Phosphorus mobilization in agroforestry: Organic anions, phosphatase activity and phosphorus fractions in the rhizosphere

In agroforestry systems on Ferralsols in the tropics, maize crop yields are low owing to a lack of P. However, some agroforestry tree species adapted to P-fixing soils may be able to utilize less available P sources and concurrently increase P availability to adjacent crops. Adaptations for enhanced P acquisition from P-fixing soils include exudation of low molecular weight (LMW) organic anions (OA) and phosphatase enzymes. We identified major organic anions, and measured acid phosphatase activity and P fractions in the rhizosphere soil of maize (Zea mays L.), as well as in the perennial species Grevillea robusta A. Cunn., Cassia spectabilis DC. (syn. Senna spectabilis (DC.) H.S. Irwin and R.C. Barneby), Tithonia diversifolia (Hemsl.) A. Gray, Eucalyptus grandis W. Hill ex Maiden and Cedrella serrata Royle. Maize and trees were grown simultaneously at field sites and in large pots. Rhizosphere soil of C. spectabilis contained at least 29 mol oxalate g¿1 soil, derived from a high exudation rate in the range of at least 5¿10 mol oxalate m¿1 root day¿1. Incubation of Ferralsols with much lower concentrations of citrate increased labile P fractions, but there was no clear relation between OA concentration and an increase in labile P fractions in rhizosphere soils, where P mobilization and P uptake occur at the same time. Acid phosphatase activity in rhizosphere soil of all species was two to five times greater in rhizosphere soil compared with bulk soil, and correlated in rhizosphere soil of G. robusta with a shift from organic P to inorganic P in soil P fractions. We conclude that organic anion exudation and acid phosphatase activity of tree roots may increase mobilization of P in the rhizosphere, the extent of which depends on the species, the organic anion and pH. However, it is unlikely that the extent of P mobilization will benefit adjacent crop plants unless crop roots exert insufficient P-mobilization effects themselves, and grow in the rhizosphere of tree roots

Cacao Quality: Highlighting Selected Attributes

World demand for cacao and the requirements for quality beans have increased every year. Research studies have developed standards for aspects of cacao quality that meet industrial criteria as well as international import and export legislation that is aimed at food security. This review focused on selected attributes of cacao bean quality. These attributes include the amount of acids, simple carbohydrates, proteins, amino acids, lipids, phenolic substances, and methylxanthines. Other attributes are bean moisture content, quantity of mineral nutrients, and presence of potentially toxic elements. Microbiological and organoleptic attributes are also of interest. Future research on cacao quality should focus on bean sampling, geographical location of plantations, genetic diversity of the cacao material, and standardized analytical methods for determination of organic and mineral substances. © 2017 Taylor & Francis.33438240