Impact of sulphur fertilisation on crop response to selenium fertilisation

UK wheat (Triticum aestivum L.) has a low selenium (Se) concentration and agronomic biofortification with Se is a proposed solution. A possible limitation is that UK wheat is routinely fertilised with sulphur (S), which may affect uptake of Se by the crop. The response of wheat to Se and S fertilisation and residual effects of Se were determined in field trials over 2 consecutive years. Selenium fertilisation at 20 g ha^?1 as sodium selenate increased grain Se by four to seven fold, up to 374 µg Se kg^?1. Sulphur fertilisation produced contrasting effects in 2 years; in year 1 when the crop was not deficient in S, grain Se concentration was significantly enhanced by S, whereas in year 2 when crop yield responded significantly to S fertilisation, grain Se concentration was decreased significantly in the S-fertilised plots. An incubation experiment showed that addition of sulphate enhanced the recovery of selenate added to soils, probably through a suppression of selenate transformation to other unavailable forms in soils. Our results demonstrate complex interactions between S and Se involving both soil and plant physiological processes; S can enhance Se availability in soil but inhibit selenate uptake by plants. Furthermore, no residual effect of Se fertiliser applied in year 1 was found on the following crop.     

Selenium biofortification of high-yielding winter wheat (Triticum aestivum L.) by liquid or granular Se fertilisation

Selenium (Se) is an essential trace element for humans and livestock. In the UK, human Se intake and status has declined since the 1980s. This is primarily due to the increased use of wheat (Triticum aestivum L.) grown in UK soils which are naturally low in Se. The aim of this study was to determine the potential for increasing grain Se concentration in a high-yielding UK wheat crop using fertilisers. The crop response of winter-wheat to Se fertilisation was determined under standard field conditions in two consecutive years at up to 10 sites. Selenium fertilisers were applied as high-volume drenches of sodium selenate solution, or as granular Se-containing products. Yield and harvest index were unaffected by Se fertilisation. Under all treatments, grain Se concentration increased by 16–26 ng Se g^?1 fresh weight (FW) per gram Se ha^?1 applied. An application of 10 g Se ha^?1 would thereby increase the Se concentration of most UK wheat grain 10-fold from current ambient levels and agronomic biofortification of UK-grown wheat is feasible. Total recovery (grain and straw) of applied Se was 20–35%. The fate of Se in the food-chain and in the soil must be determined in order to optimize the efficiency of this process.     

In-situ phytoextraction of Ni by a native population of Alyssum murale on an ultramafic site (Albania)

Ultramafic outcrops are widespread in Albania and host several Ni hyperaccumulators (e.g., Alyssum murale Waldst. &; Kit.). A field experiment was conducted in Pojske (Eastern Albania), a large ultramafic area in which native A. murale was cultivated. The experiment consisted in testing the phytoextraction potential of already installed natural vegetation (including A. murale) on crop fields with or without suitable fertilisation. The area was divided into six 36-m² plots, three of which were fertilised in April 2005 with (NPK + S). The soil (Magnesic Hypereutric Vertisol) was fully described as well as the mineralogy of horizons and the localisation of Ni bearing phases (TEM-EDX and XRD). Ni availability was also characterised by Isotopic Exchange Kinetics (IEK). The flora was fully described on both fertilised and unfertilised plots and the plant composition (major and trace elements) and biomass (shoots) harvested individually were recorded. The soil had mainly two Ni-bearing phases: high-Mg smectite (1.3% Ni) and serpentine (0.7% Ni), the first one being the source of available Ni. Ni availability was extremely high according to IEK and confirmed by Ni contents in Trifolium nigriscens Viv. reaching 1,442 mg kg^?1 (A new hyperaccumulator?). Total biomass yields were 6.3 t ha^?1 in fertilised plots and 3.2 t ha^?1 in unfertilised plots with a highly significant effect: fertilisation increased dramatically the proportion of A. murale in the plots (2.6 t ha^?1 vs. 0.2 t ha^?1). Ni content in the shoots of A. murale reached 9,129 mg kg^?1 but metal concentration was not significantly affected by fertilisation. Phytoextracted Ni in total harvest reached 25 kg Ni ha^?1 on the fertilised plots. It was significantly lower in unfertilised plots (3 kg Ni ha^?1). Extensive phytomining on such sites could be promising in the Albanian context by domesticating already installed natural populations with fertilisation.     

