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Emergent plants have been remarkably effective in reducing phosphorus (P) discharge from ecological ditches; however, the treatment and recycling of these residues is a great challenge. In this study, magnetic biochars (MBs, i.e., MB-A, MB-C, and MB-T) were fabricated from three emergent plant residues (Acorus calamus L., Canna indica L., and Thalia dealbata Fraser, respectively) and modified with Fe(II)/Fe(III). Scanning electron microscopy-energy dispersive spectroscopy and X-ray diffraction spectra confirmed the successful loading of Fe3O4 and FeO(OH) onto the surfaces of the MBs. Batch adsorption experiments showed that MBs exhibited a higher P adsorption capacity than that of the raw biochars. Within the range of 0.8–43.0 mg L−1 in solution, the adsorption capacities of P by MB-A, MB-C, and MB-T were 304.6–5658.8, 314.9–6845.6, and 292.8–5590.0 mg kg−1, with adsorption efficiencies of 95.2–32.9%, 98.4–39.8%, and 91.5–32.5%, respectively. The primary mechanisms that caused P to adsorb onto the MBs were inner-sphere complexation and electrostatic attraction. Low pH conditions were more beneficial for the P adsorption of the MBs, while co-existing anions had a negative impact with the following order: HCO3− > SO42− > Cl−≈NO3−. The P-31 nuclear magnetic resonance results further demonstrated that the main adsorbed P species on the MBs was orthophosphate, followed by orthophosphate monoesters and DNA. Overall, MBs offer a resource utilization strategy for emergent plant residues and P-laden MBs are promising alternative P fertilizers. Highlights: Emergent plant biochar modified with Fe(II)/Fe(III) enhanced P adsorption capacity. Canna indica residue-derived MB exhibited the best P adsorption efficiency. MBs promoted P adsorption mainly via inner-sphere complexation and electrostatic attraction. P species adsorbed by MBs were mainly orthophosphate followed by orthophosphate monoesters and DNA. [ABSTRACT FROM AUTHOR]

Drying and rewetting (DRW) events cause the release of colloidal phosphorus (Pcoll, 1–1000 nm) in leachate, and biochar is considered an effective inhibitor; however, the microbial mechanism remains elusive. In this study, three successive DRW cycles were performed on the soil columns to assess the effect of biochar addition on Pcoll content and its possible associates, including phosphatase-producing microbial populations (phoD- and phoC-harboring microbial communities) and alkaline/acid phosphatase (ALP/ACP) activities. Results showed that the biochar addition significantly decreased the Pcoll by 15.5–32.1% during three DRW cycles. The structural equation model (SEM) confirmed that biochar addition increased phoD- and phoC-harboring microbial communities and ALP/ACP activities, which reduces the release of Pcoll into leachate. In addition, the manure biochar was more effective than the straw biochar in promoting competition and cooperation in the co-occurrence network (2–5% nodes increased on average), and the key taxa Proteobacteria and Cyanobacteria were identified as the dominant species of potential ALP/ACP activities and Pcoll content. Our findings provide a novel understanding of biochar reducing Pcoll loss from the phosphatase perspective by regulating the phoD- and phoC-harboring communities during DRW events. Highlights: Biochar reduced Pcoll by 15.5–32.1% in leachate during DRW cycles. Pcoll decreased with increasing phoD- and phoC-harboring microbial communities. Proteobacteria and Cyanobacteria were the key taxa driving Pcoll content. [ABSTRACT FROM AUTHOR]

