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Existing studies suggest that biochar application can reduce soil nitrous oxide (N2O) emissions, mainly based on short‐term results. However, it remains unclear what the effects (i.e., legacy effects) and underlying mechanisms are on N2O emissions after many years of a single application of biochar. Here, we collected intact soil columns from plots without and with biochar application in a subtropical tea plantation 7 years ago for an incubation experiment. We used the N2O isotopocule analysis combined with ammonia oxidizer‐specific inhibitors and molecular biology approaches to investigate how the legacy effect of biochar affected soil N2O emissions. Results showed that the soil in the presence of biochar had lower N2O emissions than the control albeit statistically insignificant. The legacy effect of biochar in decreasing N2O emissions may be attributed to the reduced effectiveness of the soil substrate, nitrification and denitrification activities, and the promotion of the further reduction of N2O. The legacy effect of biochar reduced the relative contribution of nitrifier denitrification/bacterial denitrification, nitrification‐related N2O production, and the relative abundance of several microorganisms involved in the nitrogen cycle. Our global meta‐analysis also showed that the reduction of N2O by biochar increased with increasing application rate but diminished and possibly even reversed with increasing experimental time. In conclusion, our findings suggest that the abatement capacity of biochar on soil N2O emissions may weaken over time after biochar application, but this remains under further investigation. [ABSTRACT FROM AUTHOR]

High nitrogen (N) fertilizer inputs accelerate soil acidification and degradation in tea plantations, thus posing a threat to soil microbial diversity, species composition, and ecosystem service functions. The effects of organic fertilizer and biochar applications on improving soil fertility have been extensively studied on cropland; however, little is known about their effectiveness in promoting soil multifunctionality on rapidly expanding acidic soils in tea plantations. In this study, we conducted a two‐year field experiment in a subtropical tea plantation to investigate the effects of organic fertilizer substitution and biochar amendment on soil microbial communities and multifunctionality. The results showed that soil multifunctionality was enhanced in plots amended with organic fertilizer and biochar. Soil multifunctionality was significantly and positively correlated with alpha‐diversity of bacteria but not fungi. We also found that organic fertilizer substitution and biochar amendment improved soil multifunctionality by altering the abundance of keystone species. The abundance of keystone species classified as module hubs in the bacterial co‐occurrence network contributed significantly and positively to soil multifunctionality. In contrast, the keystone species categorized as module hubs in the fungal co‐occurrence network negatively affected soil multifunctionality. Soil pH was a key driver of soil microbial community composition, indicating that the increase in soil pH under organic fertilizer and biochar amendment had a crucial role in biological processes. These results suggest that organic substitution and biochar amendment are beneficial in preventing soil degradation and maintaining soil multifunctionality in subtropical tea plantations. [ABSTRACT FROM AUTHOR]

Tropical and subtropical acidic soils have been well documented as hotspots of global soil nitrogen (N) oxide (i.e., nitrous oxide (N2O) and nitric oxide (NO) emissions). While the effectiveness of possible mitigation options has been extensively examined in croplands, little is known about their effectiveness in reducing N‐oxide emissions from acidic soils of rapidly expanding tea plantations in China. Here, we conducted a 2‐year field experiment to investigate how organic substitution for synthetic fertilizer and biochar amendment affect soil N‐oxide emissions from a subtropical tea plantation. Across the 2‐year measurement period, full organic substitution for synthetic fertilizer significantly increased N2O emissions by an average of 17% while had a lower NO emission compared to synthetic fertilizer alone. Our global meta‐analysis further revealed that full or partial organic fertilizer substitution resulted in a 29% (95% confidence interval: 5%–60%) increase of N2O emissions from acidic soils. In contrast, irrespective of fertilizer type, biochar amendment significantly reduced N2O emissions by 14% in the first but not second experimental year, suggesting a transient effect. The trade‐off effect of full organic substitution on N2O and NO emissions may be attributed to the favorable conditions for N2O production due to the stimulated activity of nitrifiers and denitrifiers. The suppression of N2O emission following biochar amendment was probably due to promoted further reduction of N2O to dinitrogen. The fertilizer‐induced emission factor (EF) of N2O (2.1%) in the tea plantation was greater than the current IPCC default value, but the EF of NO (0.8%) was comparable to the global estimate. Taken together, while biochar amendment could have mitigation potential, cautions are needed when applying organic substitution for synthetic fertilizer as mitigation options for acidic soils as hotspots of N‐oxide emissions. [ABSTRACT FROM AUTHOR]