Selenium accumulation in durum wheat and spring canola as a function of amending soils with selenite, selenate and or sulphate

Aims

A comparison was performed between plant species to determine if extractable, rather than total soil Se, is more effective at predicting plant Se accumulation over a full growing season.

Methods

Durum wheat (Triticum turgidum L.) and spring canola (Brassica napus L.) were sown in potted soil amended with 0, 0.1, 1.0, or 5.0 mg kg^?1 Se as SeO₄ ^2? or SeO₃ ^2?. In addition, SeO₄ ^2?-amended soils were amended with 0 or 50 mg kg^?1 S as SO₄ ^2?. Soils were analyzed for extractable and total concentration of Se ([Se]). Twice during the growing season plants were harvested and tissue [Se] was determined.

Results

Plants exposed to SeO₃ ^2? accumulated the least Se. Fitted predictive models for whole plant accumulation based on extractable soil [Se] were similar to models based on total [Se] in soil (R²?=?0.73 or 0.74, respectively) and selenium speciation and soil [S] were important soil parameters to consider. As well, soil S amendments limited Se toxicity.

Conclusions

Soil quality guidelines (SQGs) based on extractable Se should be considered for risk assessment, particularly when Se speciation is unknown. Predictive models to estimate plant Se uptake should include soil S, a modifier of Se accumulation.     

Selenium in cereals: improving the efficiency of agronomic biofortification in the UK

Wheat, despite its relatively low selenium (Se) concentration in the UK, is still an important dietary Se source and its biofortification by use of Se fertiliser may be an efficient means to increase the relatively low Se status of the population. We need to know more about the fate of Se applied to the soil and how to ensure the efficiency of Se application, and the three studies reported in this issue of Plant and Soil are timely and informative. Selenium in soil, both globally and locally, is notoriously variable; however, the soils in these studies yielded wheat grain Se concentrations in the narrow range of 16–44 ng/g. The low plant Se levels reported here are not surprising, given that selenite is the dominant Se form in these soils. A regression equation (which used total and extractable Se and extractable S as variables) explained a high proportion of the variance in grain Se concentration. Sulphur application (a common practice on UK wheat growing soils) had variable effects on grain Se concentration, depending on soil S status, pH and possibly other factors. A fertiliser methodology study investigated ways to optimise Se application for the purpose of biofortification. It was calculated that an application of a modest 10 g Se/ha as selenate would increase the grain Se concentration of UK wheat from around 30 ng/g to 300 ng/g. The national Se fertiliser program in Finland shows that this increase would have a large effect on population Se status. However, Se recovery in grain at this application rate is only 14%, and it can be argued that large-scale agronomic biofortification of cereals with Se would be somewhat wasteful of a relatively scarce trace element. Selenium’s effects and interactions in soil, plants, animals and humans are complex and often surprising and will keep researchers busy well into the future.     

Tolerance of wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) to high soil and solution selenium levels

The fertilisation of wheat crops with Se is a cost-effective method of enhancing the concentration of organic Se in grain, in order to increase the Se intake of animals and humans. It is important to avoid phytotoxicity due to over-application of Se. Studies of phytotoxicity of Se in wheat grown in Australia, where rainfall and grain yield are usually relatively low, have not been reported previously, and overseas studies have had varied results. This study used trials conducted in the field, glasshouse and laboratory to assess Se phytotoxicity in wheat. In field trials that used rates of up to 120 g ha^–1Se as selenate, and in pilot trials that used up to 500 g ha^–1 Se soil-applied or up to 330 g ha^–1 Se foliar-applied, with soils of low S concentrations (2–5 mg kg^–1), no Se toxicity symptoms were observed. In pot trials of four weeks duration, the critical tissue level for Se toxicity was around 325 mg kg^–1 DW, a level attained by addition to the growth medium of 2.6 mg kg^–1 Se as selenate. Solution concentrations above 10 mg L^–1 Se inhibited early root growth of wheat in laboratory studies, with greater inhibition by selenite than selenate. For selenite, Se concentrations around 70 mg L^–1 were required to inhibit germination, while for selenate germination % was unaffected by a solution concentration of 150 mg L^–1 Se. Leaf S concentration and content of wheat increased three-fold with the addition of 1 mg kg^–1 Se as selenate to the growth medium. This effect is probably due to the induction of the S deficiency response of the main sulphate transporter. This study found wheat to be more Se-tolerant than did earlier studies of tobacco, soybeans and rice. We conclude that Se phytotoxicity in wheat will not be observed at the range of Se application rates that would be used to increase grain Se for human consumption (4–200 g ha^–1 Se as selenate, which would result in soil and tissue levels well below those seen in the above studies), even when – as is common in Australia – soil S concentration and grain yield are low.     

