Residence time effect on phosphorus sorption and extraction by Mehlich I in typical Oxisol clay-sized minerals

A sorção de fósforo ocorrente nos solos tropicais concorre para a diminuição da eficiência agronômica dos fertilizantes fosfatados que, por sua vez, são produzidos a partir de matérias-primas cujas reservas mundiais tendem à completa exaustão. Desse modo, o aumento do conhecimento relativo aos mecanismos de imobilização de fósforo no solo pode contribuir para a minimização desse fenômeno e assim para a maior longevidade das fontes de fósforo. No presente trabalho objetivou-se avaliar, sob condição de acidez, o efeito do tempo de contato do fósforo com a caulinita, a gibbsita, a hematita e a goethita, principais componentes da fração argila dos Latossolos, sobre a recalcitrância do fósforo sorvido à extração com a solução Mehlich I que, por sua vez, é largamente utilizada nos laboratórios brasileiros de avaliação da fertilidade do solo para a determinação do teor fito disponível de fósforo. Em paralelo, investigou-se a eventual ocorrência de difusão intrapartícula como mecanismo envolvido com a imobilização de fósforo nos minerais supracitados. Os resultados indicaram que a sorção de fósforo ocorreu lentamente na caulinita e muito rapidamente na goethita. O fósforo retido à caulinita foi praticamente todo extraído com a solução Mehlich I mesmo após 912 horas de sorção; por outro lado, a goethita se mostrou como principal mineral responsável pela formação do fósforo não-lábil. O aumento do tempo de contato de 3 para 912 horas diminuiu as frações de fósforo sorvido à gibbsita e à hematita passíveis de extração com a solução Mehlich I. A hematita contribuiu mais para a formação de fósforo não-lábil que a gibbsita. A sondagem espectroscópica dos primeiros 25 nanômetros a partir das superfícies das amostras de gibbsita, de hematita e de goethita após a extração com solução Mehlich I indicou que a difusão intrapartícula, se ocorrente, não foi o único mecanismo responsável pelas frações de fósforo sorvido não extraíveis com solução Mehlich I determinadas naqueles minerais.Phosphorus sorption in tropical soils decreases the agronomic efficiency of phosphate fertilizers which, in turn, are produced from raw materials whose world reserves tend to be completely exhausted. Thus, the increase of knowledge about the immobilization mechanisms of phosphorus in the soil can contribute to minimize this phenomenon and to increase the longevity of phosphorus sources. Here we evaluated, under acidic conditions, the residence time effect on the recalcitrance of the sorbed phosphorus on the main clay-sized minerals found in Oxisols against the extraction with Mehlich I solution, which is widely used in Brazilian soil testing laboratories to determine the soil content of plant available phosphorus. In parallel, we investigated the possible occurrence of intraparticle diffusion as a mechanism involved with the immobilization of phosphorus in the above mentioned minerals. The results indicated that phosphorus sorption was slow in kaolinite and very fast in goethite. The phosphorus retained to kaolinite was almost all extracted with Mehlich I solution even after 912 hours of sorption; on the other hand, goethite was shown to be the main mineral responsible for the formation of non-labile phosphorus. Increasing the contact time from 3 to 912 hours decreased the fractions of phosphorus sorbed to gibbsite and hematite that could be extracted with Mehlich I solution. Hematite contributed more to the formation of non-labile phosphorus than gibbsite. Spectroscopic probing of the first 25 nanometers from the surfaces of gibbsite, hematite, and goethite after the extraction with Mehlich I solution indicated that intraparticle diffusion, if occurring, was not the only mechanism responsible for the fractions of sorbed phosphorus onto those minerals that were not extractable with that solution

Silicon Application Induced Alleviation of Aluminum Toxicity in Xaraés Palisadegrass

Aluminum (Al) toxicity is a major abiotic constraint for agricultural production in acidic soils that needs a sustainable solution to deal with plant tolerance. Silicon (Si) plays important roles in alleviating the harmful effects of Al in plants. The genus Urochloa includes most important grasses and hybrids, and it is currently used as pastures in the tropical regions. Xaraés palisadegrass (Urochloa brizantha cv. Xaraés) is a forage that is relatively tolerant to Al toxicity under field-grown conditions, which might be explained by the great uptake and accumulation of Si. However, studies are needed to access the benefits of Si application to alleviate Al toxicity on Xaraés palisadegrass nutritional status, production, and chemical–bromatological composition. The study was conducted under greenhouse conditions with the effect of five Si concentrations evaluated (0, 0.3, 0.6, 1.2, and 2.4 mM) as well as with nutrient solutions containing 1 mM Al in two sampling dates (two forage cuts). The following evaluations were performed: number of tillers and leaves, shoot biomass, N, P, K, Ca, Mg, S, B, Cu, Fe, Mn, Zn, Al, and Si concentration in leaf tissue, Al and Si concentration in root tissue, neutral detergent fiber (NDF), and acid detergent fiber (ADF) content in Xaraés palisadegrass shoot. Silicon supply affected the relation between Si and Al uptake by increasing root Al concentration in detriment to Al transport to the leaves, thereby alleviating Al toxicity in Xaraés palisadegrass. The concentrations between 1.4 and 1.6 mM Si in solution decreased roots to shoots Al translocation by 259% (from 3.26 to 1.26%), which contributed to a higher number of leaves per plot and led to a greater shoot dry mass without affecting tillering. Xaraés palisadegrass could be considered one of the greatest Si accumulator plants with Si content in leaves above 4.7% of dry mass. In addition, Si supply may benefit nutrient-use efficiency with enhanced plant growth and without compromising the chemical–bromatological content of Xaraés palisadegrass

Co-Inoculations with Plant Growth-Promoting Bacteria in the Common Bean to Increase Efficiency of NPK Fertilization

Given the hypothesis that co-inoculation with plant growth-promoting bacteria (PGPB) enhances the beneficial effects of Rhizobium tropici with greater mineral nutrition, optimizes biological nitrogen fixation and reduces use of fertilizers in bean plants, the objective of this research was to evaluate the synergistic effects of Rhizobium tropici associated with Azospirillum brasilense, Bacillus subtilis, Pseudomonas fluorescens and their combinations, on increasing the efficiency of NPK fertilization to obtain high winter yields of the (irrigated) common bean in the Cerrado region. The experiment was carried out in the field over two years in a Rhodic Hapludox under a no-till system in Selvíria, Brazil. The experimental design comprised complete randomized blocks with four replications in a 3 × 7 factorial scheme. The treatments consisted of three doses of NPK fertilizer (control—0 kg ha−1 (control); 50% of the recommended dose; 100% of the recommended dose in two parts) and seven doses of inoculation or co-inoculation (control; Rhizobium tropici; R. tropici + Azospirillum brasilense; R. tropici + Bacillus subtilis; R. tropici + Pseudomonas fluorescens; R. tropici + A. brasilense + B. subtilis; R. tropici + A. brasilense + P. fluorescens). The PGPB in the co-inoculations increased the hundred-grain weight, the grain pod−1, the grain plant−1 and the grain yield following the NPK doses. The grain yield of the common bean was increased by co-inoculation with R. tropici + A. brasilense + P. fluorescens without NPK treatments, co-inoculation with R. tropici + P. fluorescens and R. tropici + A. brasilense + B. subtilis with the 50% dose of NPK and co-inoculation with R. tropici + B. subtilis with the recommended dose of NPK fertilizer (100%)