GIS-Based Variability Of Soil Phosphorus In Chengdu Plain

Based on the survey data of 117 sampling points in Chengdu Plain, the spatial variability of soil phosphorus and its influential factors, including soil particle composition, organic matter, and pH were studied by coupling geographic information system (G1S) with geostatistics. By way of field experiment, the determinacy non-mechanism model based on GIS was developed and verified, and then applied to regional phosphorus behavior simulation. Further more, the regional distribution features of phosphorus fertilizer amount, irrigation amount, together with phosphorus losses and phosphorus use efficiency under rice-wheat cropping system were studied in this paper. The main results were as follows.The contents of soil total phosphorus (STP) were 0.86±0.39g kg^-1,0.62±0.27g kg^-1 and 0.59±0.22g kg^-1 in the soil layers of 0²0cm, 20⁴0cm and 40¹00cm, respectively in Chengdu Plain in 2002, Moreover, the spatial distribution of STP content presented clot shapes in every soil layer, decreasing from the northeast to the southwest of the plain in general. The contents of soil available phosphorus (SAP) were 14.3±12.8mg kg^-1, 7.0±4.8mg kg^-1 and 7.1±5.5mg kg^-1 in soil layers of 0²0cm, 20⁴0cm and 40-100cm, respectively in Chengdu Plain in 2002. Furthermore, SAP content presented banding distribution in every soil layer, decreasing from the east to the southwest.Soil particle composition, organic matter, pH, agricultural management measures and land use types had a certain influence on the content of soil phosphorus. The results of correlation analysis showed that there were significant positive correlation between the content of STP and soil silt particle, the SAP and soil silt particle, and also between the content of soil organic matter (SOM) and STP, While, no significant difference between the SOM and SAP. Moreover, a highly significant positive correlation appeared between the STP and the pH, SAP and the pH. The equilibrium index of the cultivated land of the soil phosphorus was 1.09².53 during the past 20 years which resulted in the increase of the soil phosphorus content on the whole. In addition, the STP and SAP content of the vegetable plots' top soil in Chengdu Plain were higher than that of paddy field and dry land.The results of field experiment indicated that, the content of STP, organic phosphorus and O-P continuously increased, while the content of Al-P, Fe-P and Ca-P presented decreasing tendency in the growth duration of wheat in the topsoil in Chengdu Plain. Furthermore, during the growth duration of rice, the content of STP, organic phosphorus, Al-P, Fe-P, Ca-P, O-P presented decreasing tendency. Indifferent growth durations of the crops, soil organic phosphorus and other phosphorus forms had no significant difference, so did Ca-P, O-P and other phosphorus, whereas Al-P and Fe-P showed a significant or highly significant positive correlation in each soil layer. In different depths of soil profile, the contents of the different phosphorus forms showed various variation tendencies, but presented decreasing tendency from the top down on the whole.Based on the results of field experiment and regional soil phosphorus rough estimate, the determinacy non-mechanism transport model was set up. The model's verification results of using field experiment data and soil column simulation experiment showed that the simulated values probably coincided with the measured values. Therefore, this model could be applied to regional phosphorus transport simulation.The results of soil phosphorus transport simulation showed that the weighted mean of phosphorus cumulate flux in lm soil mass was 39.1 lkg hm"2 in Chengdu Plain in the period, occupying 18.96% of the average amount of phosphorus fertilizer. And the spatial variability presented banding, increasing from the west to the east. In addition, there was highly significant positive correlation between the amount of soil phosphorus losses and phosphorus fertilizer, irrigation amount. The utilization efficiency was between 11.78kg kg"'²1.80kg kg"1, presenting banding distribution. The highest value regions (>18kg kg"1) mainly existed in Dujiangyan and Chengdu, and from which reduced gradually towards both sides. While, the lowest value regions (<15kg kg"1) distributed in Jintang which was in the edge zone of the eastern plain.