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Aim To compare the internal balances of nutrients and the rates of nutrient cycling across nine cocoa agroforestry systems consisting of various combination of soil types (Latosols and Cambisols), production systems (cabruca and Erythrina glauca-shade) and fertilization regimes in southern Bahia, Brazil. Methods We measured nutrient stocks in litter fall production, in the accumulated litter and fruits. The internal nutrient balance for various simulations was obtained by the following expressions:(1) Balance 1=litter -fruit (seeds and husks) and (2) Balance 2=(litter+husks) -seeds. Annual litter decomposition coefficients (k) and subsequent potential of nutrient release were also investigated. The data were analyzed by principal components analysis and by Pearson correlations.

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Respecification of structural models allows evaluating new hypothesis and enhances understanding of how changes in one pool can affect the other pools in the model. The aim of this study was to evaluate the theoretical concept of P pools (unmeasurable variables-latent) using data from Hedley's sequential extraction method for a better understanding of the P cycle in tropical soils. The theoretical concept being tested is the degree to which available P is controlled by different soil P pools as measured by extraction techniques. The respecified models were adequate and able to represent a generalization of P cycling in soils. The best structural model including latent variables with multiple indicators was consistent with the theoretical concept that the indicators of soil P pools are P fractions determined by the sequential extraction method. In this 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. Thus, the model showed the interrelationships of geochemical and biological processes on the available P and it corroborated the hypothesis of the dependence of the available P pool to the organic pool in unfertilized tropical soils.

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