[Effect of Coconut Fiber Biochar and Its Nitrate Modification on Pb Passivation in Paddy Soils].

The effects of coconut fiber biochar (CFB) and nitrate-modified coconut fiber biochar (NCFB) on the passivation of exogenous lead (Pb) in paddy soils and their underlying mechanisms were investigated using soil incubation experiments combined with spectroscopic techniques such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), synchrotron radiation X-ray fluorescence (SRXRF), and Fourier transform infrared absorption spectroscopy (FTIR). The effects of NCFB and CFB on the passivation of exogenous lead (Pb) in paddy soils and its underlying mechanisms were investigated. Compared with that of CFB, the inner wall of NCFB honeycomb pores was rougher, and the amount of alcohol-phenol-ether functional groups containing the C-O structure and the amount of carboxyl groups containing the C[FY=,1]O/O[FY=,1]C-O structure on the surface of CFB was significantly decreased after nitric acid modification. Compared with that in the control (without biochar) paddy soil after 150 d of incubation, the EDTA-extracted Pb content in the paddy soil with CFB and NCFB was reduced by 39.7% and 105.4%, respectively. The carbonate-bound and Fe-Mn oxide-bound Pb contents were significantly lower, and the organic-bound and residue Pb contents were significantly higher in the NCFB-added soil. The SRXRF scans showed that the exogenous Pb was enriched in the microregions of CFB particles rich in Ca and Cu elements and relatively less so in the microregions of soil aggregates rich in the Fe, Mn, and Ti elements. In addition, the characteristic peaks of carboxylates (1384 cm-1) in A-CFBPb and A-NCFBPb were significantly enhanced in the incubation experiment in the presence of exogenous Pb compared to A-CFB and A-NCFB in the absence of exogenous Pb. The addition of CFB or NCFB was more effective in passivating exogenous Pb in paddy soils and promoted the gradual transformation of Pb from unstable to more stable forms in paddy soils to achieve the effect of passivating Pb. The greater amount of carboxyl functional groups in NCFB participated in the passivation of exogenous Pb, which made NCFB more effective than CFB in passivating Pb. NCFB was more effective than CFB in passivating exogenous Pb in paddy soils due to its rougher inner walls of honeycomb pores and abundant carboxyl functional groups. In tropical areas such as Hainan, coconut fiber biochar and its modification can be considered as an environmentally friendly candidate method for the remediation of soil Pb contamination.

[Analysis of the Traits of Nitrogen Metabolism Pathways for Several Forest Soils in Eastern China].

Nitrogen metabolism pathways mediated by microorganisms play an important role in maintaining the structure and functional stability of soil ecosystems. Clarifying the relationships between microbial communities and nitrogen metabolism pathways can expand our understanding of nitrogen metabolism pathways at a microscopic level. However, the horizontal gene transfer of microorganisms means that taxonomy-based methods cannot be easily applied. A growing number of studies have shown that functional traits affect community construction and ecosystem functions. Using methods based on functional traits to study soil microbial communities can, therefore, better characterize nitrogen metabolism pathways. Here, five typical forest soils in China, namely black soil(Harbin, Heilongjiang), dark-brown earth(Changbaishan, Jilin), yellow-brown earth(Wuhan, Hubei), red earth(Fuzhou, Fujian), and humid-thermo ferralitic soil(Ledong, Hainan), were selected to study the traits of nitrogen metabolism pathways using metagenomic technology combined with the trait-based methods. The studied nitrogen metabolism pathways were ammonia assimilation, nitrate dissimilatory reduction, nitrate assimilatory reduction, denitrification, nitrification, nitrogen fixation, and anaerobic ammonia oxidation. The results showed that bacteria dominated the metagenomic library, accounting for 98.02% of all the sequences. Across all domains, the most common pathway was ammonia assimilation. For example, an average of 2830 ammonia assimilation pathway genes were detected for every million annotated bacterial sequences. In comparison, nitrogen fixation and anaerobic ammonia oxidation were the least detected pathways, accounting for 28.3 and 10.7 per million sequences, respectively. Different microorganisms can participate in a same nitrogen metabolism pathway, and the community structure of different soils was variable. The five typical forest soils in China show the same microbial nitrogen metabolism pathway traits; however, the community structure of the microorganisms mediating these processes was found to vary.