A methodology to estimate net proton : phosphorus co-adsorption ratios for acidic soils

Despite extensive research, the behaviour of the key nutrient element, phosphorus (P), in soil is not yet fully understood. This study focussed on the outstanding issue of the co-adsorption of protons (H+ ) and P by soils. We developed a congruent set of measures to determine the net H+ :P co-adsorption ratio and tested it on goethite, for which a ratio of 1.6:1 had been estimated under CO2-free conditions for additions of NaH2PO4. Under our conditions, and using additions of KH2PO4, the net H+ :P co-adsorption ratio was estimated to be 1.44:1, i.e., in passable agreement with the published value. Application of the protocol to acidic soils resulted in a net H+ :P co-adsorption ratio of 1.92:1, and substitution of H3PO4 for KH2PO4 gave a ratio of 1.96:1. These ratios for soils differ significantly from that for goethite. The soils for which we estimated net H+ :P co-adsorption ratios had a wide range of properties and two had received previous applications of P fertiliser (Ca(H2PO4)2), which does not appear to have affected the net H+ :P co-adsorption ratios. The H+ :P co-adsorption ratio method could benefit from refinement, and further study is required to explore how these findings may apply to commercial P fertilisers under field conditions

Composted tobacco waste increases the yield and organoleptic quality of leaf mustard

Tobacco (Nicotiana tabacum) waste is produced in many countries and is phytotoxic due to the alkaloid content; in Vietnam the waste is usually burned causing air pollution. We composted tobacco waste with chicken manure in different proportions—1 t of waste ± accelerant (C1 and C2); 0.8 t of waste + 0.2 t of chicken manure ± accelerant (C3 and C4); and 0.7 t of waste + 0.3 t of chicken manure ± accelerant (C5 and C6)—for 30 d in covered heaps. Three mixtures containing the accelerant (C2, C4, and C6) reached temperatures of ∼55 °C, that 5s, hot enough to suppress disease and weeds. Composting decreased the alkaloid content from ∼6,000 to ∼200 mg kg−1, and C4 with a C/N ratio of 19:1, was used in a field trial. The compost treatments (0, 10, 15, and 20 t ha−1) were combined fertilizer with phosphorus (40 kg ha−1), nitrogen (60 kg ha−1) and potassium (90 kg ha−1) for leaf mustard (Brassica integrifolia). The yield increased from ∼17 to ∼29 t ha−1 with the amount of compost applied, and the nitrate concentration decreased concomitantly from ∼67 to ∼42 mg NO3–N kg−1 fresh weight, presumably due to ongoing composting (nitrogen drawdown). Organoleptic evaluation showed a preference for the crops grown with the compost amendments. Whether remains to be seen whether one-off compost applications >20 t ha−1 and repeated, large applications provide additional, long-term production benefits, or if the benefits may be outweighed by the accumulation of persistent, phytotoxic alkaloids

The value of on-farm rapid tests for measuring nitrate in leafy vegetables and in hydroponic solutions

Nitrate accumulates in leafy vegetables and is undesirable in terms of reducing product quality and nutritional value. In this study, the leafy vegetables pak choy [Brassica rapa L. ssp. chinensis (L.) 'Sumo'], Swiss chard [Beta vulgaris var. cicla 'Silverstar'] and cos lettuce [Lactuca sativa L. var. longifolia 'Vivian'], were grown at a lowrate (50 mg N L-1) and at a high rate of nitrate (300 mg N L-1), considered excessive in hydroponics but nonetheless used by some growers. The high nitrate supply was associated with high concentrations of shoot nitrate, and in some shoots with reduced fresh weight. This highlights that nitrate monitoring on farm could be used to prevent yield loss associated with excessive fertiliser application. To complement this study we reviewed the performance of two handheld tools in terms of their suitability for measuring nitrate on-farm: nitrate-selective electrodes and nitrate-sensitive test strips. Substances, including chloride, can interfere with measurements using nitrate-selective electrodes and the effects are potentially greater in xylem or shoot sap than in hydroponic solutions, i.e., test strips are the more likely to offer better quality on-farm measurements of sap nitrate

Stabilization of molybdate reactive phosphorus in 0.5 M sodium bicarbonate extracts of soils