Earthworms and biochar tend to have opposite effects on heavy metal bioavailability in soil. However, the influence and controlling process of earthworms on the immobilisation effect of biochar remain poorly understood. Through the co-cultivation of earthworms with rice-husk biochar and sludge biochar in heavy metal-contaminated soil and desorption experiments involving simulated earthworm gut, we explored the factors that earthworms influence the heavy metal immobilisation ability of biochar. Our results showed that rice-husk biochar and sludge biochar effectively immobilized heavy metals in soil, whereas earthworm activity mobilised heavy metals in biochar-treated soil, which weakens the immobilisation of biochar. The soil pH reduction effect of earthworms by increasing the abundance of soil ammonia-oxidising bacteria to promote soil nitrification is an important mechanism through which earthworms mobilise heavy metals; however, this process did not occur within 10 days of incubation. Nitrification inhibitors effectively inhibit the mobilisation of heavy metals in soil by earthworms. In addition, the bioavailability of heavy metals in earthworm casts was significantly higher than those in the surrounding soil and earthworm-free soil. Moreover, simulated earthworm gut fluid promoted the re-release of heavy metals from the soil and biochar particles. These results suggest that the gut digestion of earthworms is another important mechanism by which earthworms mobilise soil heavy metals and weaken the immobilisation of biochar. Therefore, earthworms weakened the immobilisation effect of biochar mainly by promoting nitrification to reduce soil pH and through gut digestion. [Display omitted] • Earthworms and biochar have opposite effects on heavy metal bioavailability in soil. • Earthworm activity weakens the immobilisation effect of biochar on heavy metals. • Reduction of soil pH by earthworms weakens immobilisation of biochar. • Earthworms promote soil nitrification to reduce soil pH by increasing AOB abundance. • Earthworm gut digestion is another way to weaken the immobilisation of biochar. [ABSTRACT FROM AUTHOR]

Biochar as a carbon-rich coproduct of pyrolyzing biomass, its amendment has been advocated as a potential strategy to soil carbon (C) sequestration. Updated data derived from 50 papers with 395 paired observations were reviewed using meta-analysis procedures to examine responses of soil carbon dioxide (CO2) fluxes, soil organic C (SOC), and soil microbial biomass C (MBC) contents to biochar amendment. When averaged across all studies, biochar amendment had no significant effect on soil CO2 fluxes, but it significantly enhanced SOC content by 40% and MBC content by 18%. A positive response of soil CO2 fluxes to biochar amendment was found in rice paddies, laboratory incubation studies, soils without vegetation, and unfertilized soils. Biochar amendment significantly increased soil MBC content in field studies, N-fertilized soils, and soils with vegetation. Enhancement of SOC content following biochar amendment was the greatest in rice paddies among different land-use types. Responses of soil CO2 fluxes and MBC to biochar amendment varied with soil texture and pH. The use of biochar in combination with synthetic N fertilizer and waste compost fertilizer led to the greatest increases in soil CO2 fluxes and MBC content, respectively. Both soil CO2 fluxes and MBC responses to biochar amendment decreased with biochar application rate, pyrolysis temperature, or C/N ratio of biochar, while each increased SOC content enhancement. Among different biochar feedstock sources, positive responses of soil CO2 fluxes and MBC were the highest for manure and crop residue feedstock sources, respectively. Soil CO2 flux responses to biochar amendment decreased with pH of biochar, while biochars with pH of 8.1-9.0 had the greatest enhancement of SOC and MBC contents. Therefore, soil properties, land-use type, agricultural practice, and biochar characteristics should be taken into account to assess the practical potential of biochar for mitigating climate change. [ABSTRACT FROM AUTHOR]

