Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 93

between polyphenols and proteins occur. Irreversible interactions are based
on the covalent binding of polyphenols to proteins resulting in complexes
which are significantly more stable than those formed by the reversible
interactions of hydrogen bonding and hydrophobic association. The
interactions occur via the transformation of phenols to quinones, highly
reactive intermediates which subsequently react with nucleophilic groups
on proteins or other macromolecules to form covalently bound complexes
(Beart et al ., 1985). Since the conditions required for this irreversible
interaction are relatively mild, it is entirely reasonable to predict the
occurrence of covalently bound polyphenol-protein complexes in situations
such as the microbial decomposition of plant tissues in the soil environment
(Haslam, 1989). This is essentially the well-known 'polyphenol theory' of
humus formation. The chemical structure of the recalcitrant forms of SOM
(the 'humic substances') reveals the presence of linked aromatic rings, and
phenolic acids are released when strong oxidation methods are employed
(Haynes, 1986). This, and other evidence, led to the 'polyphenol theory' of
soil organic matter formation which postulated that phenolic compounds
formed by degradation of lignins, together with microbially derived
phenols, are oxidized to quinones which polymerize with amino acids and
other products of degradation to form 'humus'. Although this theory fails
to account for the wide variety of structures found in the humic substances
(in particular the large amount of aliphatic C chains), the role of phenolic
compounds in reacting with other organic compounds to form stable
complexes is generally accepted (Wild, 1988).
Whetton (1999) investigated the difference in decomposition of
polyphenol-protein complexes where polyphenols were extracted from
leaves of Calliandra calothyrsus and Leucaena leucocephala . The extracted
polyphenols were purified by a series of steps including partitioning
between ethyl acetate (EA) and water (AW), column chromatography
using Sephadex LH20 and preparative high-performance liquid chroma-
tography (HPLC). Although all of the polyphenol fractions had the ability
to precipitate protein and had some degree of protein binding, analysis of
the crude polyphenol fractions showed that those fractions with very high
condensed tannin content and those with the highest molecular weight
were the most active in protein binding. In direct comparison, fractions
which contained condensed tannins of large molecular weight were shown
to be some 20-fold more biologically active than those containing simple
flavonoids. Complexes formed from the polyphenol fractions of Calliandra
and Leucaena leaves and crude leave proteins (CLP) of G. sepium were
added to an acid upland soil from Sumatra. The complexes formed from
small molecular weight polyphenols, such as flavonoids, procyanidins and
anthocyanins (fractions EA1 and AW1, Fig. 3.3), and CLP were found to
degrade rapidly in the soil. Microbial respiration was only slightly reduced
when compared with that of CLP alone, with 80% of added C recovered