Structure-function relationship of vermicompost humic fractions for use in agriculture

Abstract

The use of humic substances (HS) in agriculture is beneficial and has positive environmental impacts. However, to optimize the use of HS possible links between their structural characteristics and bioactivity must be shown. The goal of this study is to evaluate the bioactivity of different humic fractions extracted from vermicompost (VC) in rice plants and to shed light to possible structure-function relationships. Humic-like fractions were obtained from cattle manure vermicompost processed by African nightcrawlers (Eudrilus eugeniae spp.). Humic-like acid fraction using only water as extractor (HLAw), HLA fraction extracted following the International Humic Substances Society (IHSS) recommended method, and the solid residue (humified residual (HR)) after extraction of HLA were characterized using complementary chemical, physic, and spectroscopic technics (elemental composition, UV-Vis and Fourier transform infrared spectroscopy (FTIR) spectroscopies, 13C-CP MAS NMR, and MEV). Biological activity of the three HS was conducted in growth chambers and measured in roots using WinRhizo Arabidopsis software. Principal component analysis (PCA) was used to find a grouping pattern between the structural variables evaluated and the obtained root parameters. Differences were found in elemental composition among HS with larger C/N ratio in HR than in HLA and HLAw. HLA and HLAw FTIR spectra showed carboxyl band at 1714.66 cm−1 better resolved than in HR. Bands at 1642 cm−1 (amide I) and 1510 cm−1 (lignin), were better resolved in HLA. 13C-NMR showed the following order of aromaticity: HLA > HLAw > HR. For HLAw bioactivity, the structures CAlkyl-H,R, CC=O, and CCOO-H,R correlated with the number and growth of smaller root. The aromatic CAr-H,R, CAr-O,N, and aliphatic CAlkyl-O,N, CAlkyl-O, and CAlkyl-di-O structures in HLA, correlated with larger roots growth. HR also stimulated root growth and development in rice plants. Aliphatic and oxygenated structures in HLAw showed a relation with induction of initial root emissions, whereas the presence of aromatic compounds in HLA was related with root growth stimulation activity. Higher concentration of HLAw was necessary to produce an equivalent stimulus compared with HLA; it could indicate that, although both fractions showed similar types of structures in their composition, differences in the predominant structures may be determining different effects on the root.