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Agglomeration Determines Effects of Carbonaceous Nanomaterials on Soybean Nodulation, Dinitrogen Fixation Potential, and Growth in Soil
Wang, Y., C. Hyun Chang, Z. Ji, D. Bouchard, R. Nisbet, J. Schimel, J. Gardea-Torresdey, AND P. Holden. Agglomeration Determines Effects of Carbonaceous Nanomaterials on Soybean Nodulation, Dinitrogen Fixation Potential, and Growth in Soil. ACS Nano. American Chemical Society, Washington, DC, 11(6):5753-5765, (2017).
The production and use of carbonaceous nanomaterials (CNMs), including carbon nanotubes (CNTs) and graphene, have experienced rapid increase over the past decade.1-2 Annual global production of CNTs (including single- and multi-walled, SWCNTs and MWCNTs, respectively) has attained several thousand metric tons;1, 3 graphene production has been predicted to exceed one thousand metric tons annually by 2019.2 Once released into the environment, CNMs will ultimately accumulate in soils.4-5 CNM-containing biosolids will be applied to agricultural lands6 while CNM-containing products will be used to remediate soils7 and both fertilize and protect crops8. Thus, terrestrial plants, including food crops, are at potential risk of exposure, which indicates the need to further evaluate the possible hazards of CNMs to plants.9-11.
The potential effects of carbonaceous nanomaterials (CNMs) on agricultural plants are of concern. However, little research has been performed using plants cultivated to maturity in soils contaminated with various CNMs at different concentrations. Here, we grew soybean for 39 days to seed production in soil amended with 0.1, 100, or 1000 mg kg–1 of either multiwalled carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs), or carbon black (CB) and studied plant growth, nodulation, and dinitrogen (N2) fixation potential. Plants in all CNM treatments flowered earlier (producing 60% to 372% more flowers when reproduction started) than the unamended controls. The low MWCNT-treated plants were shorter (by 15%) with slower leaf cover expansion (by 26%) and less final leaf area (by 24%) than the controls. Nodulation and N2 fixation potential appeared negatively impacted by CNMs, with stronger effects at lower CNM concentrations. All CNM treatments reduced the whole-plant N2 fixation potential, with the highest reductions (by over 91%) in the low and medium CB and the low MWCNT treatments. CB and GNPs appeared to accumulate inside nodules as observed by transmission electron microscopy. CNM dispersal in aqueous soil extracts was studied to explain the inverse dose–response relationships, showing that CNMs at higher concentrations were more agglomerated (over 90% CNMs settled as agglomerates >3 μm after 12 h) and therefore proportionally less bioavailable. Overall, our findings suggest that lower concentrations of CNMs in soils could be more impactful to leguminous N2 fixation, owing to greater CNM dispersal and therefore increased bioavailability at lower concentrations.