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Brazil is the world's largest sugarcane producer and soils in the northeastern part of the country have been cultivated with the crop for over 450 years. However, so far, there has been no study on the status of heavy metal accumulation in these long-history cultivated soils. To fill the gap, we collect soil samples from 60 sugarcane fields in order to determine the contents of Cd, Cr, Cu, Ni, Pb, and Zn. We used multivariate analysis to distinguish between natural and anthropogenic sources of these metals in soils. Analytical determinations were performed in ICP-OES after microwave acid solution digestion. Mean concentrations of Cd, Cr, Cu, Ni, Pb, and Zn were 1.9, 18.8, 6.4, 4.9, 11.2, and 16.2 mg kg(-1), respectively. The principal component one was associated with lithogenic origin and comprised the metals Cr, Cu, Ni, and Zn. Cluster analysis confirmed that 68 % of the evaluated sites have soil heavy metal concentrations close to the natural background. The Cd concentration (principal component two) was clearly associated with anthropogenic sources with P fertilization being the most likely source of Cd to soils. On the other hand, the third component (Pb concentration) indicates a mixed origin for this metal (natural and anthropogenic); hence, Pb concentrations are probably related not only to the soil parent material but also to industrial emissions and urbanization in the vicinity of the agricultural areas.

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Micronutrient research for sugarcane in northeastern Brazil is scarce and most works on this issue date back to the 70's and 80's. The objectives of this study were to assess the available and reserve pools of Fe, Mn, Cu, and Zn in soils cultivated with sugarcane under three geological contexts in northeastern Brazil as well as to diagnose the micronutrient nutritional status of sugarcane grown on these areas in order to identify pedogeological conditions in which micronutrient deficiencies are likely. Results showed that the soils cultivated with sugarcane in the states of Paraíba and Pernambuco posed available and reserve contents of micronutrients related to the parent materials and soil textural classes. The reserve contents of Fe, Mn, Zn, and Cu in all the soil samples analyzed were below the background values established for the region, which indicates a continuous exportation of micronutrients through cultivation. The mean and median contents of Mn, Zn, and Cu in diagnostic leaves of sugarcane were below their respective nutritional optimum ranges recommended to Brazil while Fe contents achieved the crop nutritional requirement. This is the first time such an approach based on pedogeological contexts is used to study the available and reserve pools of micronutrients in soils of Northeast Brazil.

Silicon (Si) plays an important role in sugarcane, but there is a gap of information regarding the potential of diatomite as a Si fertilizer to this crop. Here, we assessed the effects of five rates (equivalents to 75; 150; 225; 300 and 375 kg ha-1 Si) of a diatomite-based fertilizer on the biomass yield and concentration of nutrients and Si in sugarcane plants grown on a sandy soil. The efficiency of the extractants acetic acid and calcium chloride in predicting Si uptake was also assessed. Results showed that diatomite was efficient in supplying available Si in the soil and increased Si concentration in the plant. The addition of diatomite significantly increased the available contents of P, K, Mn, Cu, and the cationic exchangeable capacity of the soil. The concentrations of N, P, Mn, Cu, and Zn in shoots were elevated, suggesting that the fertilizer can play a role in the efficient use of nutrients by sugarcane. The extractant 0.01 mol L-1 CaCl 2 was better correlated with Si concentration in plants than 0.5 mol L-1 acetic acid. Therefore, CaCl 2 is the recommended extractant to estimate Si availability because of its higher efficiency and lower cost.

The input of trace elements (TEs) into agricultural soils from phosphate fertilizers may compromise the sustainability of agroecosystems and pose a risk to human health. We evaluated 53 sources of P marketed in Brazil in regard to As, Cd, Cr, Hg, and Pb concentrations in order to assess the risks associated with their application to soils. Based on the concentrations of TEs and the consumption of different sources, we estimated the amounts of TEs added annually to agricultural soils. As, Cr, Pb, and Hg concentrations did not exceed those found in fertilizers sold in other regions of the world. Phosphate rocks had Cd concentrations that were twice as high as any other source. The potential risk of Cd affecting soil quality is at least nine times higher than that of the other TEs assessed. Due to high consumption, monoammonium phosphates and single superphosphates are the main sources of TEs to Brazilian soils. In general, TEs in P-containing fertilizers sold in Brazil did not pose a high human health risk in a medium time frame. Nevertheless, intensive application of fertilizers with high concentrations of TEs requires monitoring of the TE accumulation in soils to ensure the sustainability of agroecosystems.

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