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The concentrations of nine heavy metals (Fe, Mn, Zn, Cu, Ni, Cd, Cr, Co, and Pb) in soil samples of Arabian Gulf coast, Saudi Arabia, were investigated. Sediment Quality Guideline (SQGs), SQG-Quotient (SQG-Q), toxicity degree index (TDI), enrichment factor (EF), and multivariate analysis, including principle component analysis (PCA) and hierarchical cluster analysis (HCA), were used to measure heavy metals of concern in the study area and to identify their possible sources. The results showed that the concentrations of different heavy metals were 530–5,700 mg kg-1 for Fe; 9-150 mg kg−1 for Mn and 8-69 mg kg−1 for Zn; 1-21 mg kg−1 for Cu and not detectable-17 mg kg−1 for Ni; 6.9-130 mg kg−1 for Cr and not detectable-5 mg kg−1 for Co; and not detectable-24 mg kg−1 for Pb. Based on SQGs, only the maximum and mean Cr concentrations of study area were in heavy and moderate rates of pollution, respectively. The values of SQG-Q and TDI revealed that the investigated sampling points showed the lowest potential of adverse biological effects. The considerable number of collected soil samples has relatively higher EF values of 5–20 for Zn, Cu, Cr, and Pb, suggesting that these four metals may be derived from anthropogenic origin. Multivariate analysis also confirmed this finding that the sources of Zn, Cu, and Pb resulted primarily from anthropogenic sources, whereas Co, Ni, Fe, and Mn were mainly attributed to lithogenic sources. It could be generally concluded that it is possible to use multivariate analyses in combination with EF values as useful tools to identify the natural or anthropogenic sources of heavy metals in soils.

Many studies have reported the positive effect of biochar on soil carbon sequestration and soil fertility improvement in acidic soils. However, biochar may have different impacts on calcareous sandy soils. A 90-day incubation experiment was conducted to quantify the effects of woody waste biochar (10 g kg-1) on CO2–C emissions, K2SO4-extractable C and macro-(N, P and K) and micro-(Fe, Mn, Zn and Cu) nutrient availability in the presence or absence of poultry manure (5 g kg-1 soil). The following six treatments were applied: (1) conocarpus (Conocarpus erectus L.) waste (CW), (2) conocarpus biochar (BC), (3) poultry manure (PM), (4) PM + CW, (5) PM + BC and (6) untreated soil (CK). Poultry manure increased CO2–C emissions and K2SO4-extractable C, and the highest increases in CO2–C emission rate and cumulative CO2–C and K2SO4-extractable C were observed for the PM + CW treatment. On the contrary, treatments with BC halted the CO2–C emission rate, indicating that the contribution of BC to CO2–C emissions is negligible compared with the soils amended with CW and PM. Furthermore, the combined addition of PM + BC increased available N, P and K compared with the PM or BC treatments. Overall, the incorporation of biochar into calcareous soils might have benefits in carbon sequestration and soil fertility improvement.

Biochar is a carbon-rich product obtained by biomass pyrolysis and considered a mean of carbon sequestration. In this research, a sandy calcareous soil from the Farm of the College of Food & Agriculture Sciences, King Saud University, Saudi Arabia, was amended with either woody waste of Conocarpus erectus L. (CW) or the biochar (BC) produced from CW at rates of 0 (control), 10, 30 and 50 g kg−1. The effects of the amendments on soil pH, dissolved organic carbon (DOC), microbial biomass carbon (MBC), CO2 emission and metabolic quotient (qCO2) of the sandy calcareous soil were studied in a 60-d incubation experiment. The results showed that the addition of CW led to a significant decrease in soil pH compared to the control and the addition of BC. The CO2-C emission rate was higher in the first few days of incubation than when the incubation time progressed. The cumulative CO2-C emission from the soil amended with CW, especially at higher rates, was higher (approximately 3- to 6-fold) than that from the control and the soil amended with BC. The BC-amended soil showed significant increases in CO2-C emission rate during the first days of incubation as compared to the non-amended soil, but the increase in cumulative CO2-C emission was not significant after 60 d of incubation. On the other hand, CW applications resulted in considerably higher cumulative CO2-C emission, MBC and DOC than the control and BC applications. With the exception of 0 day (after 1 h of incubation), both CW and BC applications led to lower values of qCO2 as compared to the control. The power function kinetic model satisfactorily described the cumulative CO2-C emission. Generally, the lowest values of CO2 emission were observed in the soil with BC, suggesting that the contribution of BC to CO2 emission was very small as compared to that of CW.

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Two field experiments were executed during the seasons of 2012 and 2013 to test response of common bean to inoculation with rhizobia plus soil yeast strains Saccharomyce cerevisiae, Candida sake, Saccharomyces exiguous, Pichia membranifaciens and Cryptococcus lourentii. The results showed an the improvement of bean nodulation and plant growth parameters by the mixed inoculation treatments with Rhizobium plus the yeast strains. The highest improvement was recorded for yeast strain S. cerevisiae or S. exiguous. Inoculation with the yeast strain S. cerevisiae mixed with Rhizobium induced the following increases in nodule numbers, straw and seed yield: 51.94 %,10.92 % and 16.65 % in the firest season, and 50.94 %, 10.32 and 31.37 % in the second season, respectively over the inoculation with Rhizobium alone. The recorded enhancements are probably due to yeast hormonal production like indol acetic acid (IAA) on root growth and lateral roots leading to increased nodulation and nutrient uptake and subsequently increased yield. Also, by increasing mineral nutrients solubilization like phosphorous and iron.

Bacillus megaterium mutants selected after NTG treatments were evaluated for their efficiency in improving growth, p-uptake and Fe-uptake by tomato in greenhouse. Only five mutants out 1000 colonies were found to solubilize phosphates more than wild type strain on the basis of the solubilization index on Pikovskyaya's solid medium. They designed as M1, M2, M3, M4, M5 and their solubilization index were 2.56, 2.42, 2.28, 2.49 and 2.31, respectively. The highest phosphate solubilization index was recorded by mutant strain M1 compared with the wild type strain (2.13). The results show that all tested strains produced; in their cultures; organic substances that chelated iron in the calcareous soil. In greenhouse experiment the results showed that inoculation of tomato cultivar Super marmand with wild strain or mutant M1 with or without the addition of rock phosphate had a significant effect on shoot and root dry weights, p-uptake and Fe-uptake in shoot compared with uninoculated treatment (control), or amended with rock phosphate alone. Similar results were recorded with Burchard cultivar, but in general, the cultivar Super marmand was more responsive to wild type and mutant M1 inoculation than the cultivar Burchard. Application of both mutant and rock phosphate resulted in the highest availability of P and Fe in the soil, resulted in increases about 26 % in P and 36 % in Fe as compared to control

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