中文摘要：

以黑土为试验材料,通过室内培养实验,研究了不同冻融循环（-25—5℃、-10—5℃）对黑土氮素转化速率和土壤酶活性的影响。结果表明：随着冻融频次的增加,铵态氮含量和硝态氮含量均逐渐增加。除了中强度冻融（-10—5℃）的土壤铵态氮在培养第1周期内的含量最低外,剩余培养周期内,铵态氮含量均表现为中强度冻融（-10—5℃）〉高强度冻融（-25—5℃）〉对照组（5℃）;硝态氮含量对不同冻融温度的响应差异显著。与对照组相比较,冻融循环整体上降低了土壤的矿化速率和硝化速率,但实验末期有促进硝化的趋势,不同强度冻融对矿化速率和硝化速率影响显著。受土壤温度和冻融交替的影响,土壤的脲酶活性和转化酶活性均低于对照组,且脲酶活性和转化酶活性均表现为中强度冻融〉高强度冻融（第1周期除外）。综上,冻融作用对黑土的供氮能力和土壤酶活性具有重要影响。

英文摘要：

The black soil area in China is typically characterized by freeze-thaw alternation,however,little is known about impacts of the freeze-thaw cycles on N availability in black soil. An indoor bacth experiment was conducted to investigate effects of freeze-thaw cycles different in temperature range（- 25—5 ℃ and- 10—5 ℃） on net nitrogen transformation and soil enzyme activity in black soil. Soil ammonium nitrogen and nitrate nitrogen content was determined with the2 mol L- 1（ 1∶5 soil liquid ratio） KCl extraction method,and analyzed with the automatic chemical analyzer,Smartchem200. Soil urease activity measured with the indophenol blue colorimetric method and soil invertase activity determined with the 3,5-dinitrosalicylic acid assay method. Results show that the contents of ammonium nitrogen and nitrate nitrogen increased gradually with frequency of the freezing and thawing alternation and varied with intensity of the cycle. In terms content of ammonium nitrogen,the following order was observed： cycles moderate in intensity（- 10—5 ℃） cycles high in intensity（- 25—5 ℃） control（ 5 ℃）,except for the first round of freeze-thaw cycle moderate in intensity.However,freezing-thawing cycles did not have much influence on soil nitrate nitrogen. On the whole,freeze-thaw cycles reduced soil nitrogen mineralization and nitrification rates,and the effect was obviously affected by the intensity of freezing and thawing,but at the end of the experiment,the cycles displayed a tendency of promoting nitrification. As affected by low temperature and the alternation,the activities of urease and invertase in the soil under the cycles moderate in intensity were higher than that in the soil under the cycles high in intensity（ except for the first cycle）,however,both were lower than control. So the effect of soil freezing and thawing on black soils has an important influence on nitrogen transformation and soil enzyme activity. Urease activity and soil nitrogen supply capability are significantly