Maize yield and nutrition during 4 years after biochar application to a Colombian savanna oxisol

The application of biochar (biomass-derived black carbon) to soil has been shown to improve crop yields, but the reasons for this are often not clearly demonstrated. Here, we studied the effect of a single application of 0, 8 and 20 t ha^?1 of biochar to a Colombian savanna Oxisol for 4 years (2003–2006), under a maize-soybean rotation. Soil sampling to 30 cm was carried out after maize harvest in all years but 2005, maize tissue samples were collected and crop biomass was measured at harvest. Maize grain yield did not significantly increase in the first year, but increases in the 20 t ha^?1 plots over the control were 28, 30 and 140% for 2004, 2005 and 2006, respectively. The availability of nutrients such as Ca and Mg was greater with biochar, and crop tissue analyses showed that Ca and Mg were limiting in this system. Soil pH increased, and exchangeable acidity showed a decreasing trend with biochar application. We attribute the greater crop yield and nutrient uptake primarily to the 77–320% greater available Ca and Mg in soil where biochar was applied.     

Long-term effects of boron and copper on phenolics and monoterpenes in Scots pine (Pinus sylvestris L.) needles

Backgroud and aims

Plant boron (B) status is known to affect plant secondary metabolites but most studies have been short termed and in controlled environments. Copper (Cu) effects on phenolics are better known at toxic than at low levels. Here, the chemistry of Scots pine (Pinus sylvestris L.) needles was studied 20 years after fertilisation with B and Cu in a long-term field experiment on a drained boreal peatland.

Methods

Phenolic compounds were analysed from three needle year classes using high performance liquid chromatography (HPLC) and condensed tannins with modified acid-butanol assay. Monoterpenes in the youngest needles were analysed by gas chromatography–mass spectrometry (GC–MS).

Results

Needle B concentrations were at deficient level in controls (5.7 μg g^?1), but at the optimum level (12 μg g^?1) still 20 years after fertilisation. Copper concentrations were low but not deficient (4.0 μg g^?1 in unfertilised, 4.8 μg g^?1 in fertilised). Needle ageing increased the concentrations of individual phenolics in most cases, but decreased the concentration of condensed tannins. The concentrations of several individual phenolics were reduced by B fertilisation compared to B-deficient control, significantly in the cases of (+)-catechin and a neolignan. The concentrations of eight compounds and the sum of small-molecule phenolics were higher in Cu fertilised trees. Condensed tannins and monoterpenes were not affected by the micronutrients.

Conclusions

Boron and copper additions affected mostly the same phenolic compounds, but B decreased while Cu increased their concentrations, Cu effects being clearer. The higher phenolic concentrations in B deficient trees were not likely large enough to explain leader dieback in B-deficient trees. The effects and interactions of these micronutrients need to be further studied in field conditions to establish firstly if the changes in phenolics are consistent among species, and secondly what mechanisms lead to the changes. Although small, the changes in phenolic concentrations may affect the interactions of the trees with their biotic and abiotic environment, when consistent over many years.     

Foliar application of selenium increases fertility and grain yield in bread wheat under contrasting water availability regimes

The study was conducted to assess whether selenium (Se) application modulates fertility to alter grain yield in bread wheat grown under different moisture regimes. Seeds of wheat cultivar Millat-2011 were sown in the plots using a randomized complete block design with three replicates per treatment. After germination, the plants were exposed to six moisture regimes, viz. no irrigation after germination, irrigation at boot stage, irrigations at boot and grain-filling stages, irrigations at crown root, boot and grain-filling stages, irrigations at crown root, boot, heading and grain-filling stages and irrigations at crown root, stem elongation, boot, heading and grain-filling stages. At the heading stage, foliar spray of sodium selenate (0, 2 and 4 mg Se L^?1) was done. Withholding water at early growth stages significantly increased oxidative stress and decreased growth and grain yield. Irrespective of moisture regimes, foliar application of Se (2 mg L^?1) decreased oxidative stress, modulated photosynthetic pigments and fertility and increased grain yield in wheat. The Se-mediated increase in grain yield was attributed to the increase in chlorophyll and ascorbic acid contents and fertility coupled with decrease of oxidative stress under different moisture regimes. The results could be helpful to manage wheat production in the semi-arid environments.     

