Estimating persistent oil contamination in tropical region using vegetation indices and random forest regression.

The persistence of soil contamination after cessation of oil activities remains a major environmental issue in tropical regions. The assessment of the contamination is particularly difficult on vegetated sites, but promising advances in reflectance spectroscopy have recently emerged for this purpose. This study aimed to exploit vegetation reflectance for estimating low concentrations of Total Petroleum Hydrocarbons (TPH) in soils. A greenhouse experiment was carried out for 42 days on Cenchrus alopecuroides (L.) under realistic tropical conditions. The species was grown on oil-contaminated mud pit soils from industrial sites, with various concentrations of TPH. After 42 days, a significant decrease in plant growth and leaf chlorophyll and carotenoid contents was observed for plants exposed to 5-19 g kg-1 TPH in comparison to the controls (p < 0.05). Conversely, pigment contents were higher for plants exposed to 1 g kg-1 TPH (hormesis phenomenon). These modifications proportionally affected the reflectance of C. alopecuroides at leaf and plant scales, especially in the visible region around 550 and 700 nm. 33 vegetation indices were used for linking the biochemical and spectral responses of the species to oil using elastic net regressions. The established models indicated that chlorophylls a and b and ß-carotene were the main pigments involved in the modifications of reflectance (R2 > 0.7). The same indices also succeeded in estimating the concentrations of TPH using random forest regression, at leaf and plant scales (RMSE = 1.46 and 1.63 g kg-1 and RPD = 5.09 and 4.44, respectively). Four out of the 33 indices contributed the most to the models (>75%). This study opens up encouraging perspectives for monitoring the cessation of oil activities in tropical regions. Further researches will focus on the application of our approach at larger scale, on airborne and satellite imagery.

Application of PROSPECT for estimating total petroleum hydrocarbons in contaminated soils from leaf optical properties.

Recent advances in hyperspectral spectroscopy suggest making use of leaf optical properties for monitoring soil contamination in oil production regions by detecting pigment alterations induced by Total Petroleum Hydrocarbons (TPH). However, this provides no quantitative information about the level of contamination. To achieve this, we propose an approach based on the inversion of the PROSPECT model. 1620 leaves from five species were collected on a site contaminated by 16 to 77 g.kg-1 of TPH over a 14-month period. Their spectral signature was measured and used in PROSPECT model inversions to retrieve leaf biochemistry. The model performed well for simulating the spectral signatures (RMSE < 2%) and for estimating leaf pigment contents (RMSE ≤ 2.95 µg.cm-2 for chlorophylls). Four out of the five species exhibited alterations in pigment contents when exposed to TPH. A strong correlation was established between leaf chlorophyll content and soil TPH concentrations (R2 ≥ 0.74) for three of them, allowing accurate predictions of TPH (RMSE =3.20 g.kg-1 and RPD = 5.17). The accuracy of predictions varied by season and improved after the growing period. This study demonstrates the capacity of PROSPECT to estimate oil contamination and opens up promising perspectives for larger-scale applications.

Detection and discrimination of various oil-contaminated soils using vegetation reflectance.

The use of hyperspectral spectroscopy for oil detection recently sparked a growing interest for risk assessment over vegetated areas. In a perspective of image applications, we conducted a greenhouse experiment on a brownfield-established species, Rubus fruticosus L. (bramble), to evaluate the potential of vegetation reflectance to detect and discriminate among various oil-contaminated soils. The species was grown for 32 days on four different soils with mixtures of petroleum hydrocarbons and heavy metals. Additional plants were grown on either uncontaminated control or water-deficient soils for comparison. Repeated reflectance measurements indicated modified spectral signatures under both oil and water-deficit exposure, from leaf to multi-plant scales. The amplitude of the response varied with mixture composition, exposure time, acquisition scale and spectrum region. Reflectance changes were linked to alterations in chlorophyll, carotenoid and water contents using vegetation indices. These indices were used to catch spectral similarities among acquisition scales and to discriminate among treatments using Kendall's coefficient of concordance (W) and regularized logistic regression. Of the 33 vegetation indices tested, 14 were concordant from leaf to multi-plant scales (W > 0.75, p < 0.05) and strongly related to leaf biochemistry (R2 > 0.7). The 14 indices allowed discriminating between each mixture and the control treatment with no or minor confusions (≤5%) at all acquisition scales, depending on exposure time. Some of the mixtures remained difficult to discriminate among them and from the water-deficit treatment. The approach was tested at the canopy scale under natural conditions and performed well for identifying bramble exposed to either one of the experimentally-tested mixtures (90% accuracy) or to uncontaminated soil (83% accuracy). This study provided better understanding of vegetation spectral response to oil mixtures and opens up promising perspectives for future applications.

