P transformations in cacao agroforests soils in the Atlantic forest region of Bahia, Brazil

Abstract

Little is known about the phosphorus (P) fractions and P lability of agroforest soils in tropical regions, particularly those of cacao (Theobroma cacao L.) agroforests. We hypothesized that the effect of P fertilization on the distribution of P fractions in the soil based on the source-sink relationship differs for different cacao agroforestry systems. The cacao agroforestry systems studied were the following: open cacao-cabruca, closed cacao-cabruca, cacao + erythrina, mixed cacao + rubber tree, and cacao + rubber tree intercropping. A natural forest and an unfertilized pasture were used as reference systems. The P fractions were determined using the Hedley sequential extraction method, and the P transformation processes were evaluated via structural equation modeling. The impact of low P fertilizer input on the P fractions varied according to the specific environmental conditions of each cacao production system. Consequently, there was high dissimilarity among all of the cacao sites. In all of the cacao agroforestry systems, there was an increase in inorganic P (Pi), especially the labile fraction (resin-Pi and NaHCO3-Pi), but organic P (Po) increased only in the cacao + erythrina system and in the rubber tree planting row of the cacao + rubber tree intercropping system. As a result, the fitted structural models indicated that the inter-relationships of the geochemical processes were more important for determining the P availability than the biological processes. However, the Po concentrations and relative proportion were high in all of the cacao agroforestry systems, thereby revealing the high potential of supplying P to plants via the mineralization process in the eventual removal of mineral fertilization.