Effects of Humic Acids on Calorific Value and Chemical Composition of Maize Biomass in Iron-Contaminated Soil Phytostabilisation

An interesting feedstock for energy purposes is plant biomass due to its renewability, widespread availability and relatively low cost. One plant with a high and versatile use potential is maize. Plants used for energy production can be grown in polluted areas, e.g., with iron. The aim of the study was to determine the effect of humic acids (HAs) on the yield, calorific value and other energy parameters and chemical composition of maize biomass applied as a phytostabiliser on iron-contaminated soil. The soil was contaminated with iron at 0, 250, 500 and 750 mg kg −1 . The HAs were added to the soil in the following amounts: 0, 0.3, 0.6 and 0.9 g kg −1 of soil. Soil contamination with iron had relatively little effect on the heat of combustion and calorific value of biomass and very strongly reduced plant height (42%), dry matter yield (95%) and energy production of maize biomass (90%), the SPAD index at the fifth leaf unfolded stage (44%) (as opposed to the stem elongation stage), sodium, magnesium and phosphorus contents, and increased calcium, potassium and nitrogen contents of maize. The application of HAs to the soil had a positive and very large effect on both the height and biomass parameters studied, resulting in an increase in plant height (22%), dry matter yield (67%) and energy production from maize biomass (62%). Changes in the heat of combustion and calorific value of the biomass were minimal but positive. HAs contributed to a decrease in the value of the SPAD index during the stem elongation phase of maize and in the content of all macronutrients in maize biomass as a result of a reduction in the effect of iron on macronutrient content and to a significant increase in maize dry matter yield in plots with their application. The application of HAs appears to be an effective adjunct in the phytostabilisation of iron-contaminated soils by growing crops for energy purposes.