Shutan Ma

Hexavalent chromium (Cr(VI)) is a ubiquitous and highly toxic heavy metal contaminant widely discharged from industrial process. Fe-based reductant is candidate for Cr(VI) removal but facing challenges of high cost, low activity, and non-reusability. Herein, an encapsulation-pyrolysis strategy was adopted to synthesize core-shell structured phosphated-Fe@N-doped carbon (P-Fe@CN) reductant using active sludge (AS) and waste iron powder (W-Fe). This approach simultaneously achieves activation, phosphorization, and encapsulation of W-Fe in a single thermal process, eliminating separate phosphate and carbon precursor addition. The as-prepared P-Fe@CN shows excellent Cr(VI) reduction performance with an initial activity of 7.58 mg･g-1･min-1, Cr(VI) removal capacity of (628.91 mgCr(VI)/gFe), and Fe utilization of 66.82%, which outperformed W-Fe powder (0.22 mg･g-1･min-1, 3.93 mgCr(VI)/gFe and 3.41%), Fe@CN (4.08 mg･g-1･min-1, 218.43 mgCr(VI)/gFe and 23.31%), and commercial zero-valent iron (5.99 mg･g-1･min-1, 61.56 mgCr(VI)/gFe and 6.58%). P-Fe@CN demonstrates long-term stability over multiple cycles and regeneration without obvious Fe leakage. Mechanistic investigations indicate that Fe-P interface acts as capacitor for storing Fe electrons, shortening the electron transfer path, thereby enhancing the reaction activity. CN shell and Fe-P layer suppresses Fe dissolution, endowing it with excellent regeneration and reusability. This study establishes a sustainable ”treating waste with waste” platform that synchronously realizes solid waste valorization and Cr(VI) remediation, representing a significant advance toward green and practical water treatment technology.

The substrate-induced respiration-inhibition (SIRIN) method has been used to estimate fungi-derived N₂O emissions, but its contribution to soil N₂O emissions remains unclear. There is a need to quantify the fungal fraction of N₂O production more precisely. Here, using isotopocule analysis, we assessed the relative contribution of fungi to soil N₂O production potential under denitrifying conditions, where key limiting factors of denitrification (soil moisture, soil NO₃⁻, and electron donor) were removed. The result showed that the ratio of fungi-derived N₂O emissions (R_F) was 0.83~4.28% in paddy soils, 13.80~23.21% in vineyard soils, and 15.34~65.94% in vegetable field soils, respectively. This indicated that the bacteria were the dominator of soil N₂O production potential in most cases, but fungal pathways could be significant in vegetable field soils. The experiment with bactericide addition showed that inhibitors could affect non-target microorganisms in the SIRIN method. Our further analyses suggest that it is worth to explore the effect of soil organic carbon and microbial synergies on fungi-derived N₂O emissions.

Biochar is an efficacious amendment for mitigating nitrous oxide (N ₂ O) emissions in soils. Nevertheless, the underlying mechanisms responsible for reduced N ₂ O emissions by biochar in paddy soils remain inadequately elucidated. Here, using two typical paddy soils with contrasting pH values (5.40 and 7.56), the N ₂ and N ₂ O fluxes and the associated functional genes were investigated in soil amended with varying amounts of biochar (0%, 0.5%, and 5%, weight / weight ) via soil slurry incubation integrated with the N ₂ /Ar technique and q PCR analysis. The results showed that N ₂ O fluxes were significantly ( p < 0.05) reduced by 0.65–3.64 times following biochar amendment, concomitant with a significant ( p < 0.05) increase in N ₂ fluxes (5.47–46.14%) in both acidic and alkaline paddy soils. As a result, the N ₂ O/(N ₂ O + N ₂ ) ratios were significantly ( p < 0.05) reduced by 1.53–4.65 fold in both soil types. In acidic paddy soils, the enhanced denitrification rates and the decreased N ₂ O/(N ₂ O + N ₂ ) ratios exhibited a strong correlation with increased pH values. In alkaline paddy soil, these changes were ascribed to the enhanced nosZ Clade I gene abundance and nosZ /( nirS + nirK ) ratio. Our findings reveal that biochar primarily mitigates N ₂ O emissions in paddy soils by promoting its reduction to N ₂ .