Measurement of metal bioaccessibility in vegetables to improve human exposure assessments: field study of soil-plant-atmosphere transfers in urban areas, South China.

The quality of cultivated consumed vegetables in relation to environmental pollution is a crucial issue for urban and peri-urban areas, which host the majority of people at the global scale. In order to evaluate the fate of metals in urban soil-plant-atmosphere systems and their consequences on human exposure, a field study was conducted at two different sites near a waste incinerator (site A) and a highway (site B). Metal concentrations were measured in the soil, settled atmospheric particulate matter (PM) and vegetables. A risk assessment was performed using both total and bioaccessible metal concentrations in vegetables. Total metal concentrations in PM were (mg kg-1): (site A) 417 Cr, 354 Cu, 931 Zn, 6.3 Cd and 168 Pb; (site B) 145 Cr, 444 Cu, 3289 Zn, 2.9 Cd and 396 Pb. Several total soil Cd and Pb concentrations exceeded China's Environmental Quality Standards. At both sites, there was significant metal enrichment from the atmosphere to the leafy vegetables (correlation between Pb concentrations in PM and leaves: r = 0.52, p < 0.05) which depended on the plant species. Total Cr, Cd and Pb concentrations in vegetables were therefore above or just under the maximum limit levels for foodstuffs according to Chinese and European Commission regulations. High metal bioaccessibility in the vegetables (60-79 %, with maximum value for Cd) was also observed. The bioaccessible hazard index was only above 1 for site B, due to moderate Pb and Cd pollution from the highway. In contrast, site A was considered as relatively safe for urban agriculture.

Genetic variability and identification of quantitative trait loci affecting plant growth and chlorophyll fluorescence parameters in the model legume Medicago truncatula under control and salt stress conditions.

Salinity is one of the major stresses that limits crop production worldwide and affects most physiological activities in plants. In order to study the genetic control of salt stress in the model legume Medicago truncatula Gaertn., an experiment was undertaken to determine the genetic variability and to identify quantitative trait loci (QTLs) controlling several traits related to plant growth and physiology in a population of recombinant inbred lines. Shoot and root DW, relative water content, leaf area, chlorophyll content, chlorophyll fluorescence parameters, and Na+ and K+ in shoots and roots were measured. The experiment was carried out with three replications. ANOVA showed a large genetic variation and transgressive segregation for the traits studied, suggesting putative complex tolerance mechanisms. A total of 21 QTLs were detected under control conditions and 19 QTLs were identified under 100mm salt stress conditions, with three QTLs being common to both situations. The percentage of total phenotypic variance explained by the QTLs ranged from 4.6% to 23.01%. Overlapping QTLs for different traits were also observed, which enables us to discriminate independent traits from linked ones. The results should be helpful information for further functional analysis of salt tolerance in M. truncatula.

Relationship between the Al resistance of grasses and their adaptation to an infertile habitat.

BACKGROUND AND AIMS: Original data on Al resistance, relative growth rate and leaf traits of five European grasses as well as literature data on Al resistance, habitat preference and traits of grasses were considered to determine whether (a) Al resistance is correlated to a growth conservative strategy and (b) species occurrence could be useful to assess Al toxicity in meadows on acid soils. METHODS: The Al resistance of 15 species was represented by the Al activity in nutrient solution that resulted in a 50 % decrease in root length, [Al(3+)](50), or, for published values, in root or plant biomass. The correlations between Al resistance and acidity or nitrogen indices and the correlation between Al resistance and selected traits (relative growth rate, leaf dry matter content, specific leaf area and leaf thickness) were calculated. Principal component analysis was used for the characterization of the relationships between Al resistance and measured traits. KEY RESULTS: The [Al(3+)](50) values of the resistant species Molinia caerulea and Sieglingia decumbens were 13 and 26 microm [Al(3+)](50), respectively. The known Al resistance of 15 species that were mainly of the intermediate strategy competitor-stress tolerator-ruderal (C-S-R) type and of the S type was correlated with Ellenberg's nitrogen and acidity indices. For the whole set of species, the correlation between Al resistance and traits was not significant. CONCLUSION: The Al resistance of the C-S-R species was variable and independent of their traits. S-type species, adapted to acid soils and with traits of conservative strategy, displayed Al resistance. The large difference in Al resistance between grasses may help assess Al soil toxicity by using the abundance of grasses.