Soil organic matter is one of the largest stores of carbon and nitrogen in various ecosystems that can crucially improve plant nutrition. The present study was conducted to investigate the effect of organic matter sources on carbon and nitrogen mineralization and also the contemporary comparison of carbon and nitrogen mineralization and nitrogen immobilization in soils amended with different organic treatments. Accordingly, six types of organic amendments were applied to three types of soils, under a constant temperature and moisture at the laboratory scale. The treated soils were incubated 60 and 120 days for nitrogen and carbon mineralization study, respectively. Results showed two decomposition stages, an initial rapid increase in decomposition in the first 20 days followed by a slower stage reaching a steady state condition. The amount of mineralized carbon ranged from 980 to 6860 mg/kg of dry soil for soils incorporating with different types of organic matters. The carbon mineralization kinetic data was well fitted to a second-order kinetic model showing the differences in microbial decomposition resistance of residues contents. The kinetic of carbon and nitrogen mineralization fluctuated similarly but in different scales. Net ammonification of fertilizer group, sheep manure, cow manure, and vermicompost was decreased, whereas in plant residue group, plane tree leaves, corn leaves, and wheat straw was increased. Two nitrification periods (first 10 days and 35 to 60 days) and an immobilization period (10 to 35 days) were observed. After 60 days of incubation of amended clayey soil, the nitrification values in plane tree leaves, corn leaves, wheat straw, municipal waste vermicompost, cow manure, and sheep manure were − 13.98, 14.06, 13.22, 34.88, − 12.25, and − 12.15 mg kg −1 , respectively. The carbon and nitrogen mineralization trends fluctuated due to probably different resistance of substance components to microbial decomposition. Since the nitrate is the main part of inorganic N, therefore, net N mineralization had a similar trend as nitrification in different soils. Those organic residues with high ratios of C/N immobilized the mineral nitrogen, due to the presence of more carbon and a higher nitrogen decomposition rate, resulting in suppressed levels of nitrogen in the amended soil.

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Background: In the present study, different concentrations of lead (factor A; 0, 15, and 30 mg L −1 ) and cadmium (factor B; 0 and 5 mg L −1 ) were applied via irrigation water during 6 months to evaluate their effects on growth of European hackberry (Celtis australis L.) plants. The experiment was arranged in factorial with completely randomized design and four replications under greenhouse conditions. Results: Application of 5 mg L −1 Cd or the lead levels (15 and 30 mg L −1 ) significantly reduced new shoot growth, plant leaf area, SPAD value, leaf water conductance and leaf photosynthesis, whereas significant increase in number of chlorotic and necrotic leaves, leaf transpiration rate, leaf proline and soluble sugars was observed. Higher reduction in new shoot growth and leaf water conductance and higher increase in leaf soluble sugars and proline was observed over the interaction of 5 mg L −1 Cd and 30 mg L −1 Pb. The highest soluble sugars and proline were in Pb30Cd5 (a3b2) treatment. Leaf Pb or Cd concentration was increased following their treatment. Application of cadmium significantly reduced leaf Pb, and similarly leaf Cd was significantly reduced by application of lead at both levels compared to untreated plants. Conclusion:The results showed that the hackberry growth was influenced by positive and negative interactions of Pb and Cd applied in irrigation water. The extent of growth reduction indicates that hackberry represents a relatively tolerant ornamental tree to high Pb and Cd levels.

The environmental toxicity of heavy metals in particular cadmium is a public concern. Cadmium is toxic for all living organisms including plants; however, plant species may show different tolerance to the presence of cadmium in their root medium. Adopting practical strategies may reduce cadmium bioavailability or increase the plant tolerance. In the present study, interaction of nitrate was investigated on cadmium treatment in hackberry (Celtis australis L.) seedlings. Different levels of nitrate (0, 50 and 100 mg/L) and cadmium (0 and 5 mg/L) were applied to seedlings via irrigation water during two consequence years. The treatments were arranged in a factorial with completely randomized design in four replications. The results of ANOVA showed that the cadmium-nitrate interaction was significant on leaf Cd concentration and root dry weight at P ¼ 0.01, and on carotenoids and leaf dry weight at P ¼ 0.05, while it was not significant on the rest of traits. Application of cadmium had no significant effect on new shoot growth, leaf chlorophyll and leaf fresh weight; however, it significantly reduced stomatal water conductance and photosynthesis rate, while it increased leaf transpiration rate, root and stem fresh weights, leaf Cd and proline concentrations. Application of nitrate levels, on the other hand, constantly increased the leaf nitrate concentration, new shoot growth, leaf fresh and dry weights, root fresh weight, stomatal water conductance and photosynthesis rate, whereas it reduced the necrotic points of leaves. The results indicated that the growth characteristics of hackberry seedlings were mainly influenced by nitrate but not cadmium application, and this ornamental tree is a tolerant species to high soil Cd levels.

Landscape plantations are significantly water demanding in many parts of the world, particularly in dry regions. Adequate water supply is the main limiting factor behind landscape development, maintenance and beauty in such environments. On the other hand, monitoring the growth response of ornamental and landscape plants to irrigation water containing heavy metals can be useful in management and application of municipal wastewaters to these plantations instead of application in edible vegetable crops production, which is common in many urban areas. In the present study, one year old seedlings of Judas tree (Cercis siliquastrum) were irrigated for two years with water containing 0, 15 or 30 mg L–1 of lead (Pb) in absence or presence of cadmium (0 or 5 mg L–1) in a factorial design, and under greenhouse conditions. Heavy metal treatments had no significant effect on leaf SPAD value; however, plants treated with Pb15Cd0 combination had significantly higher SPAD value than Pb30Cd5 combination that showed the lowest leaf SPAD value. Increasing the lead and cadmium levels of irrigation water increased the number of chlorotic and necrotic leaves of plant than control, whereas it reduced the average leaf area and new shoot growth. The control plants or plants treated with lower level of heavy metals showed the highest leaf area and new shoot growth. Leaf photosynthesis rate was significantly reduced in all heavy metal treated plants than control plants, as the lowest amount was in Pb30Cd5 combination treatment. All heavy metal treatments showed higher leaf soluble carbohydrates and proline content than in control plants. Leaf soluble carbohydrates were highest in Pb30Cd0 and Pb30Cd5 treatments, and leaf proline was highest in Pb30Cd5 and Pb15Cd5 treatments. The results indicate that Judas tree is a relatively tolerant species to high concentrations of Pb and Cd in root medium added through the irrigation water, and long term dual application of these heavy metals can have additive harmful effects on plant growth.