Spatial variability of phosphorus fractions in paddy fields

Background and Objectives: Phosphorus (P) has been one of the major nutrients limiting agricultural production in many rice growing regions of the world, and certainly this is the case in north of Iran, where soils deficient in proper and balanced P management in the cultivated paddy fields. Phosphorus (P) chemistry in paddy soils is complicated due to various chemical and biological processes involve in its availability through various P fraction pools. Therefore, P fractions play a crucial role on soil available P supplying capacity especially in paddy fields. Proper and effective managerial practices of P and assessment of their effects on rice production requires an understanding of their variability in concentration across the fields. The spatial variability of soil properties and P content is the outcome of the interaction of several soil characters and processes and in agricultural fields involves also effects of management practices. Thus, the objective of this study was to conduct a large-scale analysis of the spatial variability of available and organic P, and also inorganic P fractions, and the important physical and chemical soil characters on P factions’ distribution for better fertilizers managerial practices.

Materials and Methods: This experiment was conducted at Some-e-Sara city in Guilan province. Soil samples were collected from 103 paddy fields with uniform geographical distribution in grids of 2x2 km. Soil samples were analyzed to determine available P, organic P, their effective physical and chemical properties (Texture, pH, CEC, T.N.V, Clay and O.C), and also sequentially extracted to determine inorganic P fractions(soluble P, Al-P, Fe-P and Ca-P). Descriptive and spatial variability analysis, and mapping was done by using SPSS (ver. 16), GS+ (ver. 5.1) and Arc GIS (Ver. 9.1), respectively.

Results: The highest coefficients of variation belonged to available P and its controlling P fractions, Al and Fe bounded P, about 132 and 194, respectively, whereas, pH showed the lowest CV, 10.6. The nugget to sill ratio ranged from 12 to 60% for almost all the studied data sets, showing moderate to strong pattern of spatial dependence. The small to moderate nugget effects also indicate that the sampling grid used was proper to reflect the spatial dependence of the studied soil properties and P fractions. Analysis of semivariogram corresponding to experimental data sets showed that the values for the range of spatial dependence of the semivariogram models varied from 2700 to 6000 m. Based on the results obtained from linear correlation analyses and spatial distribution maps, available P strongly correlated with both Al and Fe-P fractions and OC, about 0.84, 0.94 and 0.40, respectively.

Conclusions: Based on the results obtained from linear correlation analyses and spatial distribution maps, available P strongly correlated with both Al and Fe-P fractions and OC. Therefore, we concluded that determination of spatial correlation’s area of these soil chemical properties can effectively improve P fertilizers management in paddy fields. In this case, incorporation of remained rice straw with soil through shallow ploughing two weeks after grain harvesting enhance the oxidative condition of soil and consequent better straw decomposition increase the soil available P balance. These two mechanisms considerably decrease application of P fertilizers. Moreover, due to lack of, and/ or, insufficient P application in studied area, continuous flooding influence on available P condition by altering soil oxidative status to reduced, naturalizing the pH, and increasing P minerals stability.