Spectroscopic behaviour of 14C-labeled humic acids in a long-term field experiment with three cropping systems

Abstract

The stabilisation of 14C-labelled farmyard manure was investigated under different cropping systems (crop rotation, monoculture, and bare fallow) in a long-term field experiment established in 1967. Solid-state 13C-NMR of bulk soils yielded a gradient of increasing aromatic properties in the order: straw manure–crop rotation < straw manure–monoculture < straw and farmyard manure–bare fallow. The opposite trend was observed for O-alkyl groups. The farmyard manure–bare fallow treatment was used to investigate changes of humic acids (HAs) with time. The FT-IR bands of aromatics, carbonyl groups, and a band of methyls and benzene rings increased over the 36 years of the experiment, whereas 2 amide bands and a band of sulfone and/or ester groups decreased. Fluorescence spectroscopy verified the increase in aromatic properties with age. Consequently, during soil organic matter stabilisation, HAs showed increasing properties of carbonyl and aromatic groups, whereas amidic groups decreased. The dynamic character of HAs, as shown by 14C, was also reflected by distinct spectroscopic changes over the period of investigation.