Molybdate-reactive phosphorus (MRP) in soil-free 0.5 M sodium bicarbonate (NaHCO3) extracts of 20 soils held at 22° C increased 0-25% (median 11%) after 24 h and by 5-120% (median 43%) after 72 h. Addition of 2.5 mL L-1 of chloroform (1) or 0.25-1 g L-1 of thymol (2), phenyl mercury acetate (3), sodium cyanide (4), or sodium azide (5) showed that only (2)-(5) at 1 g L-1 stabilized the MRP for 72 h. Five of the soils were re-extracted with 0.5 M NaHCO3 containing 1 g L-1 of (2)-(5): all stabilized MRP for 72 h, and (2) increased MRP for three soils (P < 0.05). For 92 additional soils (0.5-200 mg MRP kg-1) extracted with 0.5 M NaHCO3 ± 1 g of sodium azide L-1, the azide had a negligible effect on MRP extraction, and in its presence, extracted MRP was unchanged after 72 h at 22° C. We recommend this extractant for wider evaluation

Effect of elevated atmospheric CO2 concentration on growth and physiology of wheat and sorghum under cadmium stress

Elevated concentrations of carbon dioxide (e[CO2]) affect plant growth and physiological characteristics, including metal accumulation, and the activity of anti-oxidant enzymes. These effects were investigated in cadmium (Cd) tolerant wheat (Triticum aestivum L.) and sorghum (Sorghum bicolor (L.) Moench.) cultivars. Plants were grown at the ambient and elevated CO2 levels, with four concentrations of Cd (0, 10, 20 and 40 mg kg−1) added to the soil. After 60 days, subsamples were tested for chlorophylls and carotenoids, protein, enzyme activities and morphological characteristics. Results showed that e[CO2] increased plant height, leaf area, and the dry weight of shoots and roots (P < 0.01). In addition, it decreased the Cd concentration in the shoots and roots of wheat, and increased the same concentrations for sorghum. With increasing Cd, the activities of the anti-oxidants, SOD and GSH-px increased in wheat. The differences in enzyme activity parallel the changes in Cd concentration in the plants of both species

Reactions to cadmium stress in a cadmium-tolerant variety of cabbage (Brassica oleracea L.) : is cadmium tolerance necessarily desirable in food crops?

Cadmium is a cumulative, chronic toxicant in humans for which the main exposure pathway is via plant foods. Cadmium-tolerant plants may be used to create healthier food products, provided that the tolerance is associated with the exclusion of Cd from the edible portion of the plant. An earlier study identified the cabbage (Brassica oleracea L.) variety, Pluto, as relatively Cd tolerant. We exposed the roots of intact, 4-week-old seedlings of Pluto to Cd (control ∼1 mg L−1 treatment 500 μg L−1) for 4 weeks in flowing nutrient solutions and observed plant responses. Exposure began when leaf 3 started to emerge, plants were harvested after 4 weeks of Cd exposure and the high Cd treatment affected all measured parameters. The elongation rate of leaves 4–8, but not the duration of elongation was reduced; consequently, individual leaf area was also reduced (P0.1). Phytochelatins (PCs) and glutathione (GSH) were present in the roots even at the lowest Cd concentration in the nutrient medium, i.e. ∼1 μg Cd L−1, which would not be considered contaminated if it were a soil solution. The Cd concentration in these roots was unexpectedly high (5 mg kg−1 DW) and the molar ratio of –SH (in PCs plus GSH) to Cd was large (>100:1). In these control plants, the Cd concentration in the leaves was 1.1 mg kg−1 DW, and PCs were undetectable. For the high Cd treatment, the concentration of Cd in roots exceeded 680 mg kg−1 DWand the molar –SH to Cd ratio fell to ∼1.5:1. For these plants, Cd flooded into the leaves (107 mg kg−1 DW) where it probably induced synthesis of PCs, and the molar –SH to Cd ratio was ∼3:1. Nonetheless, this was insufficient to sequester all the Cd, as evidenced by the toxic effects on photosynthesis and growth noted above. Lastly, Cd accumulation in the leaves was associated with lowered concentrations of some trace elements, such as Zn, a combination of traits that is highly undesirable in food plants

Oxygen exchange during the reaction of POCl₃ and water

To investigate O exchange during the reaction of POCl₃ and water, natural abundance POCl₃ was reacted with water highly enriched in ¹⁸O, and the resulting H₃PO₄ was isolated as KH₂PO₄. This reaction was conducted with and without tetrahydrofuran (THF) as a solvent, and was controlled in THF and violent in its absence. Approximately 5 x 10⁻⁴M aqueous solutions of the KH₂PO₄ were analyzed using electrospray ionization mass spectrometry, to estimate the proportions of the mass-clumped ¹⁶ ¹⁷ ¹⁸O isotope analogues of [H₂PO₄]⁻. During analysis, ~29% of [H₂PO₄]⁻ dehydrated to PO₃]⁻, for which the proportions of the O isotope analogues were also measured. These proportions were compared with those predicted for O exchange at either four or three positions on the P atom of POCl₃. The data strongly support O exchange at all four positions, whether or not THF was used to moderate conditions during the reaction. This result clears the way for safe, predictable synthesis of heavy-O labelled orthophosphate from POCl₃ and ¹⁸O enriched water for evaluation as an environmental and biochemical tracer

