Characteristics of organo-mineral complexes in contaminated soils with long-term biochar application

Long-term studies on the environmental effects following biochar additions to soils, while plentiful, are predominantly focused on the soil fertility, whereas few are on the soil organo-mineral complexes. This study examines the changes of organo-mineral complexes in an acidic paddy soil and a saline-alkali soil which were remediated using biochar for approximately 8 years and 3 years, respectively. The results showed that loosely combined humus increased by 30.1% and 25.1% with the application of 40 t ha^-1 biochar in the acidic paddy soil and the saline-alkali soil, respectively. Meanwhile, an increase of cement (Fe-oxides) was the contributor to the rise of the complexes content. Complex iron in the saline-alkali soil were 30% higher than in the acidic paddy soil with the application of 40 t ha^-1 biochar. Fourier Transform Infrared Spectroscopy showed oxygen-containing functional groups on the surface of the biochar separated from the remediated field. X-ray diffraction analysis indicated that both complexation and sedimentation were involved in heavy metal immobilization. It was found that biochar amendment mitigated the effect of acid rain leaching and reduced vertical migration of the Fe/Al-bound complex, which can prevent soil from podzolization and thus improve its fertility.