Abstract: A field experiment was carried out using a PVC tube in-situ incubation method to explore effect of harvest cutting on soil net nitrogen (N)  mineralization rate and net nitrification rate in temperate secondary forests one year after the felling operation varying in intensity in the Maoershan region of Heilongjiang Province. Results show that in terms of means of soil net N mineralization rates and net nitrification rates in the 0 - 10 soil layer, the treatments followed an order of afforestation after clear cutting > farming after clear cutting > 50% of the stand volume remove>25% of the stand volume removed > control. Soil water content in the 0-12 cm soil layer was  significantly higher in the harvested plots than in the control.Both net N mineralization rate and net nitrification rate were significantly and positively related to soil water content. Harvesting also increased soil organic carbon content. All the findings suggest that the increased soil water and organic carbon are the main causes of the increase in soil net N mineralization rate and net nitrification rate after harvest cutting. In addition, harvest cutting also triggered change in soil inorganic N content. During the growing season, the means of NO₃^--N contents in the harvested plots were significantly higher than that in the control, while the means of NH₄⁺-N were quite the contrary. Soil inorganic N was dominated with NO₃^--N in the harvested plots, and with NH₄⁺-N in the control. The above findings suggest that harvest cutting may increase NO₃^--N content in the soil, and hence, the risk of losing soil nitrogen in this region.

Key words: net N mineralization, net N nitrification, harvest cutting, secondary forest