Accumulation of multiple heavy metals in plants grown on soil treated with sewage sludge for more than 50 years presents health risks and an opportunity for phyto-remediation

Long-term application of sewage leads to heavy metal accumulation in soils, causing serious risks to plants, humans, animals and the environment, and phytoremediation could be essential. A study was conducted to determine the concentration of heavy metals in self-seeding vegetables, amaranthus (Amaranthus dubius), tomato (Solanum lycopersicum), black nightshade (Solanum nigrum), Rumex pulcher and turf grass, grown on land treated with sewage sludge for over 50 years. A pot experiment was conducted to determine phytoremediation potential of Indian mustard (Brassica juncea), lucern (Medicago sativa), vetch (Vicia sativa), rape (Brassica napus) and ryegrass (Lolium perenne), using the same soil. Another pot experiment was conducted to determine effects on tissue metal composition of Indian mustard of adding increasing concentrations of EDTA. All the self-seeding vegetables had tissue Zn, Cu, Cr, Ni, Cd and Pb concentrations higher than toxicity thresholds. Turf grass tissue had higher concentrations of all the metals than all the self-seeding vegetables growing on the soil. Indian mustard and rape had the highest biomass and tissue concentration of most of the metals studied. Addition of EDTA to the soil drastically increased uptake of Zn, Cu, Cd and Pb but not Cr and Ni. The findings of this study imply that self-seeding vegetables and turf grass growing on the polluted soils pose serious health risks and that Indian mustard, and to some extent rape, have potential for phytoremediation, especially if grown on the soil treated with EDTA.Keywords: heavy metals, phytoremediation, risk, sewage sludge, turf grass, uptak

Assessing soil carbon and carbon dioxide effluxes under different vegetation cover conditions in the Eastern Cape Province, South Africa