Evaluation of the Potential of Peas (Pisum sativum L.) to Be Used in Selenium Biofortification Programs Under Mediterranean Conditions

Selenium (Se), which has antioxidant, anticancer, and antiviral properties, is an essential micronutrient for humans and animals. This micronutrient is found in high quantity in legumes. Peas have an ever-increasing importance in Spain, and to increase their nutritional value, two foliar Se fertilizers: sodium selenate and sodium selenite, at five different rates: 0, 10, 20, 40, 80 g?ha^?1, were studied during the 2010/2011 crop season on semiarid Mediterranean conditions. Sodium selenate was much more effectively taken up by plants compared to sodium selenite. There was a strong linear relationship between the total Se content and Se rate in both sodium selenate and selenite. For each gram of Se fertilization as either sodium selenate or sodium selenite, the increase of total Se concentration in the grain was 148 and 19 μg Se?kg^?1 dry weight, respectively. Ingestion of 100 g of peas previously fertilized with 10 g of sodium selenate per hectare would result in an intake of 179 μg of Se. This is almost 90 % of the daily recommended dose needed to reduce the chance of some cancers and about 179 % of the minimum concentration required to prevent Se deficiency diseases in animals. The pea has shown to have a strong ability to uptake and accumulate Se under Mediterranean conditions; therefore, this would make it a very strong candidate for inclusion in biofortification programs aiming to increase Se in the food chain.     

Accumulation of arsenic in soil and rice under wetland condition in Bangladesh

Shallow tube well (STW) water, often contaminated with arsenic (As), is used extensively in Bangladesh for irrigating rice fields in the dry season, leading to potential As accumulation in soils. In the current study the consequences of arsenic from irrigation water and direct surface (0–15 cm) soil application were studied under field conditions with wetland rice culture over 2 years. Twenty PVC cylinders (30-cm length and 30-cm diameter) were installed in field plots to evaluate the mobility and vertical distribution of soil As, As mass balance, and the resulting influences on rice yield and plant-As concentration in Boro (dry season) and transplanted (T.) Aman (wet season) rice over the 2-year growth cycle. Treatments included irrigation-water As concentrations of 0, 1 and 2 mg L^?1 (Boro season only) and soil-As concentrations of 10 and 20 mg kg^?1. Following the 2-year cropping sequence the major portion (39.3–47.6%) of the applied arsenic was retained within the rooting zone at 0–15 cm depth, with 14.7–19.5% of the total applied As at the 5–10 cm and 10–15 cm soil depths compared to 1.3–3.6% at the 35–40 cm soil depth. These results indicate the relatively low mobility of applied As and the likely continued detrimental accumulation of As within the rooting zone. Arsenic addition in either irrigation water or as soil-applied As resulted in yield reductions from 21 to 74 % in Boro rice and 8 to 80 % in T. Aman rice, the latter indicating the strong residual effect of As on subsequent crops. The As concentrations in rice grain (0.22 to 0.81 µg g^?1), straw (2.64 to 12.52 µg g^?1) and husk (1.20 to 2.48 µg g^?1) increased with increasing addition of As. These results indicate the detrimental impacts of continued long-term irrigation with As-contaminated water on agricultural sustainability, food security and food quality in Bangladesh. A critical need exists for the development of crop and water management strategies to minimize potential As hazard in wetland rice production.     