Application of biochar to soil may stabilize soil organic carbon (SOC), concomitantly increasing nutrient retention. However, the interactive effect of biochar and nutrients on SOC and the underlying microbial mechanisms remain poorly understood, particularly in intensively managed forests where decarbonization is substantial after converting from natural forests. This 80-day incubation experiment aimed to quantify native SOC mineralization as affected by biochar (B) and nutrients [nitrogen (N) or phosphorus (P)], linking to the chemical composition of SOC, soil microbial community composition, and enzyme activities within a subtropical Moso bamboo (Phyllostachys edulis) forest soil. Results presented that compared to the control (nil-nutrient), nutrients (N, P, and NP) significantly destabilized native SOC [positive priming effect (PE); 20–98% increase in SOC mineralization], whereas such destabilization effect was significantly reduced by biochar (6.0–19%). The positive PE by nutrient was due to the increases in O-alkyl C, microbial biomass C, available mineral N, soil pH, β-glucosidase, and invertase activities. Meanwhile, the greater PE by N than P could be attributed to (i) decreases in diversity of bacterial and fungal communities; and (ii) increases in the relative abundances of microbial taxa such as Bacilli, Planctomycetes, and Alphaproteobacteria. Importantly, biochar's stabilization effect was because biochar not only lowered NH4+-N and NO3−-N and β-glucosidase activity, but also increased the activity of C-fixing enzyme (RubisCO) and polyphenol oxidase activity. Furthermore, biochar significantly decreased soil O-alkyl C that possibly resulted in less labile SOC mineralization, but increased aromatic C resulting in lower fungal diversity. We conclude that the biochar significantly reduces the destabilization effects of nutrients on SOC, highlighting that the biochar application is an effective approach to mitigate soil CO2 emissions within subtropical forest. [ABSTRACT FROM AUTHOR]

Purpose : Biochar application to soil has gained great interest as a land-based climate change mitigation solution. However, it lacks long-term field assessment on the effectiveness of biochar compared with other widely applied land management—such as stubble retention—on soil organic carbon (SOC) accumulation and carbon (C) distribution and persistence in the soil matrix. Materials and method: Here, we conducted a 9-year field trial in a temperate agroecosystem of North China to identify and quantify the location of C residing in the soil matrix (determined by two physical fractionation methods), as affected by land management—stubble removed (control), stubble returned at 15 t ha−1 year−1 (SR), two biochar doses at 4.5 t ha−1 year−1 (B4.5; equivalent to feedstock in SR) and 9.0 t ha−1 year−1 (B9.0). Results and discussion: The results showed that biochar application significantly increased SOC in the free and occluded particulate organic matter (POM) and mineral-associated organic matter (MAOM) fractions. Compared to B4.5 and B9.0, SR was less effective in soil C accrual in the occluded POM and MAOM, although the largest increase of C occurred in the free POM (about 10–17 g kg–1 soil). Consistent with this, biochar rather than stubble retention significantly increased C in (i) coarse POM (i.e., unprotected POM, > 250 μm) by 190–210%, (ii) microaggregates (μAgg) by 56–70%, and (iii) MAOM in silt–clay fraction (iMAOM) by 4–12%, but the biochar dose effect was statistically insignificant. Importantly, biochar significantly increased chemically recalcitrant C in soils that were further protected in the μAgg and iMAOM fractions. Conclusions: We conclude that biochar application was more beneficial for SOC accumulation and preservation than stubble management, particularly in the microaggregates and organo-mineral complexes, under the intensive cropping systems. [ABSTRACT FROM AUTHOR]

