| The black soil region that is in northeast China is located in the temperate semi-humid monsoon climate zone, with the climate characteristics of cold and dry, winter wet and heat summer in northeast China. During six-month winter and spring season, the soil is in a cycling of freezing and thawing, with an obvious seasonal freezing-thawing phenomenon, the farmland soil moisture changes frequently due to the intensive rain and high temperature during the summer, therefore drying-rewetting alternate is a widespread phenomenon in this area. The processes of freezing-thawing and drying-rewetting are the important external agent of nitrogen migration, and an important driver of material exchange and energy change in the black soil.The simulated experiments were conducted by using the black soil collected from the black soil region in northeast China. The objectives of this study were to reveal the influence of different soil moisture and drying-rewetting frequency,as well as long-term and short-term of freezing-thawing on nitrogen transformation and soil enzyme activities in black soil under different hydrothermal conditions, and to preliminarily to discuss the mechanism. The main results found in this study were as follows:1. The effects of soil moisture. The contents of both ammonium nitrogen and nitrate nitrogen in soils with the low treatment(20%) were lower than those with the high soil moisture(35% and 60%) when the samples were cultivated for the same time. While the nitrogen transformation velocities increased with the increase of soil moisture. The velocity of ammonification, mineralization and nitrification in all the treatments decreased gradually with the increase of cultivation time. The urease activity changed greatly at the beginning of cultivation and then it maintained steady in different soil moisture treatment groups. However, the invertase activity, the pattern was fluctuant during the whole process, but significant difference was not found in different soil moisture processing group of urease activities and invertase activities.2. The effects of drying-rewetting alternation. The contents of ammonium nitrogen and nitrate nitrogen decreased during the whole culture period, and the similar trend was also observed in the control group. Compared with the soils in control group, ammonium nitrogen and nitrate nitrogen contents increased under the effects of drying-rewetting alternation. With the increase of training cycles, nitrogen transformation rates were fluctuated after a sharp drop, also the transformation rates in drying-rewetting treatment were higher than that of constant soil moisture treatment. Influenced by soil moisture and drying-rewetting alternation, urease activities were lower than that of the control group(except for 0.5 and 3 cycles). During the rest training cycles, urease activity was shown as: the experimental group > the control group. And invertase activities also showed the same pattern, except for the eighth cycle in which the invertase activities of control group were higher than that of the experimental group.3. The effects of freeze-thaw cycles. The contents of ammonium nitrogen and nitrate nitrogen increased gradually with the increase of freezing and thawing cycle. The content of ammonium nitrogen presented in the following order: the freeze-thaw cycles in low intensity(-10-5 ℃) > vigorous intensity(-25-5 ℃) > the control group(5 ℃), except for the first round in the low intensity freeze-thaw cycles. Similarly, freezing-thawing cycles exerted significant influence on the nitrate nitrogen content. The freeze-thaw cycles reduced the velocity of mineralization and nitrification. The activities of urease and invertase in soils with low intensity freeze-thaw cycles were higher than that of vigorous intensity(except for the first cycle), however, the enzyme activities of soil under both the treatments were lower than the control group due to the limitation of soil temperature.4. The effects of long-term freeze-thaw cycles. The function of long-term freeze-thaw was beneficial to accumulate ammonium nitrogen content in the soil, while, nitrate nitrogen content showed a falling trend. With the increase of incubation time, the ammonification velocity increased as well as the mineralization velocity, while nitrification velocity finally presented a negative increase in the outdoor freezing and thawing treatment. Compared with the soils in control group, only the urease and invertase activities in the 3rd parallel soil sample with higher soil moisture have certain enhancement, however, urease and invertase activities in the other two parallel samples both decreased.5. The calculation of Pearson correlation analysis indicated, the relevant index of nitrogen transformation in soils had significant correlation with urease activity, except for different moisture conditions, with no significant correlation between ammonium nitrogen and short-term freezing and thawing process that had poor correlation with the mineralization velocity. While, besides the invertase activities appeared no significant correlation to the relevant index of nitrogen transformation under dry-wet alternate condition, nitrogen transformation velocities under different water contents and freezing-thawing action, also inorganic nitrogen(ammonium nitrogen, nitrate nitrogen) in freeze-thaw cycles had better correlation with invertase activities.In a word, the dry-wet alternation and freeze-thaw processes have important influence on soil nitrogen transformation and soil enzyme activities. |
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