Crop nitrogen use and soil mineral nitrogen accumulation under different crop combinations and patterns of strip intercropping in northwest China

Increasing crop nitrogen use efficiency while also simultaneously decreasing nitrogen accumulation in the soil would be key steps in controlling nitrogen pollution from agricultural systems. Long-term field experiments were started in 2003 to study the effects of intercropping on crop N use and soil mineral N accumulation in wheat (Triticum aestivum L. cv 2014)/maize (Zea mays L. cv Shendan16), wheat/faba bean (Vicia faba L. cv Lincan No. 5) and maize/faba bean intercropping and monocropping systems. Monocropping was compared with two types of strip intercropping: continuous intercropping (two crops intercropped continuously on the same strips of land every year) and rotational intercropping (two crops grown adjacently and rotated to the other crop??s strip every year). Maize/faba bean intercropping had greater crop N uptake than did wheat/faba bean or wheat/maize. Wheat/maize accumulated more mineral N in the top 140 cm of the soil profile during the co-growth stage from maize emergence to maturity of wheat or faba bean. Continuously intercropped maize substantially decreased soil mineral N accumulation under wheat and faba bean rows (60?C100 cm soil depth) at maize harvest. Soil mineral N accumulation under wheat rows increased with rotational intercropping with faba bean. Rotational intercropping may potentially alleviate the adverse effects of wheat on N use by other crops and increase the nitrogen harvest index of wheat, maize and faba bean. Intercropping using species with different maturity dates may be more effective in increasing crop N use efficiency and decreasing soil mineral N accumulation.     

Susceptibility to take‐all of cereal and grass species,and their effects on pathogen inoculum

Two field trials were conducted to investigate different herbage grasses and cereals for their susceptibility to the disease take‐all, for their impact on concentrations of the pathogen, Gaeumannomyces graminis var. tritici (Ggt), in soil and for their effect on development of take‐all in a subsequent wheat crop. In the herbage grass trial, Bromus willdenowii was highly susceptible to Ggt, produced the greatest post‐senescence Ggt concentrations in soil and highest incidence of take‐all in following wheat crop. Lolium perenne, Lolium multiflorum and Festuca arundinacea supported low Ggt soil concentrations and fallow the least. The relationship between susceptibility to Ggt and post‐senescence concentrations in soil differed between pasture grasses and cereals. In a trial in which Ggt was added to half the plots and where wheat, barley, triticale, rye or fallow were compared, the susceptibility of the cereals to take‐all was not clearly linked to post‐harvest soil Ggt concentrations. In particular, triticale and rye had low and negligible take‐all infection respectively, but greater post‐harvest soil Ggt concentrations than barley or wheat. This indicates that low Ggt concentrations on roots may build up during crop senescence on some cereals. Soil Ggt concentrations were greater following harvest in inoculated plots sown to cereals, but in the second year there was more take‐all in the previously non‐inoculated than inoculated plots. Thus, the grass and cereal species differed in susceptibility to take‐all, in their impact on Ggt multiplication and in associated take‐all severity in following wheat crop.     

Long term effects of whole tree harvesting on soil carbon and nutrient sustainability in the UK

The practice of harvesting forest residues is rapidly increasing due to rising demand for renewable energy. However, major concerns have been raised about the sustainability of this practice and its net impact on long term soil ability to support forest productivity, particularly through second and subsequent rotations. In this study, soil chemical properties such as acidity, total N and C, available NO₃–N and NH₄–N and exchangeable cations were measured in all horizons in peaty gleys soils under one of the oldest experiments in Europe—a 28-year-old second rotation stand of Sitka spruce (Picea sitchensis), in Kielder forest, UK. Treatments included Whole Tree Harvesting (WTH—of all above ground biomass), Conventional stem-only harvesting (CH) of the first rotation crop, and repeated Fertilisation (FE) after the planting of the second rotation forest. This study demonstrates the soil changes underpinning the reduced second rotation tree productivity on these acidic upland sites under WTH, a further 18 years after the investigation by Proe and Dutch (1994). Overall, WTH increased soil acidity significantly (p < 0.05) and reduced soil base saturation whilst FE reduced soil acidity (p < 0.05) and increased soil base saturation as compared to CH. Soil moisture was significantly higher (p < 0.01) under WTH compared to CH and FE plots. There was no evidence that WTH decreased soil organic carbon (SOC) and soil nitrogen (N), but to the contrary there were significantly (p < 0.01) higher concentrations and stocks of total C and N in the WTH soils compared with CH and FE. The depletion of SOC and N in CH and FE plots was attributed to much higher soil mineralisation rates associated with the brash and fertilisation as compared to the WTH plots, where significantly less soil available NO₃–N (p < 0.01) was found. In the long term WTH on peaty gley soils appears positive for soil C and N storage. However, WTH had a long term negative impact on soil and tree nutrition of K⁺ and P, which are currently at deficient levels, but has had a stabilising effect on tree N nutrition as measured in twigs and needles. These results suggest that whilst WTH lead to a reduction in aboveground tree biomass compared to conventional harvest, these practices on selected soil types and certain sites may be beneficial for soil C and N sequestration. The overall findings of this study imply that cost benefit analyses for each site should be carried out before decisions are made on the appropriate type of forest operations (harvesting and replanting), considering both geology and soils in order to serve both environmental benefits, long term sustainability and the available biomass production for timber and biofuel.     