Currently, the research and application of biochar in the remediation of heavy metal contaminated soil has become a hotspot, especially regarding the remediation of agricultural land. Biochar has been proved to be effective in reducing the content of available heavy metals in the soil as well as the heavy metals in plants. However, the long-term effectiveness of biochar immobilization has not been widely studied. In this review, retrospective search was carried out on the published literature results concerning remediation effects of biochar on different areas of heavy metal contaminated soil in the recent years, its application in field remediation (several years), and some potential abiotic and biotic factors that may weaken the immobilization effects of biochar. This results indicate that: (1) biochar is widely used in the remediation of heavy metal contaminated soil in different areas and has excellent immobilization effect. (2) Most of the research demonstrate that the immobilization effect of biochar is effective for 2–3 years or according to few results even for 5 years. However, there have been various reports claiming that the immobilization effect of biochar decreases with time. (3) Abiotic factors such as acid rain, flooded environment, changes in soil condition (pH, redox and dissolved organic matter) and changes in biochar (Cl− and alkali leaching) can significantly weaken the immobilization effect of biochar. (4) Biotic factors such as plant roots, earthworms and soil microorganisms can also significantly reduce the immobilization effect of biochar. Therefore, field experiments having longer time span with biochar need to be further carried out, and the developmental research of modified biochar with a more stable immobilization effect also needs further attention. Image 1 • Biochar is widely used to the remediation of heavy metal contaminated soil in different areas. • The immobilization effect of biochar is effective in multiple years, but there are also oppositive results. • Analyzed some potential factors that may weaken the immobilization effect of biochar. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe-modified biochar addition enhanced the total gross NH 4 + immobilization while decreasing the autotrophic nitrification. • AOB dominated N 2 O emissions in paddy soils with or without biochar or Fe-modified biochar amendment under aerobic conditions. • Fe-modified biochar addition decreased total N 2 O emissions mainly by inhibiting AOB activity. We investigated the potential of ferric iron-modified biochar to lessen autotrophic nitrification and lower nitrous oxide (N 2 O) emissions in paddy soils. A 15N tracing incubation was conducted to investigate the changes in soil gross nitrogen (N) transformations under various biochar amendments (control, unmodified biochar, and Fe-modified biochar). Acetylene and 1-octyne were used to assess the relative contributions of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) to N 2 O emission from paddy soil. The Fe-modified biochar increased the rate of NH 4 + immobilization by 26 % and 383 % compared to the control and unmodified biochar treatments, respectively. The gross rate of autotrophic nitrification was reduced to 5.43 μg N g−1 d−1 in the Fe-modified biochar treatment, compared to 6.74 μg N g−1 d−1 in the control treatment and 9.38 μg N g−1 d−1 in the unmodified biochar treatment. Soil pH had varying effects on N 2 O emissions involving AOB and AOA. The N 2 O yields of AOA were more sensitive to Fe-modified biochar applications. AOB, specifically the Nitrosopira -AOB genus, dominated N 2 O production in all treatments. Overall, this study suggests that Fe-modified biochar holds greater potential than unmodified biochar in reducing N 2 O emissions from paddy soils by stimulating NH 4 + adsorption, restraining autotrophic nitrification rates, and AOB-dominant N 2 O production pathways. [ABSTRACT FROM AUTHOR]

