Study On The Dynamic Changes Of Soil N In Forest Land Returned From Farmland

The spatial and temporal changes of soil nitrogen in forest land where farmland was returned to Triploid Populus tomentosa, Cunninghamia Lanceolata, and Bambusapervariabilis X Dendrocalamopsisvalidus forest land were analyzed such as the characters of soil N, the rules of changes, the characters of soil profile and the temporal change, the results showed that:(1) The soil N had conspicuous difference between the forest land and farmland, but the soil organic matter content did not. Among the forest land NH4+-N, NOs'-N and available N content had conspicuous difference, but the total N and the hydrolysable N did not whose content in China fir land was the highest.(2) To compare after returning forest with before, the change tendency of soil N was complicate in the forest land. The organic matter, NH4+-N, available N content have conspicuously increased; the soil total N in Populus, Bamboo lands increased, conversely it reduced in China fir land; the variations of NOs'-N and the hydrolysable N were on the contrary to the total N.(3) In farmland and forest land, the profile character of soil N showed differently: the organic matter and total N content in the upper soil was much higher than hi the below; with the soil depth's increasing the soil NFLj+-N content appeared reducing and its change was not obvious hi Bamboo land; the change of the soil NOa'-N content was complicate that was divided into increasing type, decreasing type, and moderate type; the available N's variation was the same as NCV-N's.(4) Temporal changes of each soil layer were different in farmland and forest lands. The organic matter and total N's variation of the upper and below layer were identical whose range remained constant in each season; the soil NlV-N content in the upper layer was higher than hi the below in Spring and Autumn, but this change was complicated hi Summer and Winter owing to different forest measures; the peaks and valleys of NCV-N hi the upper and below soil layer appeared in the same tune, but the change showed no laws in different forest lands; the variation of the available N was approximate to NHU+-N; the range of each layer's change of the hydrolysable N was identical that appearedparallel lines in figure.(5) Soil nitrogen's temporal changes had obvious dynamic characters. The organic matter decreased in Autumn and reached to a valley in Winter, then it raised to the peak in Summer and Autumn; the total N fluctuated which change range is smaller than 5%; the soil NHt+-N rose firstly, reduced in Autumn and Winter, then rose in Spring; additionally, its peak appeared in Summer and valley in Winter; the soil NCV-N's change was more complicated than NHU+-N and fluctuated with season whose peak appeared in rain season; the available N's change tendency was similar to NH4+-N, it decreased in May; the hydrolysable N fluctuated in smaller range, and its content decreased steadily and showed no apparent peak in a year.(6) The spatial and temporal changes of soil nitrogen were the effects of environment and vegetation types. Rainfall, soil physical and chemical character, vegetation types and farming activities affected the soil nitrogen content in some degree, and the degree of effect should be researched deeply.