Summary: Application of biochar to rice has shown to elicit positive environmental and agricultural impacts due to its physicochemical properties. However, the relationship between greenhouse gas (GHG) emissions, rice yield, and soil nutrient status under biochar amendment remains unclear. In this study, rice yield and methane (CH4) and nitrous oxide (N2O) emissions were quantified in response to biochar application rate (0, 10, 20, and 40 t ha−1) to early and late subtropical rice cropping systems. We found that application of 10 t of biochar ha−1 to early rice reduced average CH4 emission fluxes, while all biochar application rates diminished average emissions in late rice paddy. Total global warming potential (GWP) and GHG intensity (GHGI) were inherently greater in late rice than early rice cropping. In early rice, GWP and GHGI were found to be similar between soil control, 10 and 20 t of biochar ha−1 treatments, although the largest occurred in the 40 t of biochar ha−1 treatment, whereas in late rice cropping, they were not affected by biochar application rates. Compared to the nil-biochar application, biochar application at varied rates did not affect rice yield. However, compared to 10 t biochar ha−1, increasing biochar application rate to 40 t ha−1 significantly decreased total rice yield (sum of early and late cropping). Generally, application of biochar increased soil salinity and total Fe and Fe2+ content while reducing soil bulk density. Temporal effects of biochar application were noted on CH4 emission flux, soil temperature, and soil Fe2+ and Fe3+ in early rice; and soil temperature, salinity, NH4+-N, NO3−-N, and soil Fe2+ and Fe3+ in late rice. This study confirms that the application of biochar at the lower rate of 10 t ha−1 is optimal for maintaining rice yield while reducing GHG emissions. Moreover, the study demonstrates the potential benefit of biochar in sustainable subtropical rice production. [ABSTRACT FROM AUTHOR]

Background and aims: Incorporation of biochar into the soil sequesters C for millennia, but the concomitant effects on plant rhizodepositions and nutrient (e.g., nitrogen; N) trade-offs via interactions of heterotrophic microbiota, might offset this sequestration. Methods: Ryegrass (Lolium perenne L.) with and without biochar amendment were pulse labelled in a 13CO2 atmosphere and 15N fertilizer added. Ryegrass and soils were destructively sampled at 16 and 30 days after seedling emergence. Isotope analysis was coupled with MiSeq sequencing of bacterial (16s rRNA) and fungal (ITS) genes to identify the effect of biochar on the associated microbiota involved in 13C allocation into soil aggregates and promotion of 15N uptake by L. perenne. Results: Biochar increased root biomass and 15N uptake but decreased rhizodeposited-13C recovery from large and small macroaggregates (by 12–57% and 57–72%, respectively). These changes in 13C flow and 15N uptake were accompanied by an increase in microbial biomass, and enhanced negative correlations between bacteria and fungi. O2PLS indicated members of seventeen genera that were correlated with soil stabilization of rhizodeposits in soil and plant N-uptake. For instance, Xanthomonadales (Proteobacteria) and RB41 (Acidobacteria), previously reported to be plant growth promoting rhizobacteria, were found to be positively correlated with 15N uptake by L. perenne. Conclusions: Our research explored the genera associated with biochar-modified 15N uptake by Lolium perenne and photosynthate 13C allocation into soil aggregates. Future research with SIP is required to fully assess microbial turnover, the ubiquity of similar rhizosphere microbiota and their fundamental importance for sequestration in the plant-soil-microbe-biochar systems. [ABSTRACT FROM AUTHOR]

There is overwhelming evidence for the long-term persistence of biochar in soil. However, the partitioning of biochar into light and heavy carbon (C) fractions and microbial biomass C (MBC), and the dynamics of C use efficiency (CUE E : net incorporation of biochar into MBC per unit of biochar-C consumed, including microbial death and recycling of biochar-derived microbial metabolites) in planted soil systems are poorly understood. A 13 C-labelled wood biochar (δ 13 C: −36.7‰) was incorporated into topsoil (0–10 cm) in an Arenosol, Cambisol and Ferralsol under C 3 dominated temperate pastures (δ 13 C: −25 to −27‰). The partitioning of biochar-C into the various soil C pools and CUE E were measured at 4, 8 and 12 months. The results showed that 8.6–28.2% of the biochar-C in the top soils was distributed to the heavy fraction (HF) within 4 months, which increased to 11.0–33.3% at 8 and 12 months. Biochar-C recovery in the HF was the highest in the Ferralsol ( cf. Arenosol and Cambisol), possibly due to greater interaction of biochar and biochar-derived microbial metabolites with soil minerals. Biochar significantly increased MBC across the three soils. Biochar-derived MBC ranged from 22 to 93 mg C kg −1 soil over time (Arenosol < Cambisol < Ferralsol), representing 11–20% of the total MBC pool. Biochar CUE E was 0.20–0.27 at 4 months, which decreased over time, possibly due to lowering of biochar-C availability to microbes. Further, although biochar-derived MBC was higher, biochar CUE E was lower in the Ferralsol ( cf. Arenosol and Cambisol), likely supported by higher microbial respiration and turnover, and lower recycling of microbial metabolites via greater organo-mineral interaction. Here, the study advanced our understanding of key C cycling processes, such as CUE E and the temporal fate of biochar-derived C in an organo-mineral fraction with relevance for biochar sequestration in contrasting soils under planted field conditions. [ABSTRACT FROM AUTHOR]

