Sulphur K-edge X-ray absorption near edge structure (XANES) spectroscopy was used to quantify S species in humic substance extracts from ten soils from the UK, China and New Zealand, which differ in land use and agricultural management. XANES spectroscopy showed the presence of most reduced (sulphides, disulphides, thiols and thiophenes), intermediate (sulphoxides and sulphonates) and highly oxidised S (ester sulphates) forms, with the three groups representing 14–32%, 33–50% and 22–53% of the organic S in the humic substance extracts, respectively. Land use had a profound influence on the relative proportions of S species. Well-drained arable soils generally had a higher proportion of organic S present in the most oxidised form than the grassland soils collected nearby, whereas paddy soils showed a more reduced profile due to episodic flooding. In the Broadbalk Classical Experiment at Rothamsted, reversion of an arable system to grassland or woodland in the 1880s resulted in an increase of the most reduced and intermediate S species at the expense of the most oxidised S species. Long-term applications of farmyard manure to an arable plot also shifted S species from the most oxidised to the intermediate and the most reduced species. Sulphur immobilisation and gross mineralisation were determined in seven soils using the 35S isotope dilution method. Gross mineralisation during a 53-day incubation correlated more closely with the amounts of the most reduced and intermediate S species than with the most oxidised S species, suggesting that the former (C-bonded S) were the main source of organic S for mineralisation in the short-term.
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To understand the threat posed by arsenic in rice to human food chains, the quantity and chemical species of arsenic ingested by an individual or population have to be assessed, along with gut bioavailability and the hazard associated with that arsenic. This chapter...
No abstract is provided for this article.
No abstract is provided for this article.
Sulphur transformations were monitored in a unique set of arable, grassland and woodland soils from the Broadbalk Classical Experiment, which started in 1843. In an open incubation experiment with periodic leaching, 14–35mg SO4 2−-Skg−1 was mineralised in 28weeks at 25°C, equivalent to 4.4–8.3% soil organic S. Cumulative amounts of S mineralised increased linearly during the 28weeks, indicating constant rates of mineralisation. The rate of mineralisation was the greatest in the woodland soil (170μg SO4-Skg−1 day−1), followed by the grassland (120μg SO4-Skg−1 day−1) and the arable soil from the farmyard manure (FYM) plot (110μg SO4-Skg−1 day−1). Three soils from arable plots receiving different inorganic fertiliser treatments but no FYM had similar rates of S mineralisation (~70μg SO4-Skg−1 day−1). In an incubation experiment with 35SO4 2−, addition of glucose greatly enhanced S immobilisation. In 132days, the woodland and grassland soils immobilised more S than the arable soils, with or without glucose amendment. Immobilisation and mineralisation of S occurred concurrently, and both were stimulated by glucose addition. The results show that S mineralisation and immobilisation were influenced strongly by the type of land-use and long-term organic manuring, whereas annual application of sulphate-containing fertilisers for over 150years had few effects on short-term S transformations.
The mechanisms of arsenic (As) hyperaccumulation inPteris vittata, the first identified As hyperaccumulator, are unknown. We investigated the interactions of arsenate and phosphate on the uptake and distribution of As and phosphorus (P), and As speciation in P. vittata. In an 18-d hydroponic experiment with varying concentrations of arsenate and phosphate, P. vittata accumulated As in the fronds up to 27,000 mg As kg−1 dry weight, and the frond As to root As concentration ratio varied between 1.3 and 6.7. Increasing phosphate supply decreased As uptake markedly, with the effect being greater on root As concentration than on shoot As concentration. Increasing arsenate supply decreased the P concentration in the roots, but not in the fronds. Presence of phosphate in the uptake solution decreased arsenate influx markedly, whereas P starvation for 8 d increased the maximum net influx by 2.5-fold. The rate of arsenite uptake was 10% of that for arsenate in the absence of phosphate. Neither P starvation nor the presence of phosphate affected arsenite uptake. Within 8 h, 50% to 78% of the As taken up was distributed to the fronds, with a higher translocation efficiency for arsenite than for arsenate. In fronds, 49% to 94% of the As was extracted with a phosphate buffer (pH 5.6). Speciation analysis using high-performance liquid chromatography-inductively coupled plasma mass spectroscopy showed that >85% of the extracted As was in the form of arsenite, and the remaining mostly as arsenate. We conclude that arsenate is taken up by P. vittata via the phosphate transporters, reduced to arsenite, and sequestered in the fronds primarily as As(III).
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Ganges, an ecotype of Thlaspi caerulescens from southern France accumulates both Zn and Cd to extraordinary levels in the shoots. The interactions between Zn and Cd in this ecotype were investigated using solution culture and pot experiments with soil. Results from both relatively long-term (4 weeks) and short-term (1 week) solution culture experiments indicated that Cd accumulation in the shoots was not affected by the supply of a 4 to 10fold excess of Zn, whereas the Cd concentration of the roots decreased with increasing Zn concentration in the solution. By contrast, the supply of an equimolar concentration of Cd significantly reduced the Zn concentration in both roots and shoots. However, when 4-fold more Zn than Cd was supplied, Cd-induced decrease in Zn accumulation was not observed. In soil treated with Cd or Zn, addition of one metal increased the metal concentration of the other in the soil solution. However, Zn accumulation by T. caerulescens decreased by Cd addition, whereas Cd accumulation was not affected by Zn addition. These results suggested that the Ganges ecotype of T. caerulescens displayed different uptake systems for Cd and Zn, and that Cd competed with Zn uptake while Zn did not compete with Cd uptake
Molybdenum (Mo) is an essential micronutrient for almost all living organisms. The Mo uptake process in plants has been well investigated. However, the mechanisms controlling Mo translocation and remobilization among different plant tissues are largely unknown, especially the allocation of Mo to rice grains that are the major dietary source of Mo for humans. In this study, we characterized the functions of a molybdate transporter, OsMOT1;2, in the interorgan allocation of Mo in rice. Heterologous expression in yeast established the molybdate transport activity of OsMOT1;2. OsMOT1;2 was highly expressed in the blades of the flag leaf and the second leaf during the grain filling stage. Subcellular localization revealed that OsMOT1;2 localizes to the tonoplast. Knockout of OsMOT1;2 led to more Mo accumulation in roots and less Mo translocation to shoots at the seedling stage and to grains at the maturity stage. The remobilization of Mo from older leaves to young leaves under molybdate-depleted condition was also decreased in the osmot1;2 knockout mutant. In contrast, overexpression of OsMOT1;2 enhanced the translocation of Mo from roots to shoots at the seedling stage. The remobilization of Mo from upper leaves to grains was also enhanced in the overexpression lines during grain filling. Our results suggest that OsMOT1;2 may function as a vacuolar molybdate exporter facilitating the efflux of Mo from the vacuole into the cytoplasm, and thus, it plays an important role in the root-to-shoot translocation of Mo and the remobilization of Mo from leaves to grains.