Albany thicket is prevalent in the Eastern Cape Province of South Africa. Its spread has diminished through overgrazing and heavy browsing by animals, land clearance and urban expansion. The result is highly degraded land characterized by invasion of alien species. There is a wealth of documented evidence on the high carbon sequestration ability of thicket biome, but not much has been done to assess its effect on carbon dioxide emissions from the soil. Given that the concentration of atmospheric greenhouse gases has been constantly rising since the industrial era, it is imperative to assess the influence of thicket biome as a source or sink of these gases. There is evidence of shifts in the climate in southern Africa as reflected by changes in rainfall patterns, increased temperatures, recurrent droughts and fires. As such, the historical rainfall variability in an Albany thicket region and its interaction with the temporal land use / cover changes was studied. This served to give some background information about the study area for more detailed study on C and carbon dioxide effluxes in thicket vegetation under different levels of degradation. This study thus aimed to determine the influence of thicket vegetation at various levels of degradation on soil carbon and carbon dioxide fluxes. The impact of plant photosynthetic pathway on soil C residence time and gas effluxes were analysed to elucidate on the land-use and cover patterns occurring in the area. All this was done to shed some light on the role of soil and thicket vegetation on carbon dioxide emissions and C storage in the spectrum of a shifting climate. The main area of research was Amakhala reserve in an Albany thicket in Eastern Cape Province; and it concentrated on three land cover types namely intact thicket, degraded thicket and grassland. The objectives mentioned above were achieved by assessing historical rainfall variability from 1970 to 2010 through trend and time series analysis at nine rainfall stations located at Amakhala reserve, Grahamstown, Bathurst, Port Alfred, Uitenhage and Port Elizabeth. The land use changes that have occurred in the Albany thicket region covering Amakhala reserve, Grahamstown, Bathurst and Port Alfred were also assessed for 1989, 1999 and 2009 through satellite image analysis with Idrisi Andes GIS software; then their interaction with rainfall variability were determined. To elucidate on the vegetation species composition and land use / cover changes that have occurred in the study area, plant biomass as well carbon (C) and nitrogen (N) isotope measurements were done. Plant biomass was assessed for the dominant species through use of pre-existing allometric equations that required data on plant basal diameter, canopy area, stem numbers and height. The plant carbon was then estimated through use of a conversion factor of 0.48 on above-ground biomass, while soil organic C was determined through the modified Walkely - Black method. Carbon and N isotope ratios were determined from the foliar material of three replicate samples of dominant plant species then analyzed through mass spectrometry. Soil carbon dioxide effluxes were then monitored in each of the intact thicket (IT), degraded thicket (DT) and grassland (G) over a 10 month period; by measuring the net carbon dioxide exchange rate (NCER) through the dynamic chamber method. An automated carbon dioxide exchange analyzer, coupled to a soil temperature probe and photosynthetic active radiation (PAR) sensor was used; with NCER measurements taken every 20-30 days. Soil temperature, moisture, penetration resistance and PAR readings were taken during each assay and later used to interpret the NCER. Results showed that long term variability in annual rainfall had a declining trend at Grahamstown (r = -0.59), Uitenhage and Bathurst stations (r = -0.32 at both stations), but was not significant at Amakhala, Port Alfred and Port Elizabeth stations. Most reductions in rainfall occurred in the 1980s and 1990s with the autumn, winter and summer rainfalls, the daily rainfall index and the daily rainfall subclasses of 10 mm and above showing a similar trend. The land use change detection gave a significant increase in proportion of degraded and transformed (moderately degraded) land between 1989 and 2009 with most of the increases occurring from 1989 to 1999, while farmland area decreased by 1.8 percent over the years. Thus the Albany region had over 30 percent of its land occupied by transformed vegetation, with heavy browsing and uncontrolled grazing being attributed to the destruction of pristine vegetation. Land-use change to game ranching and goat pastoralism was attributed to the reduction in farmland. Rainfall variability – land use change linkages were most significant in 1999 that recorded the least rainfall and had the lowest mean, maximum and sum of the NDVI. Grahamstown had the most significant rainfall-NDVI trends as it had the lowest NDVIs in 1999 when rainfall was lowest, the highest NDVI in 1989 when rainfall was highest and moderate NDVIs in 2009 when rainfall was moderate. Vegetation at the IT was characterized by a dense thicket with diverse growth forms of canopy trees, woody shrubs, succulent shrubs and ephemerals which mostly had the C3 type of pathway. This was in contrast with the IT soil isotopy that showed more positive C isotope ratios, indicating a switch between C3 and CAM photosynthesis in original vegetation. Most of the canopy trees had disappeared in the DT to be replaced by herbs, shrubs and grasses. As such, there was a huge difference in isotope ratios between DT plants and soils with the plants having mostly C3 metabolism while the soil showed a predominance of CAM plants in previous vegetation, indicating significant changes in land cover. The G site mostly comprised the grasses Themeda triandra and Panicum maximum and a few herbs. It maintained a dominance of C4 metabolism in both plants and soils showing very little change in species composition over the years. Because of the higher species diversity at IT, its soil organic C was quite high reaching levels of 3.4 percent (i.e. 3.4 t C / ha) in the top 10 cm then decreasing with depth (p < 0.001); but was moderate at DT (1.1-1.3 percent) and very low at G ( 0.5 percent C) (p < 0.001). In the same manner above-ground biomass was highest at IT i.e. 330 000 kg/ha; but was only 22 000 kg/ha in DT and as low as 6 700 kg/ha in G vegetation. High biomass at IT was mostly attributed to the succulent shrub Portulacaria afra and the canopy trees Euclea undulate, Rhus longispina and Schotia afra. This above-ground biomass translated to biomass C amounts of 158 000 kg/ha at IT, 10 600 kg/ha at DT and 3 200 kg/ha at G. Thus the IT had the highest while G the least and DT moderate plant and soil C sequestration ability. In all, the conversion of IT to DT led to a net loss of 147 000 Kg of biomass C / ha and 12 000kg less organic C / ha of land. Soil carbon dioxide effluxes were however variable between seasons as they were affected by differences in soil properties and seasonal weather patterns. High soil moisture levels (up to 16 percent gravimetric moisture) resulted in reduced soil penetration resistance (1 to 4 Kg/cm2) which raised effluxes at G and DT sites (up to 1.2 μmols m-2 sec-1) in winter, while low moisture (2 percent) resulted in hard dry soil (14 Kgm-2 penetration resistance) with suppressed CO2 effluxes in spring (0.2 μmols m-2 sec-1) especially in DT and G soils. Rising temperature generally caused accelerated gas emissions but only when moisture was not limiting (as was the case in IT). Thus the high summer temperatures (up to 40oC) gave lower effluxes especially in DT and G (< 1 μm-2sec-1) due to limited moisture supply (< 10 percent); while the Autumn period that had very high temperature (up to 48 oC) and good moisture (up to 16 percent) saw accelerated soil CO2 emissions (averaging 2 μmols m-2 sec-1) from all cover types. The high biomass and litter fall at IT served as ready substrate for soil respiration as long as moisture was not limiting and temperatures were favourable, while reduced cover at DT resulted in poor moisture conservation and creation of hard dry soils in spring and summer with reduced respiration. It was concluded that the DT had high CO2 effluxes in winter and reduced emissions in summer; while the opposite was true for the IT. All the cover types had minimal CO2 effluxes in spring and accelerated emissions in autumn. The grassland on the other hand was a fairly moderate source or sink of CO2 in most seasons compared with the other two covers. It was observed that an environment of good moisture and low-moderate temperatures (such as that in the winter) minimises effluxes while maintaining good plant productivity. It was concluded that thicket vegetation is a good sink of carbon that should be preserved in its natural condition to optimize its carbon sequestration potential. All three land covers served as sources or sinks of CO2 depending on soil and seasonal conditions. Thus high moisture and low penetration resistance generally increased effluxes of thicket ecosystems. The effect of increasing temperature on effluxes was only significant when moisture was not limiting. Conditions of good moisture and low-moderate temperatures gave reasonable amounts of effluxes while maintaining good plant productivity. Though the dry soil conditions significantly reduced effluxes in all land covers; they were not desirable since they decreased plant productivity and ultimately its C sequestration potential. Moreover, prolonged dry conditions only serve to exacerbate recovery of thicket plants as they increase mortality of canopy species in degraded and transformed areas in comparison with intact thicket