Understanding the temperature sensitivity (Q 10 ) of carbon (C) mineralization and priming of organic matter with different stabilities in a soil with aged biochar is required to enable better forecasting of biochar C sequestration potential under a warming climate. Here, we quantified the Q 10 and priming of C mineralization in a Vertisol from: (i) newly added labile organic matter (LOM) in the presence of “aged biochars”; and (ii) stable (“aged”) native soil organic matter in the presence of aged biochars or new LOM. We also quantified the Q 10 of aged biochar-C (BC) or aged soil organic carbon (SOC)+BC mineralization. Leaf litter from Eucalyptus saligna (a source of LOM) was applied at 4% w/w (δ 13 C −38‰) to a Vertisol (δ 13 C −14‰), containing either wood, leaf or poultry litter biochar (δ 13 C −25 to −28‰), and nil biochar (control soil), previously incubated for 4 years. These biochar−soil mixtures and the control soil, with or without LOM, were re-incubated at 10, 20, 30 and 40 °C for 252 days. The results showed that 22–39% of LOM-C, 0.10–2.81% of aged BC and 2.4–77.0% of “aged SOC” mineralized across all temperatures over 252 days. The Q 10 of C mineralization increased with decreasing quality of C substrates in the soil, that is, LOM (1.17–1.21) < SOC (1.23–1.66), SOC + BC (1.23–1.60) < aged BC (1.92–2.26). Positive priming of SOC mineralization was greater by LOM (cf. aged biochar), causing a significant decrease in the SOC Q 10 at all temperatures. The aged biochars resulted in negative priming of LOM-C mineralization, mainly at 10 °C, with no impact on the LOM Q 10 . The results suggest that global warming and tropical climates may lower the C sequestration potential of biochar, by reducing its capacity to slow the mineralization of LOM-C, while increasing the mineralization of native SOC. [ABSTRACT FROM AUTHOR]

Pyrogenic carbon (PyC) is an important component of the global soil carbon (C) pool, but its fate, persistence, and loss dynamics in contrasting soils and environments under planted field conditions are poorly understood. To fill this knowledge gap, a 13C-labelled biochar, as a surrogate material for PyC, produced from Eucalyptus saligna by slow pyrolysis (450°C; δ13C -36.7‰) was surface (0−10 cm) applied in C3 dominated temperate pasture systems across Arenosol, Cambisol and Ferralsol. The results show a low proportion of the applied biochar-C mineralised over 12 months in a relatively clay- and C-poor Arenosol (i.e., 2.0% loss via mineralisation), followed by a clay- and C-rich Cambisol (4.6%), and clay-, C- and earthworm-rich Ferralsol (7.0%). The biochar-C mean residence time (MRT), estimated by different models, varied between 44−1079 (Arenosol), 18−172 (Cambisol), and 11−29 (Ferralsol) years, with the shorter MRT estimated by a one-pool exponential and the longer MRT by an infinite-pool power or a two-pool exponential model. The two-pool model was best fitted to biochar-C mineralisation. The biochar-C recovery in the 12−30 cm soil layer varied from between 1.2% (Arenosol), 2.5−2.7% (Cambisol) and 13.8−15.7% (Ferralsol) of the applied biochar-C after 8−12 months. There was a further migration of biochar-C below the 50-cm depth in the Arenosol, as the combined biochar-C recovery in the mineralised pool and soil profile (up to 30 or 50 cm) was 82%, in contrast to 101% in the Cambisol and 104% in the Ferralsol after 12 months. These results indicate that the downward migration of biochar-C was greatest in the Arenosol (cf. Cambisol and Ferralsol). Cumulative CO2-C emission from native soil-plant sources was lower (p <0.10) in the biochar-amended vs. non-amended Ferralsol. This field-based study shows that the downward migration of biochar-C exceeded its loss via mineralisation in the Arenosol and Ferralsol, but not in the Cambisol. It is thus important to understand biochar-soil interactions to maximise long-term biochar C sequestration potential in planted soil systems. [ABSTRACT FROM AUTHOR]

