Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 90

Table 3.1. Choice of plant quality attributes for different management purposes.

Objective Time scale Optimal residue quality

C sequestration Long-term High lignin and condensed tannins

Erosion control Immediate High N, lignin and condensed tannins, structurally protected

Microclimate Immediate High N, intermediate lignin and condensed tannins

Weed suppression Immediate Low N, intermediate lignin, alkaloids, hydrolizable

polyphenols, allelopathic substances

CEC Immediate Large specific surface area

Soil pH (Al toxicity) Intermediate Cation to anion ratio

Reduce pollution Immediate Low soluble C and N, high in active polyphenols (PBC)

Nitrogen supply Immediate low C : N ratio, low soluble C, low lignin : N, low in active

polyphenols (PBC)

Medium-term low C : N ratio, intermediate lignin and condensed tannins

Synchrony high C : N with high soluble C, mixed residues

PBC = protein-binding capacity.

estimated to release between 1.5 and 3 Gt C year −1 . Fisher et al . (1995)
suggested that improved pastures which replace native savannas through-
out South America could account for an additional sequestration of
100-500 Mt C year −1 in these tropical soils. Boddey et al . (2000) also
found increases in soil C under 9-year-old improved pastures after
rainforest clearing in Brazil but the magnitude was lower, and soil C under
degraded pastures declined. Substantial soil C inputs may be attributed to
the deep-rootedness of grasses in improved tropical pastures (Cadisch et al .,
1994). Indeed, 75% of the claimed increased C sequestration was found
below 20 cm soil depth and is thus likely to be due to root inputs. Fisher
et al . (1997) found that the large increase in SOM under improved tropical
pastures (up to 70 t ha −1 ) was associated with a substantial increase in the
C : N ratio, giving ratios in the SOM of 33 : 1 compared with usual SOM
values of ~12 : 1. It is thus likely that only partial decomposition of roots
occurred leading to the increased SOM content. Extrapolation from a fitted
double exponential decay model to laboratory incubation data of tropical
pasture materials suggested that between 43 and 47% of legume root C and
54-62% of grass roots was theoretically 'non-decomposable' (Urquiaga
et al ., 1998). Similarly, our recent data showed that after incubation of root
materials for 1 year, < 50% of C had been evolved through respiration (Fig.
3.1) whereas 80% of C of leaves of the tropical grass Brachiaria humidicola
had been lost. The factors determining residue C degradation in soils are
governed by residue quality. Brachiaria and legume roots had a higher
C : N ratio and higher lignin content than leaves (Schweizer et al ., 1999).
Lignin is known to be recalcitrant and to protect cellulose from microbial