Typological characterisation of farms in a smallholder food-cash crop production system in Zimbabwe – opportunities for livelihood sustainability

The diversity of smallholder farms in space, resource endowment, production and consumption decisions are often a hindrance to the design, targeting, implementation and scaling out of agricultural development projects. Understanding farm heterogeneity is crucial in targeting interventions that can potentially contribute to improved crop productivity, food security and livelihood sustainability. The study sought to define and understand farm typology in a resettlement smallholder food-cash crop production area in Zimbabwe. Data was collected from five focus group discussions (FGDs), and 102 household interviews. Principal component analysis (PCA), multiple correspondence analysis (MCA) and cluster analysis were used to analyse quantitative and qualitative data variables and aggregate farms into clusters according to production means, socio-economics and demographics. The three identified farm types were (i) resource-endowed, commercial oriented farms, (ii) medium resourced and (iii) resource constrained farms practising subsistence and income oriented production. Labour was cited as a major challenge, with high labour cost relevant for type I farms, while household size has more bearing for type II and III farms. Ownership of tillage implements and operations varied from mechanised on resource endowed farms, to animal drawn on some medium and resource constrained farms. The farms exhibited variable livelihood strategies and all clusters exhibited market participation, albeit to varying extents. Thus strengthening of market links is imperative. Use of multivariate methods allowed for identification of the most discriminating variables for farm delineation and subsequent clustering of farms forms the basis for further exploring variability across farm types for the targeting of management interventions for livelihood sustainability

Accumulation of multiple heavy metals in plants grown on soil treated with sewage sludge for more than 50 years presents health risks and an opportunity for phyto-remediation

Long-term application of sewage leads to heavy metal accumulation in soils, causing serious risks to plants, humans, animals and the environment, and phytoremediation could be essential. A study was conducted to determine the concentration of heavy metals in self-seeding vegetables, amaranthus (Amaranthus dubius), tomato (Solanum lycopersicum), black nightshade (Solanum nigrum), Rumex pulcher and turf grass, grown on land treated with sewage sludge for over 50 years. A pot experiment was conducted to determine phytoremediation potential of Indian mustard (Brassica juncea), lucern (Medicago sativa), vetch (Vicia sativa), rape (Brassica napus) and ryegrass (Lolium perenne), using the same soil. Another pot experiment was conducted to determine effects on tissue metal composition of Indian mustard of adding increasing concentrations of EDTA. All the self-seeding vegetables had tissue Zn, Cu, Cr, Ni, Cd and Pb concentrations higher than toxicity thresholds. Turf grass tissue had higher concentrations of all the metals than all the self-seeding vegetables growing on the soil. Indian mustard and rape had the highest biomass and tissue concentration of most of the metals studied. Addition of EDTA to the soil drastically increased uptake of Zn, Cu, Cd and Pb but not Cr and Ni. The findings of this study imply that self-seeding vegetables and turf grass growing on the polluted soils pose serious health risks and that Indian mustard, and to some extent rape, have potential for phytoremediation, especially if grown on the soil treated with EDTA

