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

Nitrogen dynamics in an Australian semiarid grassland soil

We conducted a four-week laboratory incubation of soil from a Themeda triandra Forsskal grassland to clarify mechanisms of nitrogen (N) cycling processes in relation to carbon (C) and N availability in a hot, semiarid environment. Variation in soil C and N availability was achieved by collecting soil from either under tussocks or the bare soil between tussocks, and by amending soil with Themeda litter. We measured N cycling by monitoring: dissolved organic nitrogen (DON), ammonium (NH4+), and nitrate (NO3"12) contents, gross rates of N mineralization and microbial re-mineralization, NH4+ and NO3"12 immobilization, and autotrophic and heterotrophic nitrification. We monitored C availability by measuring cumulative soil respiration and dissolved organic C (DOC). Litter-amended soil had cumulative respiration that was eightfold greater than non-amended soil (2000 compared with 250 ;Cg C/g soil) and almost twice the DOC content (54 compared with 28 ;Cg C/g soil). However, litter-amended soils had only half as much DON accumulation as non-amended soils (9 compared with 17 ;Cg N/g soil) and lower gross N rates (1 134 compared with 13 1326 ;Cg N 22[g soil]"121 22d"121) and NO3"12 accumulation (0.5 compared with 22 ;Cg N/g soil). Unamended soil from under tussocks had almost twice the soil respiration as soil from between tussocks (300 compared with 175 ;Cg C/g soil), and greater DOC content (33 compared with 24 ;Cg C/g soil). However, unamended soil from under tussocks had lower gross N rates (3 1320 compared with 17 1331 ;Cg N 22[g soil]"121 22d"121) and NO3"12 accumulation (18 compared with 25 ;Cg N/g soil) relative to soil from between tussocks. We conclude that N cycling in this grassland is mediated by both C and N limitations that arise from the patchiness of tussocks and seasonal variability in Themeda litterfall. Heterotrophic nitrification rate explained >50% of total nitrification, but this percentage was not affected by proximity to tussocks or litter amendment. A conceptual model that considers DON as central to N cycling processes provided a useful initial framework to explain results of our study. However, to fully explain N cycling in this semiarid grassland soil, the production of NO3"12 from organic N sources must be included in this model

Enzyme activities of root tips and in situ profiles of soils and rhizospheres.

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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

Formas de fósforo no solo sob leguminosas florestais, floresta secundária e pastagem no Norte Fluminense Soil phosphorus forms under leguminous tree species, secondary forest and pasture in Northern Rio de Janeiro State, Brazil

Em solos muito intemperizados, a disponibilidade de P para as plantas está estreitamente associada às formas de P orgânico, que tem o seu conteúdo alterado por influência do tipo de cobertura vegetal. O objetivo deste trabalho foi estimar os teores de fósforo de compostos orgânicos (Po-total, Po lábil e P microbiano do solo), em plantios de Acacia auriculiformis (acácia) e Mimosa caesalpiniifolia (sabiá), em floresta secundária e em pastagem. Os teores médios ponderados de Po total e Po lábil, pelo teor de argila, foram de 375 mg kg-1 de P na argila e de 40,2 mg kg-1 de P na argila, respectivamente. Os solos sob acácia e sabiá apresentaram maior teor relativo de Po (total e lábil) do que os solos sob pastagem e capoeira. O Po representou de 22,6 a 39,6 % do P total extraído dos solos das coberturas estudadas. Em relação ao P lábil total, o Po lábil representou, em média, 67,1 %. Para o P microbiano, o maior teor foi encontrado no solo sob sabiá, seguido da floresta secundária, acácia e pastagem. Contudo, foi na capoeira que houve a maior eficiência da biomassa microbiana do solo em imobilizar P. O teor de P disponível por Mehlich-1 correlacionou-se positivamente com o teor de C e o de P da biomassa microbiana. Houve estreita correlação positiva entre o P microbiano e o C microbiano.<br>In highly weathered tropical soils phosphorus (P) bioavailability to plants is closely associated with soil organic P forms, which in turn are influenced by the vegetation cover. The aim of this study was to evaluate total organic P (Po), labile P fractions and microbial biomass P in soils under Acacia auriculiformis and Mimosa caesalpiniifolia stands, secondary forest and pasture. The clay-adjusted average content of total Po and labile Po were 375 mg kg-1 P and 40.2 mg kg-1, respectively. The Po contents (total and labile) in soils under Acacia auriculiformis and Mimosa caesalpiniifolia were higher than under pasture and secondary forest soils. The proportion of Po ranged from 22.6 to 39.6 % of total extracted P in soils under the vegetation cover types studied. The labile Po represented about 67.1 % of total labile P. The greatest amount of soil microbial biomass P (MBP) was observed under Mimosa caesalpiniifolia, followed by secondary forest, Acacia auriculiformis and pasture. However, the soil microbial biomass of the secondary forest was the most efficient regarding P immobilization (MBP/total Po ratio). Available P was positively correlated with soil microbial biomass C and microbial P, and microbial P was positively related to microbial C