Effects Of Land Use Patterns And N Addition On Soil Phosphorus Fractions In The Longzhong Part Of Loess Plateau

The effects of different land uses (planted and fallowed croplands which had a history of traditional cultivation for at least 50 years and fenced and grazed grasslands which had restored from cropping for 24 years), as well as Nitrogen (N) addition on soil phosphorus fractions and their variations were investigated in Longzhong part of the Loess Plateau. The form, distribution, conversion of soil phosphorus fractions and their relations to the soil physical and chemical properties under different land uses were analysed. The main results are as follows:1、As a result of long-term fertilization, the contents of active inorganic phosphorus (H2O-Pi, NaHCO3-Pi), potential active inorganic phosphorus (NaOH-Pi), mid-stable inorganic phosphorus (DHCl-Pi) in 0-10 cm soil layer and highly stable inorganic phosphorus (HHCl-Pi) in 30-60 cm soil layer in the croplands were significantly higher than those in the grasslands. The contents of potential active organic phosphorus (NaOH-Po) in 0-10 cm soil layer in the grasslands were significantly higher than those in the croplands, but the situation was completely opposite in 20-60 cm soil layer.2、The conversions between soil phosphorus fractions mainly occurred between mid-stable inorganic phosphorus (DHCl-Pi) and other phosphorus components,while the conversions between soil phosphorus fractions in the croplands mainly occurred in 0-20 cm soil layer.3、The available phosphorus and total phosphorus were significantly correlated to H2O-Pi, NaHCO3-Pi, and NaOH-Pi (P<0.01). Total soil nitrogen and soil organic carbon were also significantly correlated to NaOH-Pi, NaOH-Po, and HHCl-Pi (P<0.01). Soil pH were significantly and negatively correlated to NaHCO3-Po (P<0.05), H2O-Pi, NaHCO3-Pi, NaOH-Pi, NaOH-Po and HHCl-Pi (P<0.01).4、N addition decreased the active inorganic phosphorus (H2O-Pi) content in 0-10 cm soil layer, it also decreased the potential active organic phosphorus (NaOH-Po) and the potential active inorganic phosphorus (NaOH-Pi) content in 10-20 cm soil layer. However, N addition increased highly resistant inorganic phosphorus (HHCl-Pi) content in 0-10 cm soil layer. The contents of both active inorganic phosphorus (H2O-Pi, NaHCO3-Pi) and the potential active inorganic phosphorus (NaOH-Pi) decreased with soil depth.5.. The mid-stable inorganic phosphorus (DHCl-Pi) and the potential active organic phosphorus (NaOH-Po) are transitional forms in the conversion processes between soil phosphorus fractions. With no N addition, the conversions between soil phosphorus fractions in 0-10 cm soil layer mainly occoured between the active inorganic phosphorus (H2O-P1) and other phosphorus fractions. N addition accelerated the conversion of stable phosphorus to mid-active phosphorus. In 20-40 cm soil layer, there were slow or even no conversions between the stable phosphorus fractions and active phosphorus fractions for each treatment.