Environmental assessment of surface runoff quality after biochar application under natural rainfall

Introduction
The non-point source pollution generated by surface runoff is recognized as a major environmental threat to the sustainability of soil and water resources. Biochar has attracted considerable attention as a soil amendment because of its ability to improve soil physical and chemical properties and reduce surface runoff. However, it remains unclear whether biochar application under field conditions improves runoff quality through enhancing soil properties or acts as a potential source of pollutants. Therefore, this study aimed to evaluate the environmental impacts of biochar application on surface runoff quality while simultaneously investigating changes in soil chemical properties under natural rainfall using standard erosion plots established on abandoned rainfed cropland.
 
Materials and Methods
This study was conducted on abandoned rainfed cropland located upstream of the Nowruzloo diversion dam in Miandoab, West Azerbaijan Province, within the Zarrinehroud Watershed, one of the major sub-watersheds of Lake Urmia. Six standard Universal Soil Loss Equation (USLE) erosion plots, each measuring 22.1 × 1.83 m², were constructed using galvanized steel sheets and installed with a one-meter spacing between adjacent plots. Biochar was produced from waste wood of Populus nigra through pyrolysis at 420°C under oxygen-limited conditions for 3 h, yielding approximately 44% of the initial feedstock mass. Two treatments, namely the control and biochar application at 32.5 t ha^-1, were established, each with three replicates. The experiment commenced in November 2021, and runoff samples for water quality analysis were collected from three effective rainfall events that generated surface runoff during the study period. Soil samples were collected at 6-month intervals following biochar application. The runoff quality parameters evaluated included BOD, COD, NO₃^-, PO₄^-3 and DOC, in addition soil components such as SOC, TN, and AP were determined.
 
Results and Discussion
Biochar application increased SOC, TN, and AP by 16.35–27.71%, 11.43–19.77%, and 2.96–36.37%, respectively, compared with the control treatment. The effect of biochar was significant for SOC and TN, whereas sampling time had a significant effect only on AP. However, neither biochar application, sampling time, nor their interaction had a significant effect on any of the investigated runoff quality indicators. The integrated assessment of soil chemical properties and runoff quality provided a more comprehensive understanding of the environmental implications of biochar application. The biochar application increased the soil SOC, TN and AP by 16.35 to 27.71, 11.43 to 19.77 and 2.96 to 36.37 percent respectively compared to control treatment, and the biochar effect on SOC and TN and the effect of time on AP was significant. The results demonstrated that biochar significantly enhanced SOC and TN and generally increased AP across most sampling periods, however these improvements were not accompanied by significant increases in organic pollutant or nutrient concentrations in surface runoff. In other words, the improvement in soil fertility resulting from biochar application did not increase the risk of surface runoff pollution. The different behavior of NO₃^- and PO₄^-3 can be attributed to the transmission mechanism of these two elements in the soil. NO₃^- due to its negative load and high mobility, it is mainly displaced by the water flow. Whereas PO₄^-3 is much less mobile due to its strong absorption on the surface of soil particles and iron and aluminum oxides. The results of the variance analysis showed that the TN component, in biochar plots, had a significant difference with the control plots with rates of 95 percent during the test period. The time factor and the interaction between biochar and time on the TN component was not significant. These results indicate that the increasing soil TN is mainly due to the biochar application and this effect had a relatively stable trend during the study period.
 
Conclusion
The results of this study showed that biochar application under natural rainfall conditions had no significant effect on runoff quality indicators, including BOD, COD, NO₃^-, PO₄^-3, and DOC, and did not increase organic pollution or nutrient transport. In contrast, biochar significantly increased SOC and TN, while also promoting a relative increase in AP, indicating an improvement in soil chemical properties and nutrient retention capacity. The integrated assessment of soil chemical properties and runoff quality demonstrated that the enhanced storage of carbon and nutrients in the soil was not accompanied by greater losses through surface runoff. These findings support the main hypothesis that biochar application does not pose significant environmental risks to runoff quality under field conditions. Overall, biochar improved soil chemical quality while preserving surface runoff quality, highlighting its potential as an environmentally sustainable soil amendment for land restoration and water resource protection in semi-arid regions.