Highlights • Biochar amendment reduced annual emission factor of N fertilizer for N 2 O + NO. • The ratio of NO-N/N 2 O-N was negatively correlated with soil water content. • Biochar enhanced the negative response of NO-N/N 2 O-N ratio to soil moisture. • Biochar rate of 20 t ha−1 gained the lowest N 2 O and NO emissions. Abstract Biochar impact on soil nitrous oxide (N 2 O) and nitric oxide (NO) emissions from intensive greenhouse vegetable cropping systems is not well established. Annual soil N 2 O and NO emissions were simultaneously measured from a typical greenhouse vegetable field treated with different biochar application rates in southeast China. Both N 2 O and NO emissions differed greatly across the individual vegetable cropping seasons, of which being higher in green soybean and tomato growing seasons, respectively. Over the annual vegetable cropping rotation, biochar amendment decreased soil N 2 O and NO emissions by 10–23% and 18–19% relative to N fertilization alone, respectively. Biochar rate of 20 t ha−1 gained the lowest N 2 O and NO emissions over the annual cycle. The ratio of NO/N 2 O was negatively correlated with soil moisture and depends on biochar addition or not. On average, annual emission factor (EF) of N 2 O and NO combined was accounted for 2.13% with biochar amendment, which was decreased by 22% compared with N fertilization alone (EF = 2.74%). Vegetable yield was significantly enhanced under the biochar combined with N fertilizer plots relative to control, while there is no difference among the N fertilization treatments with or without biochar amendment. Therefore, biochar could be used as an effective management mitigating soil N 2 O and NO emissions while maintaining crop yield in greenhouse vegetable cropping systems in southeast China. [ABSTRACT FROM AUTHOR]

The potential of biochar for mitigating climatic impacts of coastal saline bioenergy production is not well established. A full accounting of net greenhouse gas balance (NGHGB) and greenhouse gas intensity (GHGI) affected by biochar amendment combined with or without nitrogen (N) fertilizer application was examined in an annual coastal reclaimed Jerusalem artichoke bioenergy cropping system. The net ecosystem exchange of CO 2 (NEE) was determined by the difference between soil heterotrophic respiration ( R H ) and net primary production (NPP) using static chamber method. Biochar amendment raised the seasonal R H but without suppressing the NPP during the Jerusalem artichoke cropping season. Soil CH 4 emissions were 72% and 80% lower in the biochar amended than unamended plots when combined with N fertilizer application during the Jerusalem artichoke cropping and non-cropping seasons, respectively. The biochar-induced soil N 2 O mitigation efficiency was weakened by N fertilizer input over the annual cycle. Annual NGHGB and GHGI were negative for all the field treatments and were significantly lower in biochar amended than in unamended soils, suggesting that Jerusalem artichoke cropping system served as a net sink of GHGs due to net ecosystem CO 2 and biochar-induced C sequestration exceeding CO 2 -equivalents released as CH 4 and N 2 O emissions. On average, biochar amendment significantly enhanced GHGs sink capacity by resulting in almost 4–5 folds decrease in annual NGHGB or GHGI when combined with N fertilizer application or not. Therefore, higher biomass gain as potential alternative source of biofuels but lower climatic impacts of bioenergy production would be reconciled by biochar use in southeast coastal China. [ABSTRACT FROM AUTHOR]

Biochar amendment has been shown to reduce nitrous oxide (N 2 O) emissions from acidic soils in tea plantations. Given that both soil bacterial and fungal denitrifications can produce N 2 O, their relative contributions to the decrease in N 2 O emissions following biochar amendment remain unclear. Here, we examined soils sampled from a subtropical tea plantation that had received 2 years of biochar amendment. Measurements of the relative contributions of fungi and bacteria to N 2 O production were taken by the substrate-induced respiration method implemented with the addition of selective inhibitors. The abundances of total fungi, bacteria, and key N 2 O-related bacterial genes were quantified by q-PCR, and the composition of the fungal community was analyzed by 18S rRNA amplicon sequencing. The results showed that the contribution of fungi to N 2 O production (52%) was greater than that of bacteria (18%) for the N-applied acidic soils. Biochar amendment significantly decreased the fungal abundances and the fungal contribution to N 2 O production (by 28%). In contrast, biochar amendment significantly increased the abundances of N 2 O-related bacteria (e.g., ammonia-oxidizing bacteria (AOB), nirS , nosZ). Structural equation models (SEMs) revealed that biochar decreased the fungal contribution to N 2 O production through enhancing the soil pH and shifting the fungal community composition. Our results highlight that the decreased N 2 O emissions could be ascribed to the stimulated N 2 O consumption process (driven by N 2 O-consuming bacteria encoded by the nosZ gene) and suppressed fungal dominance in acidic soils from tea plantations. This study presents relatively comprehensive insights into the regulatory roles of biochar on soil microbe-mediated N 2 O production processes. • Fungi played a more important role than bacteria in N 2 O production from acidic soils. • Biochar shifted the fungal community on genus level from acidic soils. • Biochar decreased the fungal contribution to N 2 O production. [ABSTRACT FROM AUTHOR]

