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Biochar is an organic amendment used for soil remediation, there are only a few studies documenting the effects of nitrogen on the role of biochar in contaminated soils. A pot experiment was conducted to investigate the impacts of biochar (0%, 1%, and 2.5%, w/w) and nitrogen (0, 100, and 200 mg N kg) on plant growth, nutrient and cadmium (Cd) uptake of Cichorium intybus. N, P, Ca, Mg, and Cd concentrations increased with N level in 0% and 1% biochar treatments. In plants treated with 2.5% biochar, 200 mg N kg addition caused significant reductions of N, P, Ca, Mg, and Cd concentrations in comparison to 100 mg N kg treatments. Nitrogen promoted shoot biomass at all biochar treatments, while biochar had no effect on shoot biomass in 0 and 200 mg N kg addition treatments. Nitrogen also significantly increased N, P, K, Ca, Mg, and Cd contents in the 0% and 1.5% biochar addition treatments. Although soil DTPA-extractable Cd concentration showed the lowest values in 1% biochar in combination with 100 and 200 mg N kg addition treatments, lowest shoot Cd concentration, and relatively high shoot biomass occurred in the 2.5% biochar + 200 mg N kg treatment. Based on these results, biochar application at its highest rate (2.5%) in combination with high N supply (200 mg N kg) contributed to both crop yield and agricultural product safety. N input alone might increase the risk of human health, and the optimum N dose should be determined during phytostabilization process.

An incubation experiment is conducted at 20 °C for 30 days to explore the influence of biochar, nitrogen, and their interactions on CH4, CO2, and N2O emissions, as well as C and N variations in soils with milk vetch and rape residue amendment or in soils without plant residue. Both the biochar and nitrogen additions had negative effects on the cumulative CH4 and CO2 emissions in the presence of milk vetch and rape residues. Regardless of the biochar and nitrogen addition, the CH4 and CO2 emissions from soils amended with milk vetch residue are the highest, followed by those from soils amended with rape residue and those from soils with no plant residue. With the N application, N2O emissions from soils amended with rape residue are significantly higher than those from soils without plant residue, whereas the N2O emissions from soils amended with milk vetch residue are significantly lower than those from soil without plant residue. DOC is positively related with CO2 emissions. NH4+N, NO3−N, and TIN are also positively correlated with N2O emissions. In soils with the N fertilizer application, milk vetch residue, and biochar decreases the global warming potential, whereas rape residue increases the global warming potential. The results indicate that milk vetch residue in combination with biochar could benefit greenhouse gas mitigation and should be recommended for agricultural practices at high nitrogen level. In contrast, rape residue incorporated with biochar should be applied cautiously, especially in N abundant soils.