Changes In Phosphorus Fractions In The Plough Layer Of Loess Soil Under Long-term Contrasting Soil Management Regimes

In general, the soil available phosphorus(P) content is very low, which limites plant growth and development. Application of P fertilizer is the normal measure to meet crop requirement, but phosphorus resource is limited. Therefore, understanding phosphorus cycling characteristics in soil-plant systems is of great significance. This study investigated long-term effects of soil management in a Lou soil on P pools by applying for inorganic phosphorus method established by JIANG- GU under rainfed conditions and by Tiessen- Hedley P rating system under irrigation. Soil management included cropping system(Cropping), bare Fallow and cropland abandonment(Setaside). There were several nutrient management regimes under Cropping. Main results achieved were as follows:1) Under rainfed conditions, soil total inorganic phosphorus and inorganic phosphorus fraction obtained by JIANG- GU method(except Ca10-P) were significantly higher under Cropping than bare Fallow and cropland abandonment treatments, while there is no difference between latter two treatments in total inorganic P and its fractions. In addition, the total inorganic phosphorus content in the bare Fallow and cropland abandonment treatments was similar to the original value at start of experiment, except for a significant decline in the Fe-P and Al-P contents, other fractions had no significant change.2) Under rainfed conditions, long-term without application of fertilizers significantly decreased the total amount of inorganic phosphorus, Al-P and O-P contents, and other fractions were not changed compared with initial values. Long-term application of P fertilizer significantly increased total inorganic phosphorus content and all inorganic phosphorus fractions. In particular in PK treatment, due to very low soil nitrogen content which affects the absorption of phosphorus in wheat, the accumulation of a large number of soil phosphorus occurred in soil, inorganic phosphorus fractions stood at the highest level among treatments.3) Under rainfed conditions, in treatment of long-term application of chemical fertilizers with organic manure, the total inorganic phosphorus content of the soil and all inorganic phosphorus fractions were significantly higher than other treatments which received P fertilizer. The Ca10-P content slightly accumulated. Application of P fertilizer, to some extent, improved the Ca10-P content, but significantly reduced its relative content. This indicates that long-term organic and inorganic manure is beneficial to improve the availability of soil phosphorus.4) Under irrigation, after 20 years of different land-use change, conventional fertilization treatment significantly increased the Resin-P content and its proportion. The Resin-P Contents in Fallow and Setaside treatments were significantly lower than the background value, and there was no difference between above two treatments. Different land management and fertilization had no effect on Na HCO3-Po content.5) Under irrigation, data from Tiessen-Hedley method showed the HCl-P and Residual-P were main P fractions in tested soil under wheat / maize rotation system. Treatments of unfertilized control, nitrogen application alone and nitrogen combined with potassium showed the lowest Residual-P level, indicating that the depletion of phosphorus could benefit transformation of Residual-P. Long-term application of organic manure and chemical fertilizer can significantly increase the proportions of active inorganic phosphorus and medium-active inorganic phosphorus, fail to affect the proportion of organic phosphorus.