Lead (Pb) contamination in soils is found in many parts of the world. Previous research has shown that high concentration of Pb affects microbial activities and inhibits the reactions in the nitrogen (N) cycle. Over a week, we gradually contaminated soils with Pb (lead acetate) aiming for the final concentration of 0, 1000, 5000 or 9000 mg Pb kg −1 soil. 3 atom% 15 N urea was added to the soils after contamination, followed by 2-week incubation. The concentration and 15 N ratio of extractable

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... 3 -, NH4 + and immobilized N were measured on days 1, 3, 7 and 14 of the incubation period. To measure isotopic N ratio of each N pool, we chemically converted all of them to NO3by persulfate oxidation, and then to N2O using spongy cadmium with further reduction using sodium azide in an acetic acid buffer. 15 N2O laser was used for the measurement. The result showed that all the added urea quickly became NH4 + by day 3 and then turned to NO3by the end of the incubation. As for NH4 + , the concentration and 15 N ratio were not significantly different among different treatments. Significantly higher concentration and lower 15 N ratio of NO3were seen in early period of the incubation with high Pb. This suggested that with Pb, soil NO3increase was mostly due to added urea whereas soil-N contributed to the increase in NO3when Pb contamination was minor. Also, there was a trend that biomass N is lower in more contaminated soils, although it was not significant. The effect of Pb may influence the microbial utilization of soil-N and possibly on nitrification process. Stress response of the plant-soil microbial carbon continuum of a forest understory ecosystem to extreme heat and drought ○von Rein, I.