Effects of nitrogen application rate on the microorganism and soil enzyme activities with different growth stages of potato plant in the cold saline-alkali soil

Field experiment was conducted to assess the effects of the different nitrogen fertilizer application on microorganism and four kinds of soil enzyme activities in different growth stages of the cold saline-alkali soil for potato plant.Three different N application amounts were set: 0 kg ·hm- 2,90 kg· hm- 2and 180 kg· hm- 2,which were called low nitrogen,middle and high nitrogen treatments respectively.Main results were as follows: compared to low nitrogen treatment,middle and high nitrogen treatments could increase microorganism density and enzyme activities with different growth stages of the soil for the potato cultivation.With the process of the growth stage,the total amount of the microorganis displayed a patterns which ascend firstly then descend.There were no significant difference for soil enzyme activities,urease and invertase activities of soil in low nitrogen treatment but after seeding stage,neutral phosphatease activity of soil was decreased.However,catalase activity of soil was significantly increased; invertase and neutral phosphatase activities in both middle and high nitrogen treatment exhibited to rise firstly and then decrease tendency,finally increase again,while the catalase and urease activities displayed continuous increasing tendency with the plant maturation.In addition,the microorganis and enzyme activities decreased with soil depth increasing within a certain range.