A one-step extraction procedure and a leaching column experiment were performed to assess the effects of citric and tartaric acids on Cu and Zn mobilization in naturally contaminated mine soils to facilitate assisted phytoextraction. A speciation modeling of the soil solution and the metal fractionation of soils were performed to elucidate the chemical processes that affected metal desorption by organic acids. Different extracting solutions were prepared, all of which contained 0.01 M KNO(3) and different concentrations of organic acids: control without organic acids, 0.5 mM citric, 0.5 mM tartaric, 10 mM citric, 10 mM tartaric, and 5 mM citric +5 mM tartaric. The results of the extraction procedure showed that higher concentrations of organic acids increased metal desorption, and citric acid was more effective at facilitating metal desorption than tartaric acid. Metal desorption was mainly influenced by the decreasing pH and the dissolution of Fe and Mn oxides, not by the formation of soluble metal-organic complexes as was predicted by the speciation modeling. The results of the column study reported that low concentrations of organic acids did not significantly increase metal mobilization and that higher doses were also not able to mobilize Zn. However, 5-10 mM citric acid significantly promoted Cu mobilization (from 1 mg kg(-1) in the control to 42 mg kg(-1) with 10 mM citric acid) and reduced the exchangeable (from 21 to 3 mg kg(-1)) and the Fe and Mn oxides (from 443 to 277 mg kg(-1)) fractions. Citric acid could efficiently facilitate assisted phytoextraction techniques.

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Industrial fertilization with an excess of mineral fertilizer leads to water pollution and global warming. For instance, high mineral fertilization rates in coffee plantations of Ecuador result in N losses, such as emissions of N 2 O, a greenhouse gas to the atmosphere. Therefore, we conducted here field experiments to optimize fertilization. We studied the effects of mineral fertilizers on soil fertility, N 2 O emissions, productivity, and profitability of monoculture coffee systems in Loja, Ecuador. During 2 years, treatments on plots of Coffea arabica included (1) low fertilization with 70 N, 22 P, and 31 K kg/ha/year first year and then 200 N, 65 P, and 62 K second year; (2) medium fertilization with 150 N, 44 P, and 62 K first year and then 300 N, 87 P, and 125 K second year; (3) high fertilization with 225 N, 65 P, and 93 K and then 400 N, 109 P, and 187 K second year; and (4) control plots without fertilizers. Results showed that, although medium-high fertilization rates recommended by experts gave high coffee yields and income, such treatments produced high N 2 O emissions and thus led to low production/N 2 O emissions ratios of 208 for medium fertilization, and of 188 for high fertilization. Low fertilization gave a high production/N 2 O ratio of 603, and an income of 7606 USD/ha higher than control, of 3524 USD/ha. Our findings demonstrate that rates based on expert recommendations from local extension services and usually applied by Ecuadorian coffee growers, such as our medium rate, fail to achieve a sustainable production. The application of approximately half the expert recommended rates, such as our low treatment, could allow an environmental and economic sustainability in monoculture coffee plantations of Ecuador.

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Phytoremediation of contaminated mine soils requires the use of fast-growing, deep-rooted, high-biomass, and metal-tolerant plants with the application of soil amendments that promote metal uptake by plants. A pot experiment was performed to evaluate the combined use of vetiver grass (Chrysopogon zizanioides) and humic acid for phytoremediation of Cu and Zn in mine soils. Vetiver plants were grown in soil samples collected from two mine sites of Spain mixed with a commercial humic acid derived from leonardite at doses of 0, 2, 10, and 20 g kg(-1). Plant metal concentrations and biomass were measured and metal bioavailability in soils was determined by a low molecular weight organic acid extraction. Results showed that humic acid addition decreased organic acid-extractable metals in soil. Although this extraction method is used to estimate bioavailability of metals, it was not a good estimator under these conditions due to competition with the strong chelators in the added humic acid. High doses of humic acid also promoted root growth and increased Cu concentrations in plants due to formation of soluble metal-organic complexes, which enhanced removal of this metal from soil and its accumulation in roots. Although humic acid was not able to improve Zn uptake, it managed to reduce translocation of Zn and Cu to aerial parts of plants. Vetiver resulted unsuitable for phytoextraction, but our study showed that the combined use of this species with humic acid at 10-20 g kg(-1) could be an effective strategy for phytostabilization of mine soils.