear to provide a direct route to C sequestration and