The contribution of lignin to the composition of humic acids from a wheat‐straw amended soil during 3 years of incubation in pots

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Abstract
SUMMARY: An Andisol from the Vesuvian volcanic area, low in organic matter, was treated with wheat straw (20/1 w/w) and urea (500/1 w/w) in pots, and maintained at 60–90% of field capacity in an open greenhouse in order to study the yields and properties of humic acids after 30, 122 and 224 d, and 1, 2 and 3 years of incubation.During the first year the yields increased, but subsequently they showed a decrease. However, the chemical and spectroscopic properties of humic acids remained substantially constant.The chemical characteristics of modified lignin from added wheat straw were recognized in the properties of humic acids by UV‐visible, FT‐IR and 13C CP‐MAS NMR spectroscopic analysis. Evidence was obtained that these modifications included the microbial removal of carbohydrates, the oxidative destruction or transformation of the side chains of lignin units, the demethylation of mainly guaiacyl, and subsequent oxidation of catechol‐like structures to o‐quinone. The latter can be postulated to promote the nucleophilic addition of amino compounds with formation of aromatic amines in humic acids.