Tailoring agricultural practices to enhance the soil carbon (C) stock is seen as a promising mitigation tactic to offset greenhouse gas (GHG) emissions in croplands. Tea plantations are not only the important part of economic industry but also one of the crucial agricultural sources of non-carbon dioxide emissions. Although many studies have measured GHG emissions from tea plantations, it remains unknown about the effect of knowledge-based mitigation options on the entire C budget from Chinese rapidly expanding tea plantations. Thus, we carried out a 2-year field trial to provide an insight into the influence of organic fertilizer substitution for synthetic fertilizer and biochar amendment on net ecosystem carbon budget (NECB), net greenhouse gas budget (NGB), and yield-scaled greenhouse gas intensity (GHGI) from a subtropical tea plantation. Results showed that when averaged a 2-year experimental period, both full organic substitution and biochar amendment contributed significantly to the increment in NECB, mainly due to the enhanced soil organic C content in the tea field. Compared with the conventional farm practice, the application of full organic substitution can induce a 52% decrease in both NGB and GHGI. Regardless of fertilizer type, both NGB and GHGI were negative and 2.4 times lower in the treatments with biochar amendment relative to the control. In addition to their roles in maintaining soil health and alleviating soil acidification, our results suggest that organic fertilizer substitution and biochar addition may achieve low carbon development for tea plantations. Our findings will inform efforts to implement and evaluate these tailored mitigation options in tea plantations at a national scale. • The net greenhouse gas budget was assessed in a tea plantation over two years. • Organic substitution and biochar amendment decreased net greenhouse gas budget. • Organic substitution and biochar contributed to lowering yield-scaled emission. [ABSTRACT FROM AUTHOR]

• Biochar amendment decreased N 2 O and NO emissions from acidic soils. • Biochar amendment stimulated both nitrification and denitrification processes. • Soil N 2 O was produced mainly through denitrification in acidic soils. • Soil DOC and pH were key factors to N 2 O and NO emissions from acidic soils. • Soil N 2 O emissions were negatively related to nosZ gene abundances. Acidic soils are hotspots of nitrous oxide (N 2 O) and nitric oxide (NO) and biochar is documented to have the potential for mitigating N 2 O and NO. The N 2 O and NO emissions associated with soil functional genes and physicochemical properties under biochar amendment remains unclear in acidic soils. Here, we carried out a two-year field study to examine the responses of soil N 2 O and NO emissions to biochar amendment in a subtropical tea plantation in China. Measurements of N 2 O and NO fluxes were taken from inter-row soils using the static chamber method. We also measured the seasonal changes in soil key nitrogen (N)-cycling functional genes and physicochemical properties. Annual N 2 O and NO emissions averaged 27.31 kg N 2 O-N ha−1 yr−1 and 8.75 kg NO-N ha−1 yr−1 for the N fertilizer applied plots, which were decreased by 24% and 16% due to biochar application, respectively. In addition, both potential nitrification (PNR) and denitrification (PDR) rates were stimulated by biochar amendment, which significantly increased the abundances of bacterial amoA (AOB), nirK and nosZ genes. Changes in the composition of the N 2 O-related microbial functional community were closely associated with soil PNR, pH, DOC, and NO 3 −-N contents. The ratios of NO/N 2 O were mainly lower than 1, suggesting that N 2 O was produced mostly through denitrification rather than nitrification. There were negative correlations between soil N 2 O and NO emissions and soil PDR and pH, and soil N 2 O emissions were negatively correlated with nosZ gene abundances. Together, the decrease in N 2 O and NO emissions following biochar application could be largely attributed to the enhanced denitrification process, in which biochar enriched the nirK and nosZ genes abundance, resulting from the enhancement of soil DOC and pH in acidic soils. [ABSTRACT FROM AUTHOR]

