Discrepant responses of the electron transfer capacity of soil humic substances to irrigations with wastewaters from different sources.

An increasing area of agricultural land is irrigated with wastewater worldwide due to the scarcity of fresh water resources. Whether wastewater irrigations can affect the electron transfer capacity (ETC) of natural organic matter in soils is unclear. In this study, we assess the responses of the electron-accepting capacity (EAC) and electron-donating capacity (EDC) of soil humic substances (HS) to irrigations with wastewaters from different sources. We show that the EAC of soil HS increases and the EDC of soil HS decreases after irrigation with domestic wastewater. Conversely, the EAC of soil HS decreases and the EDC of soil HS increases after irrigation with industrial wastewater. The EAC and EDC of soil HS exert no changes after irrigation with pharmaceutical wastewater. Irrigations with wastewaters from different sources can cause the distinct directions of changes in the activities of lignin peroxidase and laccase by altering the content of labile organic carbon, heavy metals or antibiotics in soils, thereby changing the chemical structures and finally the ETC of HS along different directions. These results can provide insights into the role of HS in environmentally relevant processes in agricultural soils under the context of wastewater irrigations.