| Due to unitary composition of tree species, simple structure and human disturbance, pinus massoniana (P. massoniana) plantations have showed a series of problems, such as site degradation and decline productivity, etc. To solve these key scientific issues of P. massoniana plantations, cultivation of mixed forest and tending thinning in plantations were used. Thinning is one main way of forest management, and forest gaps after cutting plays an important role in the regulation of growth, structure and biological diversity in forest plantation. However, whether these gap may lead to a positive effects and the most appropriate area of gaps on the forest ecosystem of inefficient plantations or not, and these question still maintain unknown. In the present study,39-year P. massoniana plantations at the hilly region of the upper reaches of the Yangtze River were selected as objective, and the gaps with seven different size (100,225,400,625,900,1225 and 1600 m2) in these plantations were constructed. The soil samples were set as control. The effects of gaps on total nitrogen, ammonium nitrogen, and nitrate nitrogen, as well as the changes of ureaase. protease hydroxylamine. and nitrite reductase activities in soil were investigated, and the mechanism of soil nitrogen transformation in these gaps was analyzed. The present results were shown as following:Gap size has significant influence on the ammonium nitrogen content in soil, and no significant differences were observed on other nitrogen composition. The ammonium nitrogen and total nitrogen contents in the center of gaps showed rising tread with increasing gap size, and then decreased. The maximum value was observed in the gap of 400 m2. However, The ammonium nitrogen and total nitrogen contents, in the edge of gaps, gradually decreased with the rising gap size, and then raised, finally decreased. The minimum values were observed in the gap of 400 and 625 m2. Nitrate nitrogen content in the center and edge of 100 m2 was significantly higher than those of in other gaps, and they showed a shape of "M" trend in the center of 225-1600 m2. These results were corrected with the changes of protease activity in the Autumn and Winter. Nitrogen transformation enzymes in the gaps with little size, except for nitrite reductase, showed higher activity than those of in the gaps with large size. However, nitrite reductase activity showed a contrary trend. The effects of gap size on soil nutrient content and enzyme activity may be related to formation period of gap, and further long-period study still need.The hydroxylamine reductase activity were significantly different in the center and edge of different gap size. The activity in the center of gap in the Spring and in the edge of gap in the Autumn was significantly higher than those of in under forest. However, the activity, except for the edge in the Summer, was significantly less than those of in under forest. The activity in the center of gaps in the Spring and Summer was significantly higher than those of in the edge, but the opposite results were observed in the Autumn and Winter. These findings indicated that the activity in the gaps is generally higher than those of under forest. The changes of available nitrogen and enzyme activity may be related to biodiversity, soil microorganisms, moisture and temperature in the gaps. Moreover, our finding also suggested that available nitrogen and enzyme activity showed no differences between the edge and center of gaps, which may be related with no significant differences of plant diversity and hydrothermal environments during early period of gap formation, especially in little gaps. After gap formation in P. massoniana plantations, environmental heterogeneity might lead to the changes of soil enzymes activity, and these complex mechanism still need to explore.Season dynamic will lead to the changes of temperature, soil water and the nutrient content. The present results indicated that season dynamic have significant effects on total nitrogen, ammonium nitrogen, nitrate nitrogen, urea enzyme, hydroxylamine reductase, and nitrite reductase. The order of ammonium nitrogen content in the center and edge of gaps was shown as following:Spring, Summer, Autumn and Winter. The nitrate nitrogen content in the Spring and Summer is significant less than those of in the Autumn and Winter. The peak values of protease activity were observed in the Spring and Autumn, and the trough values appeared in the Summer and Winter. However, the peak values of hydroxylamine reductase activity were observed in the Winter, and the trough values occurred in the Autumn. These results are agreed with the seasonal changes of nitrite reductase and urease activity. These findings indicated that soil denitrification in the gaps is weaker than those of under forest, which will result in nitrate nitrogen loss for the utilization of plant and micro-organisms as well as leaching of rains, etc. Less protease and hydroxylamine reductase activity in the hot Summer, indicating that this may be due to decreased total nitrogen causing ammonia mineralization under high temperature. Moreover, the dynamic changes of protease activity may be also corrected with the higher ammonium nitrogen content in the Spring compared to in the Summer.In summary, the formation of gap may lead to the changes of microenvironment in the gaps, and further improve the nutrient cycling processes of soil, which might help to restoration and renewal of aboveground vegetation. Moreover, soil temperature and moisture may promote the accumulation of available nitrogen and the activation of nitrogen transformation related enzymes, and further inhibit the soil denitrification in the gaps. Thus, the present findings indicated that soil temperature and moisture may be two key factors in the nutrient cycling processes during the early formation period of gaps. |
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