Soil carbon fractions in grasslands respond differently to various levels of nitrogen enrichments

Background and Aims

Soil contains many different C fractions which have diverse physical and chemical compositions. Examining these differential soil C fractions in response to N enrichment is helpful for better understanding soil C changes under the predominantly increasing N deposition. In this study, we used a field N addition experiment in a grassland to explore the effects of various N enrichment levels on soil C fractions.

Methods

We conducted a field manipulative experiment which used a Latin square design with six N addition levels of 0, 2, 4, 8, 16 and 32 g N m^?2 year^?1 since 2003 in a semiarid grassland in northern China. Soil samples were collected in August (when plants have the greatest biomass), 2011. We measured C and N concentrations in soil light fraction, microbial biomass, extractable organic matter, heavy fraction, and total soil C and N.

Results

The results showed that total soil C and N, and heavy fraction C and N were not significantly affected by N addition after 9 years of treatments. In contrast, different N enrichment levels changed soil light fraction C and N, ranging from 4.3 to 27.7 % and 3.3–30.0 %, respectively. Moreover, both light fraction C and N had a nonlinear relationship with N addition rates, and the threshold for N-induced change in light fraction C and N was near 16 g N m^?2 year^?1 in this semiarid grassland. Increases of soil light fraction C and N primarily resulted from changes in biotic (N-stimulated aboveground biomass) and abiotic (soil temperature, moisture and pH) factors under N enrichment. Soil microbial biomass exponentially declined with increasing N, but extractable organic C showed a positive linear response to N enrichment rates. Changes in microbial biomass C and extractable organic C were primarily due to the reduced soil pH under N addition.

Conclusions

Our findings suggest that various soil C fractions differentially respond to elevated N, because different sets of biotic and abiotic factors regulate those fractions under N enrichment.     

Zinc seed coating improves the growth,grain yield and grain biofortification of bread wheat

This study, comprising three independent experiments, was conducted to optimize the zinc (Zn) application through seed coating for improving the productivity and grain biofortification of wheat. Experiment 1 was conducted in petri plates, while experiment 2 was conducted in sand-filled pots to optimize the Zn seed coating using two sources (ZnSO₄, ZnCl₂) of Zn. In the first two experiments, seeds of two wheat cultivars Lasani-2008 and Faisalabad-2008 were coated with 0.25, 0.50, 0.75, 1.00, 1.25, 1.50, 1.75 and 2.00 g Zn kg^?1 seed using ZnSO₄ and ZnCl₂ as Zn sources. The results of experiment I revealed that seed coating with 1.25 and 1.50 g Zn kg^?1 seed using both sources of Zn improved the seedling emergence. However, seed coated with 1.25 and 1.50 g Zn kg^?1 seed using ZnSO₄ was better regarding improvement in seedling growth and seedling dry weight. The results of the second experiment indicated that seed coated with 1.25 and 1.50 g Zn kg^?1 seed using ZnSO₄ improved the seedling emergence and seedling growth of tested wheat cultivars. However, seed coating beyond 1.5 g Zn kg^?1 seed using either Zn source suppressed the seedling emergence. Third experiment was carried out in glass house in soil-filled earthen pots. Seeds of both wheat cultivars were coated with pre-optimized treatments (1.25, 1.50 g Zn kg^?1 seed) using both Zn sources. Seed coating with all treatments of ZnSO₄ and seed coating with 1.25 g Zn kg^?1 seed using ZnCl₂ improved the seedling emergence and yield-related traits of wheat cultivars. Seed coating with 1.25 g Zn kg^?1 seed also improved the chlorophyll a and b contents. Maximum straw Zn contents, before and after anthesis, were recorded from seed coated with 1.5 g Zn kg^?1 seed using either Zn source. Increase in grain yield from seed coating followed the sequence 1.25 g Zn kg^?1 seed (ZnSO₄) >1.25 g Zn kg^?1 seed (ZnCl₂) >1.5 g Zn kg^?1 seed (ZnSO₄). However, increase in grain Zn contents from seed coated was 1.5 g Zn kg^?1 seed (ZnCl₂) >1.25 and 1.5 g Zn kg^?1 seed (ZnCl₂, ZnSO₄) >1.25 g Zn kg^?1 seed (ZnSO₄). Seed coating with Zn increased the grain Zn contents from 21 to 35 %, while 33–55 % improvement in grain yield was recorded. In conclusion, wheat seeds may be coated with 1.25 g Zn kg^?1 seed using either source of Zn for improving the grain yield and grain Zn biofortification.     

