Phosphorus fractions under different land uses in Sanjiang plain

Five land-uses type (soybean-filed, rice-field, abandoned cultivation, artificial forest, natural wetland) were obtained before planting and soil P fractions were determined by a modified Hedley fraction method. The result showed that the content of total P(TP) in natural wetland was higher than other soil. While the proportion of total inorganic P(TPi) in TP of farm land soil was higher than that of wetland. Adversely, the proportion of total organic P(TPo) was higher in wetland and least in rice-field. There was no obvious difference between that of abandoned field and artificial forest. The values of labile inorganic phosphorus (labile Pi) under different land-uses were similar. The contents of Resin-P and NaHCO3-Pi varied at a range of 32-36.3 mg·kg-1 and 33.77-50.42 mg·kg-1 respectively. The contents of NaOH-Pi and C.HC1-Pi in farmland were higher than those of wetland, especially in rice-field which was 152.4 mg·kg-1. But for D.HCl-Pi, the content was higher in wetland with the value of 84.3 mg·kg-1, and the contents of NaOH-Pi, C.HCl-Pi and D.HCl-Pi in abandoned field and artificial forest were in the middle of the above two land use patterns, which indicated that the restoration of soil P in abandoned farmland tended to approach the level of natural wetland. The value and percentage of Po was decreased greatly with the reclamation of wetland, and the decrease of C.HCl-Po was the most significant in all forms of organic P, and its values in soybean-filed and rice-field were only 25.96% and 19.26% of that in wetland. As the time went by the content of Po in abandoned arable land increased with very slow speed, which indicated that the restoration of wetland after reclamation would need a long time. The distribution of soil P in different P fractions has significant difference for the land-use change in Sanjiang plain.