Studies on the structure and behaviour of (poly)-maleic acid, as a model soil organic polymer
(Poly)-maleic acid (PMA) was obtained by the pyridine-catalysed homopolymerization of maleic anhydride, followed by acidification with resin. PMA was found to show many structural similarities with the humus extracted from a locally-derived podsol Bh horizon; spectroscopic (IR, NMR, U.V./Visible) and chemical degradation studies of PMA showed that although a degree or aromaticity undoubtedly exists, the predominant structural feature was an aliphatic or alicylic backbone substituted predominantly by vicinal carboxyl groups. Pyrolysis of PMA yielded as a major product, 2-cyclopenten-1-one, which has hitherto not been detected from any natural source other than soil pyrolysates. Elemental and functional group analyses, and potentiometric titration of the molecular size fractions of PMA revealed that the low molecular weight fraction resembled a "typical" fulvic acid, whereas the high molecular weight fraction resembled a "typical" humic acid. 14C-labelled radioisomers of PMA were added to soil and incubated in a closed system. Estimation of the labelled carbon dioxide liberated showed that PMA did not grossly alter microbial respiration compared to unlabelled control samples. The polymer was only slowly degraded in soil, with less than 1 per cent of the radiolabel being released as 14CO2 during eight weeks. Organic and inorganic soil components were found to adsorb PMA. The clay mineral montmorillonite strongly adsorbed PMA, which produced expansion of the interlayer. Adsorption mechanisms were discussed. The secondary clay mineral proto-imogolite was found to inhibit the incorporation of PMA into the soil humic fraction, by the production of an acid-soluble complex. It was postulated that such a complex might occur in the podsolisation process. PMA was found not to cause any gross effects on soil processes. Ryegrass grown in soil was found to be tolerant to high levels of PMA amendment. Small amounts of radioactivity, monitored in the shoots and roots of ryegrass grown in soil amended with 14C-labelled PMA, probably arise via 14C-carbon dioxide.