Yield gaps, nutrient use efficiencies and response to fertilisers by maize across heterogeneous smallholder farms of western Kenya

The need to promote fertiliser use by African smallholder farmers to counteract the current decline in per capita food production is widely recognised. But soil heterogeneity results in variable responses of crops to fertilisers within single farms. We used existing databases on maize production under farmer (F-M) and researcher management (R-M) to analyse the effect of soil heterogeneity on the different components of nutrient use efficiency by maize growing on smallholder farms in western Kenya: nutrient availability, capture and conversion efficiencies and crop biomass partitioning. Subsequently, we used the simple model QUEFTS to calculate nutrient recovery efficiencies from the R-M plots and to calculate attainable yields with and without fertilisers based on measured soil properties across heterogeneous farms. The yield gap of maize between F-M and R-M varied from 0.5 to 3 t grain ha^?1 season^?1 across field types and localities. Poor fields under R-M yielded better than F-M, even without fertilisers. Such differences, of up to 1.1 t ha^?1 greater yields under R-M conditions are attributable to improved agronomic management and germplasm. The relative response of maize to N–P–K fertilisers tended to decrease with increasing soil quality (soil C and extractable P), from a maximum of 4.4-fold to ?0.5-fold relative to the control. Soil heterogeneity affected resource use efficiencies mainly through effects on the efficiency of resource capture. Apparent recovery efficiencies varied between 0 and 70% for N, 0 and 15% for P, and 0 to 52% for K. Resource conversion efficiencies were less variable across fields and localities, with average values of 97 kg DM kg^?1 N, 558 kg DM kg^?1 P and 111 kg DM kg^?1 K taken up. Using measured soil chemical properties QUEFTS over-estimated observed yields under F-M, indicating that variable crop performance within and across farms cannot be ascribed solely to soil nutrient availability. For the R-M plots QUEFTS predicted positive crop responses to application of 30 kg P ha^?1 and 30 kg P ha^?1 + 90 kg N ha^?1 for a wide range of soil qualities, indicating that there is room to improve current crop productivity through fertiliser use. To ensure their efficient use in sub-Saharan Africa mineral fertilisers should be: (1) targeted to specific niches of soil fertility within heterogeneous farms; and (2) go hand-in-hand with the implementation of agronomic measures to improve their capture and utilisation.     

Methane oxidation potentials and fluxes in agricultural soil:Effects of fertilisation and soil compaction

We have studied the inhibiting effect offertilisation and soil compaction on CH₄oxidation by measuring gas fluxes and soil mineral Ndynamics in the field, and CH₄ oxidation rates inlaboratory-incubated soil samples. The fertilisationand soil compaction field experiment was establishedin 1985, and the gas fluxes were measured from 1992 to1994. Methane oxidation was consistently lower infertilised than in unfertilised soil, but thereapparently was no effect of repeated fertiliseradditions on the fertilised plots. The measuredmineral N in fertilised and unfertilised soil showedlarge differences in NH₄ ⁺ concentrationsjust after fertilisation, but the levels rapidlyconverged because of plant uptake and nitrification.The CH₄ oxidation rate did not reflect thesecontrasting mineral N patterns, suggesting that theCH₄ oxidation capacity remaining in the soil thathad been fertilised since 1985 was largely insensitiveto ammonia in the new fertiliser. Thus, competitiveinhibition by ammonia may have been involved in theearly stage of the field fertiliser experiment, butthe CH₄ oxidation remaining after 7 to 9 years ofcontinued fertilisation seems not to have beenaffected by ammonia. The substrate affinity of theCH₄-oxidizing microflora appeared to be the samein both the fertilised soil and the unfertilisedcontrol, as judged from the response to elevatedCH₄ concentrations (52 µl l^–1) inlaboratory incubations. Soil compaction resulted in apersistent reduction of CH₄ influx, also seen inlaboratory incubations with sieved (4-mm mesh) soilsamples. Since the sieving presumably removesdiffusion barriers created by the soil compaction, thefact that compaction effects persisted through thesieving may indicate that soil compaction has affectedthe biological potential for CH₄ oxidation in thesoil.     

