Chemical and structural characterization of soil humic substances under different land use systems in sub-tropical regions of northeast India

Four different tree or crop species (Champak, tree bean, alder_and Khasi pine in agroforestry land use; maize, potato, rice and turmeric in agriculture land use; pear, peach, Khasi mandarin and guava in horticulture land use) were selected and compared with control plots. Soil humic acids (HAs) were extracted from 0-15 cm layer and characterised by Fourier Transform Infrared Spectroscopy (FTIR), elemental composition, total acidity and functional groups analyses. The HAs showed an increase in the carbon and nitrogen contents, followed by a reduction in C/N ratios in agroforestry and horticulture land use systems over agriculture and fallow lands. Generally, C and N contents of HAs were decreasing in the order: agroforestry > horticulture > agriculture > fallow lands. The relatively low E4/E6 values (4.86) of HAs in the agroforestry land use indicated the prominence of aromatic components, suggesting that the HAs were of high molecular weight. The relatively high E4/E6 ratios (5.63) of HAs under agriculture land use indicated the dominance of aliphatic components, suggesting that HAs were of low in molecular weight. The total acidity, carboxylic (-COOH) and phenolic (-OH) groups of HAs were found to be significantly higher under agroforestry land use followed by horticulture and agriculture land uses. Conversely, adoption of various agroforestry tree species, horticulture fruit trees and agriculture crop species increased the C and N contents, total acidity, carboxylic OH and phenolic OH groups of HAs compared to fallow lands.