| Eucalyptus is the main afforestation tree species in South China. It has brought lots of economic and social benefits to society because of its large area planted and successive rotations. But we should be noted that whether these planting methods will lead to soil fertility decline and other potential risks. Soil phosphorus is one of the three essential major nutrients for plant growth. Due to the most of main region of Eucalyptus cultivation in China are lack of soil phosphorus, phosphorus has become a limiting factor for eucalyptus growth. In order to explore the effects of continuous planting of eucalyptus on soil phosphorus forms and adsorption-desorption properties, we selected the three different generations of Eucalyptus plantations and the shrub-grass land as research object for a one-year position monitoring. The main findings are as follows:(1) The variation features of soil total phosphorus content in different generations are as follows:the first generation is 0.403g-kg"’, the second generation is 0.392g-kg’’, the third generation is 0.468g-kg-’, the shrub-grass land is 0.4156g·kg-1. With the season changes, soil total phosphorus content did not change significantly (p>0.05). With the increase of Eucalyptus generations, the soil total phosphorus content increased slightly, but did not reach the significant level.(2) With the increase of Eucalyptus generations, the content of soil available phosphorus showed a decreasing trend, it followed as shrub-grass land (1.469mg·kg-1)>first generation (1.232mg·kg-1)>second generation (1.048mg·kg-1)>third generation (1.023g·kg-1). In addition to the second generation subsoil (20-40cm) phosphorus content was highest in spring, other generations and soil layer available phosphorus content were highest in winter. With the increase of Eucalyptus generations, soil Phosphorus Activation Coefficient (PAC) was decreased, suggesting that continuous planting rotation of Eucalyptus plantation may inhibit the effectiveness of soil phosphorus.(3) The content of water extractable phosphorus (H2O-P) was the lowest form of all phosphorus forms, which accounts for less than 0.5% of total phosphorus. Except the second generation subsoil (20-40cm) phosphorus content was highest in spring, other generations and soil layer available phosphorus content were highest in winter.(4) Soil sodium bicarbonate extractable inorganic phosphorus (NaHCO3-Pi) content is between 5.922-6.980mg-kg-1. With the increase of Eucalyptus generations, NaHCO3-P; content showed an upward trend, but did not reach the significant level. The content of spring and winter was higher than summer and autumn. Sodium bicarbonate to extract organic phosphorus (NaHC03-Po) content was between 14.258～16.684mg·kg-1, NaHCO3-Po content shows the same trend as NaHCO2-Pi.(5) With the increase of Eucalyptus generations, the content of Sodium hydroxide extractable inorganic phosphorus (NaOH-Pi) was first reduced and then increased. Compared with shrub-grass land, the content of NaOH-Pi in Eucalyptus plantations was slight accumulation. As the season changes, the content of NaOH-Pi at all generations are expressed as summer and autumn were higher than spring and winter. With the increase of Eucalyptus generations, sodium hydroxide extractable organic phosphorus(NaOH-Po) content showed an upward trend, NaOH-Po content shows the same trend as NaOH-Pi.(6) Compared with shrub-grass land, Eucalyptus plantation soil hydrochloric acid extractable phosphorus (HC1-P) decreased significantly (p<0.05), the difference between the different generations is not significant.(7) The content of soil residual phosphorus (Residual-P) was the highest form of all different phosphorus forms, accounting for over 50% of Total-P, Residual-P content was between 207.186～261.345mg·kg-1. With the increase of Eucalyptus generations, Residual-P content showed a trend of gradual accumulation, the content of third generation is significantly higher than the first generation.(8) Used Langmuir equation to fit the adsorption characteristic of soil phosphorus in different generations, the results showed that the correlation of the fitted curve have reached the significant level (p<0.01). When the equilibrium concentration at a low level, has a larger slope, soil has a strong adsorption capacity on phosphorus; equilibrium concentration increases, the slope of the curve decreases, soil adsorption rate is reduced and stabilized.(9) Correlation analysis results showed that the maximum adsorption capacity(Xm)of the soil, adsorption intensity factor(K), the maximum buffer capacity(MBC) was associated with different forms of phosphorus significantly correlated (p<0.05 or p<0.01). With the increase of phosphorus concentration, the adsorption capacity of soil phosphorus increased, while the adsorption rate decreased. There was a significant exponential relationship between adsorption and desorption. |
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