The oxidation of surface functional groups on biochar increases its reactivity and may contribute to the cation exchange capacity of soil. In this study, two Eucalyptus wood biochars, produced at 450 °C (B450) and 550 °C (B550), were incubated separately in each of the four contrasting soils for up to 2 years at 20 °C, 40 °C and 60 °C. Carbon functional groups of the light fraction (< 1.8 g/cm 3 ) of the control and biochar amended soils (fresh and aged for 1 and 2 years at 20 °C, 40 °C and 60 °C) were investigated using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The spectra of biochar and light fractions of the control and biochar amended soils showed two distinct peaks at ∼285.1 eV and 288.5 eV, which were attributed to the C1s-π ∗ C C transitions of aromatic C and C1s-π ∗ C O transitions of carboxylic C, carboxyamide C and carbonyl C. The proportion of aromatic C was substantially greater in the light fraction of the biochar amended soils than the corresponding light fraction of the control soils. Also, the proportion of aromatic C was much higher in the light fraction of the B550 amended soils than in the corresponding B450 amended soils. Neither NEXAFS nor XPS results show any consistent change in the proportion of aromatic C of biochar amended soils after 1 year ageing. However, XPS analysis of hand-picked biochar samples showed an increase in the proportion of carboxyl groups after ageing for 2 years, with an average value of 8.9% in the 2 year aged samples compared with 3.0% in the original biochar and 6.4% in the control soil. Our data suggest that much longer ageing time will be needed for the development of a significant amount of carboxyl groups on biochar surfaces. [ABSTRACT FROM AUTHOR]

Highlights: [•] The first study to report temperature sensitivity of biochar in contrasting soils. [•] Soil type influenced the temperature sensitivity (Q10) of biochar mineralisation. [•] Q10 of biochar-C was similar or lower than native soil organic C (SOC). [•] The presence of biochar decreased Q10 of native SOC in some soils at 20–40°C range. [•] Q10 of biochar-C in soil decreased with increasing incubation temperature. [ABSTRACT FROM AUTHOR]

