The effect of soil application of liquid dairy cattle (Bos taurus) manure on Cu, Zn, Fe, Mn, and B content and uptake by corn (Zea mays L.) and their availability in a calcareous soil was studied. The field received 9 yr of liquid cattle manure in two fertilization experiments, initially with winter wheat (Triticum aestivum L.) (1996–2000), followed by a similar experiment with corn (2002–2006). The treatments, practiced on the same plots every year since 1996, included: (i) soil incorporation of liquid dairy cattle manure before sowing at a rate equivalent to the recommended N‐P inorganic fertilization for each crop; (ii) application of the recommended inorganic N‐P fertilization for each crop, before sowing; (iii) identical to ii, but with split N application; and (iv) no fertilization (control). Surface soil samples were analyzed at the beginning of each corn‐growing season and in the spring of 2007. Corn samples, collected at the R3 growth stage, were also analyzed. The results showed that concentrations of all micronutrients in corn were not affected by fertilization. However, following manure application in 2005 and 2006, micronutrients' uptake by corn was significantly increased compared to the control, and was similar to levels for the inorganic fertilizer treatments. After 9 yr of manure application to soil, DTPA extractable Cu, Zn, and Mn and hot water extractable B were significantly increased. In addition, soil total and dissolved organic C were significantly increased and dissolved organic C was significantly correlated to DTPA extractable metals.

Coal fly ash (CFA) and municipal sewage sludge (MSS) management is a great concern worldwide. An alternative gaining high interest, is their use in agriculture and for reclamation of degraded lands. The purpose of this paper was to present very briefly the results of some case studies carried out in China and Greece related to land reclamation and agricultural use of CFA and MSS separately or combined. An experiment in Platanoulia area, central Greece showed clearly that CFA applied together with MSS at appropriate rates increased substantially wheat grain and biomass yield and improved soil quality (increased soil pH, organic matter content, total nitrogen, available P and boron). In a long-term experiment carried out in Huaibei city, Anhui province, China with a reconstructed soil in a subsided land by using CFA, it was found that physicochemical characteristics (infiltration rate, bulk density, total nitrogen, available P and extractable K) tended to be improved over time. In another experiment in Rodia area central Greece, MSS application improved soil quality of limestone mining spoils from bauxite mining activities. Several other experiments with MSS in Greece showed a clear positive effect on cotton and maize yield and on soil quality.

Soil contamination with potentially toxic elements (PTEs) is considered one of the most severe environmental threats, while among remediation strategies, research on the application of soil amendments has received important consideration. This review highlights the effects of biochar application on soil properties and the bioavailability of potentially toxic elements describing research areas of intense current and emerging activity. Using a visual scientometric analysis, our study shows that between 2019 and 2020, research sub-fields like earthworm activities and responses, greenhouse gass emissions, and low molecular weight organic acids have gained most of the attention when biochar was investigated for soil remediation purposes. Moreover, biomasses like rice straw, sewage sludge, and sawdust were found to be the most commonly used feedstocks for biochar production. The effect of biochar on soil chemistry and different mechanisms responsible for PTEs’ immobilization with biochar, are also briefly reported. Special attention is also given to specific PTEs most commonly found at contaminated soils, including Cu, Zn, Ni, Cr, Pb, Cd, and As, and therefore are more extensively revised in this paper. This review also addresses some of the issues in developing innovative methodologies for engineered biochars, introduced alongside some suggestions which intend to form a more focused soil remediation strategy.

Although Ni is officially recognized as an essential micronutrient for all higher plants, the majority of the published research on soil availability of Ni focuses on its hazardous role as a heavy metal. The objective of the study was to evaluate certain Ni soil tests in uncontaminated soils for an initial estimation of its sufficiency critical levels. Nickel was extracted from 30 cultivated soils employing the following extraction methods: DTPA, AB-DTPA, AAAc-EDTA, Mehlich-3, 0.1 M HCl, and 0.1 M HNO 3 . Ryegrass (Lolium perenne L.) was grown in pots containing the soils, harvested five times, certain plant parameters were determined, and the Cate-Nelson procedures were used for Ni critical levels determination. Among the six methods, HCl was the least reliable extractant for the evaluation of soil available Ni, whereas the most significant (p ≤ 5%) relationships between Ni concentration or Ni uptake by ryegrass and Ni soil tests were consistently obtained for AAAc-EDTA or Mehlich-3 extractable Ni. In many cases, > 80% of the variability of Ni concentration or uptake by ryegrass was explained by these two soil tests without the inclusion of other soil properties that affect Ni bioavailability. Sufficiency critical levels of Ni in soil were ≈ 2 mg kg -1 for both methods. Consequently, as an initial approach, concentrations of AAAc-EDTA or Mehlich-3 extractable Ni < 2 mg kg -1 are probably a good guide to indicate soils that will respond to Ni fertilization.

Although Ni has been officially recognized as an essential micronutrient for all higher plants since 2004, research on assessing its sufficiency critical levels with different soil tests is missing in the literature. The objective of the study was to determine Ni critical levels in unpolluted cultivated soils utilizing four methods, employing three commonly used calibration techniques. Ten soils with different physical–chemical properties and low Ni content were treated with Ni at rates of 1, 2, 4, and 8 mg kg−1. After equilibration for one month, the soils were analyzed for extractable Ni by four methods, namely DTPA, AB‐DTPA, AAAc‐EDTA, and Mehlich‐3. Response to soil‐applied Ni was assessed by a greenhouse pot experiment, with the untreated and Ni‐treated soils in three replications, using ryegrass (Lolium perenne L.). The aboveground biomass of ryegrass was harvested two months after sowing, dry weight of biomass was measured and relative biomass yield was calculated. Nickel's critical levels were determined employing the: (a) graphical technique of Brown and co‐workers, (b) Mitscherlich–Bray equation, and (c) Cate and Nelson graphical technique. According to the first technique, Ni critical levels were ≈ 2 mg kg−1 for the DTPA and AB‐DTPA methods, and 6.0 and 5.3 mg kg−1 for the AAAc‐EDTA and Mehlich‐3 methods, respectively. Similar levels were obtained by the Mitscherlich–Bray equation. However, the critical levels assessed by the Cate and Nelson technique were lower and ranged from 0.5 to 1.3 mg kg−1 for all four methods. Conclusively, Ni sufficiency critical levels for all four methods are expected to range at levels of a few mg Ni kg−1 of soil. As far as the three calibration techniques are concerned, a distinct boundary between Ni response and non‐response was accomplished by none. However, the fact that 60–74% of the soils were correctly separated into responsive and non‐responsive to added Ni by the graphical technique of Brown and co‐workers suggests that this is the most suitable technique.

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