1. Results and Discussion
RE-SPECIFICATION OF MODELS OF THE PHOSPHORUS CYCLE IN TROPICAL SOILS USING STRUCTURAL EQUATION
Marcus Vinicius da Silva Sales1*; Antonio Carlos Gama-Rodrigues, 1 Emanuela Forestieri Gama-Rodrigues1; Seldon Aleixo1.
1Universidade Estadual do Norte Fluminense Darcy Ribeiro – UENF
*Correspond Author: Av. Alberto Lamego 2000, Campos dos Goytacazes, RJ, Brasil;
Introduction
Phosphorus ( P ) is essential for plant metabolism and nutrient limitations on its availability , both at the beginning of the vegetative cycle and the production cycle can result in huge losses in agricultural productivity . In strongly weathered tropical soils in particular, the ability of fixation of P in the soil is high, which can lead to low productivity . Thus , understanding of the P cycle in soil becomes important to establish management strategies aimed at increasing the availability of this nutrient to plants in low-input production systems .
The aim of this study was to use the technique of Structural Equation Modeling (SEM) with latent variables using the SPSS AMOS 22 IBM software to re-specify hypothetical models of the P cycle, based on the method of Hedley sequential extraction of P, to check interactions among the pools of P and to identify which pools act as a P source or sink in unfertilized soils in different land uses.
Figure 1: The hypothesized structural model for the soil P cycle. .
Materials and Methods
Soil P data were collected from Gama-Rodrigues et al. (2014) and corresponded to studies that used the sequential fractionation technique developed by Hedley et al. (1982) and modified by Tiessen and Moir (1993). The database consisted of 81 observations covering a wide variation of unfertilized soil types under different land use systems in the tropical region (native forest, secondary forest, agroforest, savannah and pasture). The P fractions used were resin inorganic P (Pi­), bicarbonate P (Pi and organic phosphate (Po)), hydroxide P (Pi and Po), sonic P (Po), HCl Pi, HCl Phot and residual P.
The variables were separated by specific groups through factor analysis, thus allowing the identification of pools , representing the models presented , Figures 1 and 2 .           
The models were constructed using the AMOS 22 statistical software that works with structural equation modeling , and allows addition of a graphical visualization , to understand the processes of inter - relationships between the measured and latent variables . Measurable variables are variables that can be directly measured variables and latent variables are formed by concepts.
Figure 2. Hypothetical model of the relations of P fractions (measured variables) to pool available (latent variable) from the SEM to the cycle of soil P in different agroforestry systems in southern Bahia, Brazil. The numbers correspond to the estimated standardized parameter (P<0.05). Variable error (ɛ2-ε5 and ζ1) are normalized values​​. (X2 = 6.9, df = 6, P <0.33).
Figure 2: Respecified structural equation model for the soil P cycle. All measured variables (in boxes) are represented as effect indicators associated with latent variables (in circles). The numbers correspond to the standardized parameters estimated (P < 0.001). ** Significant at P < ° Significant at P < 0.1. Error variables (ԑ 1 – ԑ6, δ8, ζ1) are standardized values. Model χ2 = 7.02, df = 7, p =
Results and Discussion
Conclusion
In general, the respecified models were adequate and able to represent a generalization of P cycling in soils that have similar characteristics as the sampled data.
The model, not only is there a direct relationship between the Organic, Occluded and primary Mineral pools and the Available P pool, but the indirect relationships via the Organic pool were theoretically and statistically adequate.
The respecified model corroborated the hypothesis of the dependence of the Available P pool to the Organic pool in unfertilized tropical soils.
The respecified model shows good results that confirm the assumptions of the initial model, Figure 1. Organic and primary Mineral pools are sources of P by direct relations to the pool of Available P , while the Occluded pool, shows up as the sink of P direct relation to the pool of Available P . The primary Minerals pool showed high positive overall effect (β = 0.774), the Occluded pool also had a positive overall effect, though not significant (β = 0.028). The Available P pool was explained by 70% of the variations.
The respecified model presented is adjusted with χ2 = 7.019, df = 7, p = 0.427, GFI = .976, CFI = e RMSEA = 0.006, Figure 2.
The results of this study showed that no theory of the P cycle can or should be represented by a single model of structural equations. Depending on the respecification conducted, if performed correctly and without changing the general idea of the hypothetical model, very approximate results may be obtained, which will only then require sound judgment to choose the best model.
References
GAMA-RODRIGES, A.C., SALES, M.V.S., SILVA, P.S.D., COMERFORD, N.B., CROPPER, W.P., GAMA-RODRIGUES, E.F.. An exploratory analysis of phosphorus transformations in tropical soils using structural equation modeling. Biogeochemistry, 118; , 2014.
HEDLEY, M.J., STEWART, W.B., CHAUHAN, B.S.. Changes in inorganic and organic soil phosphorus fractions induced by cultivation practices and by laboratory incubations. Soil Sci Soc Am J 46:970–976, 1982.
TIESSEN, H., MOIR, J.O.. Characterization of available P by sequential extraction. In: Carter MR (ed) Soil sampling and methods of soil analysis. CRC Press, Boca Raton, FL, pp 75–86, 1993.
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