Biochar is often applied to paddy soils as a soil improver, as it retains nutrients and increases C sequestration; as such, it is a tool in the move towards C-neutral agriculture. Nitrogen (N) fertilizers have been excessively applied to rice paddies, particularly in small farms in China, because N is the major limiting factor for rice production. In paddy soils, dynamic changes in iron (Fe) continuously affect soil emissions of methane (CH 4) and carbon dioxide (CO 2); however, the links between Fe dynamics and greenhouse gas emissions, dissolved organic carbon (DOC), and rice yields following application of biochar remain unclear. The aims of this study were to examine the effects of two rates of nitrogen (N)-enriched biochar (4 and 8 t ha−1 y−1) on paddy soil C emissions and storage, rice yields, and Fe dynamics in subtropical early and late rice growing seasons. Field application of N-enriched biochar at 4 and 8 t ha−1 increased C emissions in early and late rice, whereas application at 4 t ha−1 significantly increased rice yields. The results of a culture experiment and a field experiment showed that the application of N-enriched biochar increased soil Fe 2+ concentration. There were positive correlations between Fe 2+ concentrations and soil CO 2 , CH 4 , and total C emissions, and with soil DOC concentrations. On the other way around, these correlations were negative for soil Fe 3+ concentrations. In the soil culture experiment, under the exclusion of plant growth, N-enriched biochar reduced cumulative soil emissions of CH 4 and CO 2. We conclude that moderate inputs of N-rich biochar (4 t ha−1) increase rice crop yield and biomass, and soil DOC concentrations, while moderating soil cumulative C emissions, in part, by the impacts of biochar on soil Fe dynamics. We suggest that water management strategies, such as dry-wet cycles, should be employed in rice cultivation to increase Fe2+ oxidation for the inhibition of soil CH 4 and CO 2 production. Overall, we showed that application of 4 t ha−1 of N-enriched biochar may represent a potential tool to improve sustainable food production and security, while minimizing negative environmental impacts. [Display omitted] • Application of N-enriched biochar favors the soil conversion of Fe3+ to Fe2+. • Can be a tool to improve fertilization efficiency and yield in a C neutral scenario. • At 8 t ha−1 promoted gaseous C emissions in paddy soil without yield improvement. • At 4 t ha−1 increased yield and decreased the C gas emissions in culture experiments. • Fe2+ content was positively correlated with DOC content, CO 2 and CH 4 emissions. [ABSTRACT FROM AUTHOR]

This study investigates the utility of plant δ¹3C natural labeling in predicting the impacts of environmental shifts on carbon cycling within ecosystems, particularly focusing on paddy fields treated with maize (Zea mays L.) residues and biochar. Specifically, it examines how soil δ¹3C and the sources of soil organic carbon (SOC), respond in paddy fields (which cultivate C3 plants like rice) when amended with maize residues, maize biochar, and silica‐enriched biochar (derived from C4 plants). Conducted in the Fuzhou paddy fields, the experiment included control groups and treatment groups with maize residue (4 t ha⁻¹), maize biochar (4 t ha⁻¹), and silicon‐modified maize biochar (4 t ha⁻¹) during both the early and late rice growth periods. The results indicate that all soil treatments increased soil δ¹3C. The application of maize residues notably affected the δ¹3C of the upper soil profile (0–15 cm) differently from the deeper layers (15–30 cm), and it increased soil organic C more than biochar or silicon‐modified maize biochar. Soil available P (AP) and pH emerged as significant factors linking δ¹3C, influencing rice yield through changes in soil physicochemical properties. Unlike maize residues, which reduced rice yields, applications of biochar and silicon‐modified maize biochar increased rice yields. The latter, which was particularly effective in lowering SOC decomposition rates and addressing rice's silica needs, emerged as the preferred option. The study highlights maize biochar and silicon‐modified maize biochar as sustainable alternatives to maize residues for rice cultivation, enhancing soil fertility, carbon pool stability, and yields. [ABSTRACT FROM AUTHOR]

