Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 114

Table 3.2.1. Chemical compositions of plant material used in this study.

N
(%)

C : N
ratio

Lignin
(%)

TEP
(%)

PBC

Prunings ( µ g BSA mg − 1 )

Experiment 1
Gliricidia sepium

4.0 12 20 1.3 22

Calliandracalothyrsus 3.6 13 22 3.5 317

Peltophorumdasyrrachis 2.5 20 32 3.9 245

Experiment 2
Sesbaniasesban

3.7 12 5.9 2.4 23

Macroptiliumatropurpureum 2.5 16 9.0 2.3 24

TEP = total extractable polyphenols; PBC = protein-binding capacity of plant extract; BSA = bovine
serum albumin.

Results and Discussion

Incorporation of plant material resulted in higher N 2 O emissions
( P < 0.05) than from the unamended soil (Fig. 3.2.1; Table 3.2.2). This
was probably a result of rapid stimulation of microbial decomposition,
possible creation of anaerobic microsites resulting from microbial
respiration and the increased C supply and substrate for nitrification and
denitrification. The greatest total losses were measured after incorporation
of Gliricidia leaves, with 9.7 mg N 2 O-N m −2 ( P < 0.005) emitted over 23
days (Table 3.2.2). The rapid release of N (Fig. 3.2.1) from the Gliricidia ,
providing the substrate for nitrification and denitrification, was related to
the high N and low polyphenol contents of this material. N 2 O emissions
were positively correlated with available soil N for the first 11 days after
incorporation of Gliricidia leaves ( r = 0.91 and 0.86 for NH 4 + and NO 3 − ,
respectively; P < 0.005), but thereafter were negatively correlated.
Incorporation of Calliandra and Peltophorum prunings resulted in temporary
immobilization of N and low N 2 O emissions. Emissions after addition of
Calliandra leaves were significantly lower than from the Gliricidia treatment,
despite similar C : N ratios of 13 and 12, respectively. Hence lower N 2 O
emissions and mineralization from the former prunings was attributed to
their higher polyphenol contents and protein-binding capacities. These
findings confirm that the C : N ratio alone is insufficient to predict N
release and subsequent N 2 O emissions. The polyphenols of Calliandra and
Peltophorum had a high capacity to bind the plant N, thereby reducing the
availability of N for nitrification and denitrification. Materials with a
polyphenol + lignin : N ratio of < 10 are generally found to result in fast
N release (Mafongoya et al ., 1998). The Peltophorum leaves had a ratio
of 14.4, and exhibited low N release and the lowest N 2 O emissions of the