Phosphorus released by incremental acidification of alkaline Vertosols

Phosphorus (P) in the soil solution of alkaline, vertic soils is considered to be buffered by calcium phosphate minerals (CaP), influenced by pH and the solution concentration of P and/or Ca. We measured the P released to solution as we incrementally acidified 3 alkaline Vertosols containing 300–6000 mg kg-1 of acid soluble P in the presence or absence of an anion exchange membrane (AEM). The AEM kept the concentration of P in solution below 1 μM. After removal of labile forms, the concentration of P extracted remained low as pH decreased until there was a large increase as the pH passed a threshold. The thresholds varied little between soils, and were at pH 6.0–6.3 with the AEM, and were 0.7–1.0 pH units lower without the AEM. Soil pH buffer capacity (pHBC) affected the amount of acid required to release the P. The results indicate that rhizosphere acidification may increase the availability of CaP minerals to plants, though pHBC may restrict the availability of acid soluble P

Site-specific, genotypic and temporal variation in photosynthesis and its related biochemistry in wheat (Triticum aestivum)

Photosynthesis in wheat (Triticum aestivum L.) pericarps may contribute appreciably to wheat grain yield. Consequently, we investigated the temporal variation of traits related to photosynthesis and sucrose metabolism in the pericarps and flag leaves of three wheat genotypes, Huandoy, Amurskaja 75 and Greece 25, which are reported to differ in expression of genes related to the C4 pathway in wheat grain. Significant site-specific, genotypic and temporal variation in the maximum carboxylation rate (Vcmax) and maximum rates of electron transport (Jmax) (biological capacity of carbon assimilation) were observed early in ontogeny that dissipated by late grain filling. Although the transcript abundance of rbcS and rbcL in flag leaves was significantly higher than in the pericarps, in line with their photosynthetic prominence, both organ types displayed similar expression patterns among growth stages. The higher N concentrations in the pericarps during grain enlargement suggest increased Rubisco; however, expression of rbcS and rbcL indicated the contrary. From heading to 14 days post-anthesis, wheat pericarps exhibited a strong, positive correlation between biological capacity for carbon assimilation and expression of key genes related to sucrose metabolism (SPS1, SUS1 and SPP1). The strong correlation between spike dry weight and the biological capacity for carbon assimilation along with other findings of this study suggest that metabolic processes in wheat spikes may play a major role in grain filling, total yield and quality

Transpiration influences cadmium uptake by baby leaf spinach (Spinacia oleracea L.)

Cadmium (Cd) is a toxic heavy metal whose concentration in soils is rising. This study investigated the effect of transpiration on Cd uptake by plants using soil conditions akin to those experienced by field crops. Two experiments were performed using baby leaf spinach (Spinacia oleracea L.) grown in soil containing Chloride (Cl) at a typical concentration of 70 mg kg−1. Experiment 1 explored plant response to Cd over the range ~0.04 to 10 mg kg−1 so that a non-toxic concentration could be selected for Experiment 2. Experiment 2 tested the hypothesis that transpiration and Cd uptake were related using soil amended to 70 mg Cl kg−1 and 0.24 mg Cd kg. Differences in transpiration were achieved by manipulating atmospheric CO2 (~400 and~640 µL CO2 L−1 ) and air temperature (22/14, 26/18 and 30/22°C day/night). In Experiment 1, Cd in the foliage variedfrom ~2 to ~100 mg kg−1 DW, yet there was no evidence of Cd toxicity. In Experiment 2, temperature and [CO2] caused large differences in plant growth. [Cd] was lower in chambers with increased temperature and higher in those with elevated CO2. Despite differences in growth, transpiration and foliar Cd uptake were positively and strongly correlated, even when both parameters were expressed on a DW basis. The data are consistent with Cd transport by mass flow towards the roots being a substantial contributor to Cd uptake. Higher [Cd] at elevated CO2 raises concerns about future chronic dietary exposure to Cd. The findings challenge the interpretation of earlier studies on the effects of CO2 and temperature on Cd uptake and may partly explain the inter-seasonal variation in Cd uptake by field crops