Biochars generally result in short-term positive priming of native soil organic carbon (SOC), but longer-term carbon (C) stabilization, and these effects can be altered by global warming. However, uncertainty remains about the mechanisms associated with these priming effects, temperature sensitivity of native SOC, and microbial responses to biochars of differing properties. To address these knowledge gaps, rice straw biochars (produced at 300 and 800 °C at 2% w/w application rate), and their labile (water extracted) fraction and recalcitrant (chemically oxidized) fraction (obtained from the equivalent weight of biochar) were incubated in a C4 dominated soil at 15, 25, and 35 °C. Our results showed that 300 °C biochar and its recalcitrant fraction resulted in an increased SOC mineralization due to positive priming across the incubation thermosequence. This was likely linked to an observed increase in the abundance of K -strategists (fungi and Actinobacteria). The biochar produced at 800 °C and its recalcitrant fraction resulted in the stabilization of native SOC (i.e. , negative priming) at all temperatures, likely due to the adsorptive protection of native SOC by the large surface area. The water extractable C from both biochars generally induced SOC stabilization across the thermosequence, which could be attributed to microbial shifts to r -strategists preferentially utilizing labile C components in biochar. Both biochars increased SOC stabilization with warming from 15 to 25 °C, supporting the role of biochar application in soil C sequestration in cooler regions. The lower SOC stabilization by biochars with temperature increases from 25 to 35 °C was correlated with the biochar-induced increases in fungal growth (K -strategist) under warming. The low-temperature biochar increased the abundance of aromatic C decomposers and concomitantly lowered the Q 10 and activation energy (E a) of native SOC. The findings from this study highlight that the low- and high-temperature biochars can result in various changes in native SOC mineralization, as well as temperature sensitivity, mainly by microbial population alterations and physicochemical interactions. • 300 °C biochar and its recalcitrant fraction caused SOC loss at 15, 25 and 35 °C. • The SOC loss by 300 °C biochar was likely due to increased fungi and Actinobacteria. • 800 °C biochar and its recalcitrant fraction stabilized SOC at 15, 25 and 35 °C. • Water extracts of biochar stabilized SOC, likely due to C substrate switching. • Biochars increased SOC stabilization with warming from 15 to 25 °C, but not to 35 °C. [ABSTRACT FROM AUTHOR]

• Atmospheric nitrogen (N) deposition enhanced soil heterotrophic respiration (R H). • Biochar mitigated the stimulatory effect of N deposition on soil R H. • R H was positively related to β-glucosidase and cellobiohydrolase (CBH) activities. • Reduced R H by biochar was linked to decreased CBH activity and cbh I gene abundance. Both atmospheric nitrogen (N) deposition and biochar application can markedly impact soil heterotrophic respiration (R H), an important component of the global carbon cycle. However, the interactive effects of N deposition and biochar application on soil R H in subtropical forest ecosystems remain unclear. Here, we conducted a three-year (2019–2022) field trial within a bamboo forest in subtropical China to examine the responses of soil physicochemical and microbial properties to N deposition and biochar application, and to elucidate how biochar regulates N deposition-induced change in soil R H. Nitrogen deposition stimulated soil R H by 8.1–9.8 % annually over three years compared to the control, and this stimulation was mitigated (by 8.1–8.9 % annually) with biochar addition. In the context of N deposition, the decrease of soil R H by biochar application was not through changing soil temperature, moisture or labile organic carbon content. Biochar treatment reduced the abundances of bacterial glycoside hydrolase family 48 gene (GH48) and fungal glycoside hydrolase family 7 cellobiohydrolase I gene (cbh I) and the activities of β-glucosidase and cellobiohydrolase (CBH), but increased the abundance of cbbL gene and activity of RubisCO enzyme. Furthermore, the R H was correlated positively (P < 0.01) with β-glucosidase and CBH activities and negatively (P < 0.01) with RubisCO enzyme activity. Structural equation modeling revealed that the biochar-induced reduction of soil R H under N deposition was associated with decreases in the abundance of cbh I gene and the activity of CBH in soils. We highlight that management practices can mitigate soil carbon loss in forests through modulating soil microbial functions under atmospheric N deposition, and that biochar application in Moso bamboo forests has the potential to reduce R H by approximately 7.6 × 106 t CO 2 yr−1 under atmospheric N deposition. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Introducing interfering materials affecting biochar quantification in soils. • The methods for quantifying biochar content in soils are explained in detail. • Methods and prospects for biochar quantification are compared and analyzed. Biochar is derived from the oxygen-restricted pyrolysis of biomass and is characterized as being biologically stable and having high porosity. Adding biochar to soil is a widely recognized strategy to increase soil carbon (C) stocks and improve soil chemical, physical, and biological properties. Accurate quantification of stable fractions of biochar from soil is vital to assessing its long-term C storage potential for accounting purposes. However, methods specifically developed for quantifying biochar in soils are limited with most current methods adapted from techniques for measuring black carbon (BC). This results in numerous drawbacks for the quantification of biochar. Drawing on existing techniques for determining BC, the potential biochar quantification methods are identified and ordered into four categories: thermal oxidation methods, chemical oxidation methods, benzene poly-carboxylic acids (BPCAs) molecular marker methods, and optical methods. Within this framework, this review then considers factors that might skew measurements, such as loss of biochar and interferences. Measurement principles, technical characteristics, and advantages and disadvantages associated with each of the methodologies are also examined. Ultimately, our objective is to provide researchers with a comprehensive understanding, enabling them to select the most appropriate biochar quantification methods. [ABSTRACT FROM AUTHOR]

