Freeze-thaw Cycle Effects on Nitrogen and Phosphorus Content in Typical Soils of Northeast China

The influence of freeze-thaw cycles(FTC)on the total nitrogen,available nitrogen,total phosphorus and available phosphorus of black soil(BS),dark brown soil(DS) and paddy field soil(PS)were studied.The result demonstrated that the total nitrogen degree of the three types of soil had no significant changes after 15 times of FTC,which suggested that nitrification and denitrification had no obvious co-effect during this process.After 15 times of FTC,the available nitrogen degree of the three types of soil increased as a result of intensifying of soil water releasing and water permeability caused by FTC,and was the most remarkable for the available nitrogen of the PS which was increased by 31.25%.The total phosphorus degree of the three types of soil increased significantly after the first FTC,and then showed the increased tendency gradually,which was probably due to the damage of soil aggregate by FTC and the increased opportunities of reacting with the digestive system.After 15 times of FTC,the available phosphorus degree rose by 17.72% and 56.16% in BS samples and PS samples,respectively,and descended by 35.19% in DS samples.This showed that the change of available phosphorus degree was not only influenced by the C/P degree of soil but also by the land use pattern,and the available phosphorus degree could descend due to the net immobilization of phosphorus in forest land.