Spectroscopic and biological properties of humic substances extracted from earthworm coprolites in a long-term experiment treated with manure and mineral fertilization

Humic substances (HS) are the most important natural soil conditioners because they play an important role in guaranteeing long-term fertility. In this study the structure and biological properties of HS of earthworm faeces were investigated in order to better understand how fertilization practices can influence their chemical and biological properties. The study was conducted in the Experimental Farm of Padua University, at Legnaro (NE Italy 45° 21' N; 11° 58' E; 6 m a.s.l.) as part of the longest running rotation experiment in Italy. The soil is a fluvi-calcaric cambisol (CMcf), silty or sandy loam, with a sub-basic pH. The trial considers 4 treatments with maize as the main crop. These allow the comparison between fertilization with only organic (farmyard manure, 60 t ha-1 y-1, 20% d.m.), only mineral (high mineral input, 300 kg N ha y-1, 66 kg P ha y-1, 348 kg K ha y-1) or mixed inputs (farmyard manure, 30 t ha-1 y-1, 20% d.m. plus mineral input, 150 kg N ha y-1, 33 kg P ha y-1, 174 kg K ha y-1) and no fertilization. The experimental layout is a randomized block with three replicates, on plots of 7.8 × 6 m. The coprolites taken from each plot were treated with alkaline solution to extract the HS. Gel permeation chromatography, infrared spectroscopy and surface-enhanced Raman scattering (SERS) were applied to study the structure of HS. The biological activity of HS was also investigated by assaying the auxin and gibberellic-like activity. The results obtained have shown that a relationship exists between different fertilization practices and chemical and biological properties of HS produced by earthworms.