Soil carbon dioxide effluxes from different vegetation environments in semi-arid Eastern Cape, South Africa

Though important, CO2 effluxes from non-agricultural ecosystems are poorly characterised. Given that thicket vegetation is prevalent in Eastern Cape, South Africa, we monitored soil CO2 effluxes from an intact thicket, degraded thicket (DT) and grassland (G) over 10 months, as affected by temperature, moisture and penetration resistance (PR). High soil moisture (16%) reduced PR (4 kg cm−2), which raised winter effluxes (1.2 μmol m−2 s−1), while low moisture (2%) resulted in hard dry soil (14 kg cm−2) that suppressed spring effluxes (0.2 μmol m−2 s−1). There was good interaction between PR and moisture (r = −0.53), with seasonal effluxes increasing with increasing moisture (r = 0.9, p = 0.0001) and decreasing PR (r = −0.66, p = 0.02). Temperature effects were significant under unlimited moisture supply. Thus high summer temperature (40 °C) gave lower effluxes in DT and G (&lt;1 μmol m−2 s−1) due to limited moisture (&lt;10%), whereas high autumn temperature (48 °C) and good moisture (16%) accelerated CO2 emissions (averaging 2 μmol m−2 s−1) from all covers. Although semi-arid ecosystems are limited by low moisture and sandy soils, they contribute to CO2 emissions under high moisture and increasing temperature.Keywords: Eastern Cape, grassland, soil carbon dioxide effluxes, soil organic carbon, thicke

Tillage and Urea Fertilizer Application Impacts on Soil C Fractions and Sequestration

Conservation tillage has been considered a smart agriculture practice which preserves soil organic carbon (SOC). However, little work on the labile C fractions in South Africa has been documented. As such, this work focused on C fractions under different management systems. The objective of this study was to assess the impact of different tillage techniques and fertilizer application rates on soil C fractions along the soil profile. Samples from no-till (NT), conventional tillage after 5th season (CT-Y5), and annual conventional tillage, longer than 5 years (CT-ANNUAL) at 0, 60, 120, and 240 kg N ha&minus;1 were taken at 0&ndash;10, 10&ndash;20, and 20&ndash;30 cm depths and analyzed for C fractions. The 30 cm depth was chosen as the sampling depth because of the 30 cm plough layer. At 0&ndash;10 cm, soil NT had higher total C, organic C, particulate organic C (POC), and permanganate oxidizable C (POxC) for all application rates, especially in the control treatment, compared to both the CT-Y5 and CT-ANNUAL treatments (p &lt; 0.05). At the 10&ndash;20 cm soil depth, CT-Y5 had higher POC than both NT and CT-ANNUAL at 60 kg N ha&minus;1 (p &lt; 0.05). Greater C fractions in the surface soil under NT, and at deeper depths under CT, was due to litter availability on the surface under NT and incorporation to the subsoil on CT. Higher C sequestration in NT than in CT-Y5 and CT-ANNUAL was observed because of slower organic matter (OM) turnover in NT leading to the formation and stabilization of C. A larger input over output of OM, through high crop residue accumulation over decomposition, is the reason for the increase of C fractions in the fertilized treatments. Therefore, using conservation agriculture, particular NT, with 0 kg N ha&minus;1 application rate in dryland agriculture is recommended

Selected Soil Physicochemical Properties under Different Tillage Practices and N Fertilizer Application in Maize Mono-Cropping

