Abstract: A laboratory incubation test was conducted to study the effects of litters and tannin on forest soil nitrate- and ammonium N. The addition of litters and tannic acid made the soil nitrate-and ammonium N　decreased. With the addition of fir litter, the nitrate- and ammonium N contents in red soil decreased by 6.1%-25.9% and 19.7%-68.6%, respectively, and the decrements in yellow-red soil were higher than those with the addition of bamboo litter, being significant for ammonium N. Compared with the control, the addition of tannin decreased the ammonium N content in yellow-red soil significantly, and there was a positive correlation between the concentration of added tannin and the decrement of soil ammonium N content. When the concentration of added tannin was high, the decrement of the ammonium N reached 31.9%-57.8%. With the addition of low concentration tannin, the soil nitrate N content decreased with time, and the decrement on the 84th day reached 4.5%. However, the addition of high concentration tannin increased the soil nitrate N content by 10.3%-18.5% in the first 7-28 days, but decreased it by 23.9% and 42.3% on the 56th and 85th day, respectively.

Key words: litter, tannin, nitrate nitrogen, ammonium nitrogen, forest soil, root order, fine root trait, seasonal dynamics, poplar plantation, productivity decline.