Biochar amendment has been proposed as a potential strategy to reduce soil nitrous oxide (N 2 O) emissions, although experimental studies have generated inconsistent results on N 2 O emissions following biochar amendment. Differential responses of soil N 2 O to biochar amendment may depend on soil microbial functional genes abundance and abiotic properties. Here we sampled three types of soil from fields under long-term cultivation of green tea (TG), film greenhouse vegetable cabbage (GV) and Jerusalem artichoke (JA), respectively. We conducted a microcosm experiment to examine N 2 O emissions from the different soils following biochar amendment. Results showed that biochar amendment increased N 2 O emissions from the GV soil while decreasing N 2 O emissions from the TG and JA soils in the presence of nitrogen fertilizer. Biochar amendment increased soil pH and C/N ratio across the three soils. Quantitative PCR (qPCR) analysis showed that biochar amendment also consistently increased the abundances of AOB and nosZ genes but decreased the AOA abundances for all the soils, while the effects of biochar on the abundances of nirK and nirS genes differed between the soils. Our results suggest that biochar amendment can affect the processes of both ammonia oxidation and reduction of N 2 O to N 2 for all the soils and the net effect of biochar on N 2 O emissions depended on the predominant process in a specific soil. Biochar-induced increase in N 2 O emissions in the GV soil was largely attributed to the stimulated nitrification rate, which was primarily driven by AOB. Biochar-induced decreases in N 2 O emissions in the TG and JA soils were linked to the increased nosZ gene abundances. Overall, the effectiveness of biochar for mitigating N 2 O emissions is linked to its dominant N 2 O production pathway in soils. • Biochar increased the abundances of AOB and nosZ gene across all the soils. • Effects of biochar on N 2 O emissions differed with N addition and soil type. • Biochar increased soil N 2 O emissions produced by AOB-dominated pathways. • Biochar decreased soil N 2 O emissions produced by denitrification-mediated pathways. [ABSTRACT FROM AUTHOR]

The effect of earthworms on the immobilization of heavy metals by biochar was investigated using 4-week soil pot experiments. Our results showed that both sludge and rice husk biochars applied to Cd, Pb and Zn contaminated soils significantly reduced the bioavailability of those heavy metals in soils, and their concentrations in soil pore water. The Cd and Pb concentrations in pakchoi shoots were decreased by 10.8%–11.7% and 21.5%–26.5%, respectively, in biochar treatments alone. Biochar and earthworm treatments, alone or in combination, can significantly increase pakchoi growth. However, compared with biochar-treated alone, applying earthworms into pots with biochar treatments significantly reduced soil pH by 0.1–0.19 units, increased the Cd, Pb and Zn concentrations in soil pore water, their bioavailability and total uptake in shoots. Additionally, earthworms weaken the immobilization effect of biochar on heavy metals. The results of principal component analysis and correlation analysis showed that earthworm treatment was the main cause of soil pH reduction, and soil pH was significantly negatively correlated with the bioavailable Cd, Pb and Zn concentrations in the soil. Furthermore, the effect of the earthworm digestive system (casts) on bioavailable Cd, Pb and Zn concentrations could not explain their changes in the soil. In addition, the application of biochar had no significant effect on the survival and heavy metal enrichment of earthworm. Therefore, the effect of earthworms must be considered in the process of Cd, Pb and Zn contaminated farmland soil remediation using biochar. Unlabelled Image • Sludge biochar and rice husk immobilized Cd, Pb, and Zn in a contaminated soil. • Earthworm activities increased the bioavailable concentrations of Cd, Pb, and Zn in contaminated soil. • Earthworm activities significantly reduced soil pH by 0.1–0.19 units. • Earthworms may weaken the immobilized effect of biochar by lowering soil pH. [ABSTRACT FROM AUTHOR]