No-till (NT) has been said to conserve soil moisture, maintain or increase organic matter (OM), and improve crop production compared to conventional tillage (CT). However, very few studies have explored the effect of these under dry-land agriculture with occasional tillage where ploughing is performed only after several years of NT, especially in KwaZulu&ndash;Natal. The aim of this study was to assess the effect of tillage and fertilizer application on selected physicochemical soil properties under rain-fed maize production. Soil samples from NT, conventional tillage in the 5th season (CT-Y5), and annual conventional tillage (CT-A) with 0, 60, 120, 240 kg N ha&minus;1 were taken at 0&ndash;10, 10&ndash;20, and 20&ndash;30 cm and analysed for pH, EC, exchangeable acidity, exchangeable bases, C:N, gravimetric water content, bulk density, and soil texture. Results showed that NT at 0 and 60 kg N ha&minus;1 in 0&ndash;10 cm had higher bases, gravimetric water content, pH, and EC compared CT-Y5 and CT-A (p &lt; 0.05). At 10&ndash;20 cm depth, CT-Y5 had higher gravimetric water content (0.17 gg&minus;1), followed by CT-A, (0.13 g g&minus;1), while NT had the least (0.11 g g&minus;1) (p &lt; 0.05) in the control treatment. Again at 20&ndash;30 cm depth, NT had higher (p &lt; 0.05) bases than CT-Y5 and CT-A tillage practices at 120 and 240 kg N ha&minus;1 application rate. Regression analysis of fertilizer application rate with both bases and gravimetric water content showed a strong relationship under NT. Better soil properties under both NT and CT-Y5 was attributed to residue cover and minimum disturbance of the soil, which encouraged infiltration, thus reducing runoff and evaporation from the soil surface. Accumulation of residue under conservation tillage enhances OM, which subsequently improves soil quality, whereas ploughing aerates the soil causing oxidation of OM, thus releasing H+ ions. Again, fertilizer application induces mineralization of OM, thus, higher fertilizer application rates result in low levels of carbon. NT is well-recommended in conserving soil quality while sustaining crop productivity

Diversity analysis of provitamin A maize inbred lines using single nucleotide polymorphism markers

The success of a breeding programme can be determined by the level of genetic diversity that exists among breeding materials. This study was carried out to (i) determine the level of genetic diversity that exist among the collection of tropical provitamin A maize inbred lines and (ii) evaluate the genetic structure and divergence of provitamin A maize inbred lines. Forty-six advanced provitamin A inbred lines obtained from the International Maize and Wheat Improvement Centre (CIMMYT) and International Institute of Tropical Agriculture (IITA) were genotyped using 3047 single nucleotide polymorphism (SNP) markers. Genotyping was done following an Illumina Infinium HD Assay Ultra protocol. Eight six percent of the SNPs were polymorphic with the mean polymorphic information content of 0.36. Cluster analysis displayed two distinct clusters. The average pairwise genetic distance among the inbred lines was 0.60. The average gene diversity was 0.359. Variation was partitioned into among individuals (78%), among populations (12%) and within individuals (10%). Overall results suggest the inbred lines are genetically diverse. The key implication of this study is that selection should be done from genetically diverse inbred lines in order to exploit heterosis when developing hybrids

Factors affecting crop water use efficiency: A worldwide meta-analysis

International audienceWater is a limiting natural resource for agricultural production. While it is well accepted that crop types differ in their water use efficiency (WUE), there is no consensus on the main factors affecting WUE of main field crops, which was the motivation of the current study. The effects of rainfall pattern, soil type and climatic regime on crop WUE were evaluated using data from 514 experiments around the world published in ISI journal papers. The results confirmed that crop type had a significant effect (p < 0.05) on WUE with cereals producing on average 2.37 kg of dry grain per cubic meter (m(-3)) of water followed by oilseeds (0.69 kg m(-3)), fibre crops (0.45 kg m(-3)) and legumes (0.42 kg m(-3)). Amongst cereals, maize (3.78 kg m(-3)) and sorghum (2.52 kg m(-3)) were more water-use efficient than wheat (1.02 kg m(-3)), barley (1.21 kg m(-3)) and millet (0.47 kg m(-3)). Overall, maize was the most water use efficient crop under well-watered conditions (9.90 kg m(-3)), but sorghum was the most efficient under dry conditions (5.99 kg m(-3)). WUE of crops increased from desert to tropical through sub-tropical climate. Moreover, WUE tended to correlate positively with soil organic carbon content (r = 0.86) and negatively with clay content (r=-0.78) and soil bulk density (r=-0.85). These results provide information that is important for making decisions on crop selection in a context of increased climate variability and for crop variety development with enhanced WUE. However, there is need for more research to gain more understanding of the mechanisms responsible for the observed trends and causes of the unexplained variability

A meta‐analysis of combining ability effects in wheat for agronomic traits and drought adaptation: Implications for optimizing biomass allocation

International audienc