Iron
(Fe) phases are tightly linked to the preservation rather
than the loss of organic carbon (OC) in soil; however, during redox
fluctuations, OC may be lost due to Fe phase-mediated abiotic processes.
This study examined the role of Fe phases in driving hydroxyl radical
(•OH) formation and OC transformation during redox
cycles in paddy soils. Chemical probes, sequential extraction, and
Mössbauer analyses showed that the active Fe species, such
as exchangeable and surface-bound Fe and Fe in low-crystalline minerals
(e.g., green rust-like Fe phases), predominantly regulated •OH formation during redox cycles. The •OH oxidation
strongly induced the oxidative transformation of OC, which accounted
for 15.1–30.8% of CO2 production during oxygenation.
Microbial processes contributed 7.3–12.1% of CO2 production, as estimated by chemical quenching and γ-irradiation
experiments. After five redox cycles, 30.1–71.9% of the OC
associated with active Fe species was released, whereas 5.2–7.1%
was stabilized by high-crystalline Fe phases due to the irreversible
transformation of these active Fe species during redox cycles. Collectively,
our findings might unveil the under-appreciated role of active Fe
phases in driving more loss than conservation of OC in soil redox
fluctuation events.
Biochar is the carbon-rich product obtained from the thermochemical conversion of biomass under oxygen-limited conditions. Biochar has attained extensive attention due to its agronomical and environmental benefits in agro-ecosystems. This work adopts the scientometric analysis method to assess the development trends of biochar research based on the literature data retrieved from the Web of Science over the period of 1998-2018. By analysing the basic characteristics of 6934 publications, we found that the number of publications grew rapidly since 2010. Based on a keyword analysis, it is concluded that scholars have had a fundamental recognition of biochar and preliminarily found that biochar application had agronomic and environmental benefits during the period of 1998-2010. The clustering results of keywords in documents published during 2011-2015 showed that the main research hotspots were "biochar production", "biochar and global climate change", "soil quality and plant growth", "organic pollutants removal", and "heavy metals immobilization". While in 2016-2018, beside these five main research hotspots, "biochar and composting" topic had also received greater attention, indicating that biochar utilization in organic solid waste composting is the current research hotspot. Moreover, updated reactors (e.g., microwave reactor, fixed-bed reactor, screw-feeding reactor, bubbling fluidized bed reactor, etc.) or technologies (e.g., solar pyrolysis, Thermo-Catalytic Reforming process, liquefaction technology, etc.) applied for efficient energy production and modified biochar for environmental remediation have been extensively studied recently. The findings may help the new researchers to seize the research frontier in the biochar field.
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