Effect of a Terminated Cover Crop and Aldicarb on Cotton Yield and Meloidogyne incognita Population Density

Terminated small grain cover crops are valuable in light textured soils to reduce wind and rain erosion and for protection of young cotton seedlings. A three-year study was conducted to determine the impact of terminated small grain winter cover crops, which are hosts for Meloidogyne incognita, on cotton yield, root galling and nematode midseason population density. The small plot test consisted of the cover treatment as the main plots (winter fallow, oats, rye and wheat) and rate of aldicarb applied in-furrow at-plant (0, 0.59 and 0.84 kg a.i./ha) as subplots in a split-plot design with eight replications, arranged in a randomized complete block design. Roots of 10 cotton plants per plot were examined at approximately 35 days after planting. Root galling was affected by aldicarb rate (9.1, 3.8 and 3.4 galls/root system for 0, 0.59 and 0.84 kg aldicarb/ha), but not by cover crop. Soil samples were collected in mid-July and assayed for nematodes. The winter fallow plots had a lower density of M. incognita second-stage juveniles (J2) (transformed to Log₁₀ (J2 + 1)/500 cm³ soil) than any of the cover crops (0.88, 1.58, 1.67 and 1.75 Log₁₀(J2 + 1)/500 cm³ soil for winter fallow, oats, rye and wheat, respectively). There were also fewer M. incognita eggs at midseason in the winter fallow (3,512, 7,953, 8,262 and 11,392 eggs/500 cm³ soil for winter fallow, oats, rye and wheat, respectively). Yield (kg lint per ha) was increased by application of aldicarb (1,544, 1,710 and 1,697 for 0, 0.59 and 0.84 kg aldicarb/ha), but not by any cover crop treatments. These results were consistent over three years. The soil temperature at 15 cm depth, from when soils reached 18°C to termination of the grass cover crop, averaged 9,588, 7,274 and 1,639 centigrade hours (with a minimum threshold of 10°C), in 2005, 2006 and 2007, respectively. Under these conditions, potential reproduction of M. incognita on the cover crop did not result in a yield penalty.     

Effects of Different Selenium Application Methods on Wheat (<i>Triticum aestivum</i> L.) Biofortification and Nutritional Quality

Mineral nutrient malnutrition, especially deficiency in selenium (Se), affects the health of approximately 1 billion people worldwide. Wheat, a staple food crop, plays an important role in producing Se-enriched foodstuffs to increase the Se intake of humans. This study aimed to evaluate the effects of different Se application methods on grain yield and nutritional quality, grain Se absorption and accumulation, as well as 14 other trace elements concentrations in wheat grains. A sand culture experiment was conducted via a completely randomized 3 × 2 × 1 factorial scheme (three Se levels × two methods of Se application, foliar or soil × one Se sources, selenite), with two wheat cultivars (Guizi No.1, Chinese Spring). The results showed that both foliar Se and soil Se application methods had effects on wheat pollination. Foliar Se application resulted in early flowering of wheat, while soil Se application caused early flowering of wheat at low Se levels (5 mg kg⁻¹ ) and delayed wheat flowering at high selenium levels (10 mg kg⁻¹ ), respectively. For trace elements, human essential trace elements (Fe, Zn, Mn, Cu, Cr, Mo, Co and Ni) concentrations in wheat grains were dependent of Se application methods and wheat cultivars. However, toxic trace elements (Cd, Pb, Hg, As, Li and Al) concentrations can be decreased by both methods, indicating a possible antagonistic effect. Moreover, both methods increased Se concentrations, and improved grain yield and nutritional quality, while the foliar application was better than soil. Accordingly, this study provided useful information concerning nutritional biofortification of wheat, indicating that it is feasible to apply Se to conduct Se biofortification, inhibit the heavy metal elements concentrations and improve yield and quality in crops, which caused human health benefits.