Ammonia (NH₃) volatilized from cropland significantly impacts the ecological environment and human health. The identification and quantification of atmospheric sources of NH₃ from cropland are therefore important for NH₃ emission reduction and air pollution control. Choosing appropriate nitrogen (N) fertilizer types is one of the key ways to reduce NH₃ emissions from agricultural systems due to different N fertilizers with different emission factors. The natural abundance isotope of N (δ¹⁵N) values can well identify the source of NH₃ volatilization, although there is rare research on δ¹⁵N-NH₃ values volatilized when applying different types of N fertilizers. Here, we conducted an incubation experiment to study the characteristics of δ¹⁵N-NH₃ values during the whole volatilization process after different N fertilizers were applied to the soil. The results show that the cumulative NH₃ volatilization from urea (U), urease inhibitor fertilizer (UI), compound fertilizer (CF) and ammonium nitrate phosphate fertilizer (AP) treatment was 5.25 ± 0.00, 3.11 ± 0.00, 3.22 ± 0.19 and 1.38 ± 0.12 kg N ha⁻¹ at the end of the 15-day incubation period, respectively. The average δ¹⁵N value of NH₃ volatilized from the U, UI, CF and AP treatments was −36.02 ± 4.95, −29.08 ± 9.70, −35.18 ± 4.98 and −29.42 ± 4.33‰, respectively. Generally, the δ¹⁵N-NH₃ values ranged from −41.33 to −6.30‰ during the NH₃ volatilization process. The δ¹⁵N-NH₃ value was lower in the U treatment than in the UI and AP treatments (p < 0.05), which suggests that N forms and the slow-release additions of different fertilizers, such as NO₃⁻-N and urease inhibitors, can delay or slow down NH₃ volatilization, resulting in relative isotopic enrichment. Therefore, the basic properties of different N fertilizers, the changes in soil NH₄⁺-N and cumulative NH₃ during the volatilization process significantly impacted the δ¹⁵N-NH₃ values.

Nitrogen (N) fertilization is the major contributor to nitrous oxide (N ₂ O) emissions from agricultural soil, especially in post‐harvest seasons. This study was carried out to investigate whether ryegrass serving as cover crop affects soil N ₂ O emissions and denitrifier community size. A microcosm experiment was conducted with soil planted with perennial ryegrass ( Lolium perenne L.) and bare soil, each with four levels of N fertilizer (0, 5, 10 and 20 g N m ⁻² ; applied as calcium ammonium nitrate). The closed‐chamber approach was used to measure soil N ₂ O fluxes. Real‐time PCR was used to estimate the biomass of bacteria and fungi and the abundance of genes involved in denitrification in soil. The results showed that the presence of ryegrass decreased the nitrate content in soil. Cumulative N ₂ O emissions of soil with grass were lower than in bare soil at 5 and 10 g N m ⁻² . Fertilization levels did not affect the abundance of soil bacteria and fungi. Soil with grass showed greater abundances of bacteria and fungi, as well as microorganisms carrying narG , napA , nirK , nirS and nosZ clade I genes. It is concluded that ryegrass serving as a cover crop holds the potential to mitigate soil N ₂ O emissions in soils with moderate or high NO ₃ ⁻ concentrations. This highlights the importance of cover crops for the reduction of N ₂ O emissions from soil, particularly following N fertilization. Future research should explore the full potential of ryegrass to reduce soil N ₂ O emissions under field conditions as well as in different soils. Highlights: This study was to investigate whether ryegrass serving as cover crop affects soil N ₂ O emissions and denitrifier community size; Plant reduced soil N substrates on one side, but their root exudates stimulated denitrification on the other side; N ₂ O emissions were lower in soil with grass than bare soil at medium fertilizer levels, and growing grass stimulated the proliferation of almost all the denitrifying bacteria except nosZ clade II; Ryegrass serving as a cover crop holds the potential to mitigate soil N2O emissions.

An increasing area of oilseed rape cultivation in Europe is used to produce biodiesel. However, a large amount of straw residue is often left in the field in autumn. Straw mineralization provides both carbon (C) and nitrogen (N) sources for emission of soil nitrous oxide (N ₂ O), which is an important greenhouse gas with a high warming potential. Some studies have focused on soil N ₂ O emissions immediately post‐harvest; however, straw mineralization could possibly last over winter. Most field studies in winter have focused on freeze‐thaw cycles. It is still not clear how straw mineralization affects soil N ₂ O emissions in unfrozen wintertime conditions. We carried out a field experiment in northern Germany in winter 2014, adding straw and glucose as a source of C with three rates of N fertilizer (0, 30, and 60 kg N ha ⁻¹ ). During the 26 days of observation, cumulative N ₂ O emission in treatments without C addition was negative at all N fertilizer levels. Straw addition produced –3.2, 11.2, and 5.0 mg N ₂ O‐N m ⁻² at 0, 30, and 60 kg N ha ⁻¹ , respectively. Addition of glucose surprisingly caused –1.5, 74.6, and 165 mg N ₂ O–N m ⁻² at 0, 30, and 60 kg N ha ⁻¹ , respectively. This study demonstrates that oilseed rape straw does not cause high N ₂ O emissions in wintertime when no extreme precipitation or freeze‐thaw cycles are involved, and soil organic C content is low. However, N ₂ O emission could be intensively stimulated, when both easily available organic C and nitrate are not limited and the soil temperature between 0 and 10°C. These results provide useful information on potential changes to N ₂ O emissions that may occur due to the increased use of oilseed rape for biodiesel combined with less severe winters in the northern hemisphere driven by global warming.