ABSTRACT. Phosphorus (P) is one of the most important elements used in fertilizing soils in tropical regions due to the low efficiency of phosphate fertilization. This work aimed to fractionate inorganic P (Pi) in tropical soils of different mineralogical compositions and to relate these fractions with their respective phosphate buffer capacity (PBC) indicators. The soils were characterized physically, chemically and mineralogically. Additionally, we evaluated the P concentration that remained in solution of soil after equilibrium was met; this was termed P remaining (P-rem). In general, the Pi fractions of soils did not correlate with the PBC indicators. The P-H2O fraction showed a negative correlation with the P-Al fraction. Ferric minerals did not influence P fixation. P-rem showed a strong correlation with the maximum P adsorption capacity, adsorption energy, and the amorphous and crystalline forms of Fe. The minerals of aluminum contributed the most to P fixation. P-rem was the best estimator of PBC. The soils with high, moderate and low rates of P fixation showed high amounts of the fractions P-Al, P-Ca, and P-Fe, respectively. The results showed that P fixation was influenced by the fractions of P in the soil, suggesting that the efficiency of phosphate fertilizers in tropical soils depends on the mineralogy of the clay fraction within those soils.

Abstract This study aimed to evaluate decomposition and release of C and of N from the litter of forest Caatinga species in the humification phase, as well as to estimate the residence time of C and N. Litter decomposition of the species was evaluated in 2014/2015 in the municipality of Floresta, Pernambuco, Brazil, using a method of the litter bags at 270; 300; 330; and 360 day. Litter from species of the Fabaceae family presented different decomposition groups, suggesting that decomposition was more influenced by the C/N ratio than N concentration only. The N release was 6.25% greater than the release of C. C will remain in the litter for more time, while N will be almost totally released in approximately 1.14 years. The species litter decomposition were shared, suggesting that the nutrients cycling is fundamental in the preservation of the Caatinga, mainly due to the longer residence time of C.

Abstract Grass from the genus Brachiaria spp. predominates in pastures with low fertile soils. This scenario highlights the importance of the association with microorganisms to foster plant growth, which becomes essential to the successful establishment of this forage in such environments. This study aimed to evaluate the genetic variability and identify the mechanisms of plant growth promotion, in vitro, of bacteria associated with Brachiaria decumbens Stapf. and Brachiaria humidicola (Rendle.) Schweickerdt in Pernambuco, Brazil. We evaluated 20 isolates of diazotrophic bacteria obtained from the endophyte or rhizosphere communities. The genetic characteristics were determined via sequencing the 16S rRNA region, which allowed us to identify ten different bacterial genera: Bacillus sp., Burkholderia sp., Enterobacter sp., Klebsiella sp., Microbacterium sp., Pantoea sp., Ralstonia sp., Rhizobium sp., Sinomonas sp., and Sphingomonas sp., with a specificity of the genus Rhizobium sp. to Brachiaria decumbens Stapf.. The phenotypic and functional characteristics revealed that 100% of the bacterial strains produced indol-3-acetic acid (IAA) with the addition of L-tryptophan, and 60% presented IAA production independent of the L-tryptophan pathway. We also detected that 70% of the isolated bacteria possessed the capacity to solubilize phosphorus. The analysis of the enzymatic output revealed that 30% of the bacterial isolates produced cellulase, 60% produced pectate lyase, 15% produced polygalacturonase, and 30% produced amylase. We also detected the production of N-acyl homoserine lactones in 65% of bacterial strains. In summary, our results showed that plants of B. decumbens Stapf. and B. humidicola (Rendle.) Schweickerdt interacted with different bacterial genera capable of promoting plant growth.

Resumen Pastos del género Brachiaria spp. predomina en pasturas con suelos poco fértiles. Este escenario resalta la importancia de la asociación con microorganismos para fomentar el cultivo de plantas. Este estudio tuvo como objetivo evaluar la variabilidad genética e identificar los mecanismos de promoción del crecimiento de las plantas, in vitro, de bacterias asociadas con Brachiaria decumbens Stapf. y Brachiaria humidicola (Rendle.) Schweickerdt en Pernambuco, Brasil. Evaluamos 20 aislamientos de bacterias diazotróficas obtenidas de la comunidad de endofitas o rizosfera. Las características genéticas se determinaron mediante la secuenciación de la región 16S rRNA, lo que permitió identificar diez géneros bacterianos diferentes: Bacillus sp., Burkholderia sp., Enterobacter sp., Klebsiella sp., Microbacterium sp., Pantoea sp., Ralstonia sp., Rhizobium sp., Sinomonas sp., y Sphingomonas sp., con una especificidad del género Rhizobium sp. a Brachiaria decumbens Stapf.. Las características fenotípicas y funcionales revelaron que el 100% de las cepas bacterianas producían ácido indol-3-acético (AIA) con la adición de L-triptófano y el 60% presentaban producción de AIA independientemente de la vía de L-triptófano. También detectamos que el 70% de las bacterias aisladas poseían la capacidad de solubilizar el fósforo. El análisis de la producción enzimática reveló que el 30% de los aislados producían celulasa, 60% pectato liasa, 15% poligalacturonasa y el 30% producía amilasa. También detectamos la producción de N-acyl homoserine lactones en el 65% de las cepas bacterianas. En resumen, las plantas de B. decumbens Stapf. y B. humidicola (Rendle.) Schweickerdt interactuó con diferentes géneros de bacterias capaces de promover el crecimiento de la planta.