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The use of carbon nanoparticles (CNPs) as a fertilizer synergist to enhance crop growth has attracted increasing interest. However, current understanding about plant growth and soil response to CNPs is limited. In the present study, we investigated the effects of CNPs at different application rates on soil properties, the plant growth and nutrient use efficiency (NUE) of corn (Zea mays L.) in two agricultural soils (Spodosol and Alfisol). The results showed that CNPs affected corn growth in a dose-dependent manner, augmenting and retarding growth at low and at high concentrations, respectively. The amendment at the optimal rate of 200 mg CNPs kg−1 significantly enhanced corn growth as indicated by improved plant height, biomass yield, nutrient uptake and nutrient use efficiency, which could be explained by the higher availability of phosphorus and nitrogen in the amended soils. The application of CNPs largely stimulated soil urease activity irrespectively of soil types. However, the responses of dehydrogenase and phosphatase to CNPs were dose dependent; their activity significantly increased with the increasing application rates of CNPs up to 200 mg kg−1 but declined at higher rates (>400 mg kg−1). These findings have important implications in the field application of CNPs for enhancing nutrient use efficiency and crop production in tropical/subtropical regions.

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Willows (Salix spp.) have shown high potential for the phytoextraction of heavy metals. This study compares variations in copper (Cu) and zinc (Zn) tolerance and accumulation potential among 12 willow clones grown in a nutrient solution treated with 50 μmol/L of Cu or Zn, respectively. The results showed differences in the tolerance and accumulation of Cu and Zn with respect to different species/clones. The biomass variation among clones in response to Cu or Zn exposure ranged from the stimulation of growth to inhibition, and all of the clones tested showed higher tolerance to Cu than to Zn. The clones exhibited less variation in Cu accumulation but larger variation in Zn accumulation. Based on translocation factors, it was found that most of the Cu was retained in the roots and that Zn was more mobile than Cu for all clones. It is concluded that most willow clones are good accumulators of Zn and Cu.

Composts are considered one of major sources that contribute heavy metals to the environment. The objective of this study was to investigate historical changes and spatial variation of metal concentrations in Chinese composts by analysing representative compost samples and published data from 2002 to 2013. Mean concentrations of cadmium (Cd), chromium (Cr), lead (Pb), arsenic (As), mercury (Hg), zinc (Zn), nickel (Ni), and copper (Cu) in the composts were 2.42, 35.52, 32.38, 16.33, 0.32, 258.1, 9.71, and 72.24 mg kg, respectively. The percentages of samples with metal concentration exceeding critical levels of Chinese standards for organic fertilizers (NY525-2012) were 28.7, 18.3, 9.6, 1.7, and 0.9%, respectively, for Cd, As, Pb, Cr, and Hg. Modelling predicts that it would take 18, 24, 29, and 48 years for Cu, Zn, Hg, and Cd to reach Category I Environmental Capacity of China (GB15618-1995) according to current application rate. During the 2002-2013 period, concentration of Cd in composts tended to decrease with time, whereas those of Zn, Cu, and As were opposite, which is likely due to less or no control for these elements. Concentrations of Cu, Zn, As, and Cd were generally higher in composts from the economically developed regions in China, where more commercial farms were built that use metal-spiked feeds. Manures from the commercial farms generally had higher metal concentration, as compared to the subsistence farms. Further research is needed to monitor metal concentrations from source materials to composts and assess long-term impact of compost application on soil quality.

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