Colloidal phosphorus (P coll) in paddy soils can pose a serious threat to the water environment. Biochar amendment not only directly absorb P coll to reduce the runoff loss, but also create hotspots for microbial communities which simultaneously affects soil P coll. However, despite the crucial role of microorganisms, it remains elusive regarding how biochar and its feedstock types affect the relationships of soil microbial communities and P coll in soil matrix (such as at soil aggregate level). To address the knowledge gap, we explored the (in)direct effects of biochar on the soil P coll in physically separated fractions including micro- (53–250 μm) and macroaggregates (250–2000 μm). Results showed that straw and manure biochars decreased the soil P coll content by 55.2–56.7% in microaggregates and 41.2–48.4% in macroaggregates after 120 days of incubation, compared to the respective control. The fungal communities showed a significantly correlation (0.34, p < 0.05) with P coll content in the macroaggregates, whereas the bacterial communities were extremely significantly correlated (0.66, p < 0.001) with P coll content in the microaggregates. Furthermore, the partial least squares path model analysis indicated that biochar amendments directly increased P coll content (0.76 and 0.61) in micro- and macroaggregates, but the reduced P coll content by biochar was mainly derived from indirect effects, such as changed soil biological characteristics carbon (C)/P (−0.69), microbial biomass C (−0.63), microbial biomass P (−0.68), keystone taxa Proteobacteria (−0.63), and Ascomycota (−0.59), particularly for the macroaggregates. This study highlights that to some extent, biochar addition can reduce soil P coll content by affecting microbial communities (some keystone taxa), and soil biological characteristics at soil aggregate level. [Display omitted] • Biochar reduced P coll content by 41.2–56.7% in micro- and macroaggregates. • Soil P coll has a negative response to Proteobacteria and Ascomycota in aggregates. • Biochar reduced soil P coll content via altering soil biological characteristics. [ABSTRACT FROM AUTHOR]

Applying industrial waste amendments, such as steel slag and biochar, to soils is an increasingly common practice to improve soil fertility, crop yield, and soil carbon (C) pool storage and stability. However, the effects of separate and combined applications of slag and biochar on total and active soil organic C (SOC) pools and the associated relationships between soil microorganisms and C cycling are unclear. Therefore, this study examined the effects of steel slag, rice straw biochar, and slag+biochar amendments to subtropical early and late rice paddy soils on total and active SOC concentrations and microbial abundance and diversity. The results showed that slag+biochar increased SOC content in early and late rice, with increases in dissolved organic C of 28.7 and 52.6 % in the early and late rice jointing stages, respectively. Soil organic C was positively correlated with soil C:N ratios in the two crop seasons. Applications of slag and biochar alone increased the diversity of soil bacteria in early rice but decreased in late rice. Meanwhile, the combined application of slag and biochar decreased diversity of soil bacterial community diversity and increased the relative abundance of some beneficial bacterial taxa, such as Pseudomonas, Bacillus, Flavisolibacter , and Ferruginibacte which are related to SOC cycling and sequestration. The results highlight that the combined slag and biochar amendments can improve soil specific properties, such as pH and SOC stocks, which elicit alterations in soil bacterial composition, especially of some keystone genera, altogether resulting in enhanced soil C sequestration and C pool stability. [Display omitted] • Combined slag and biochar amendment increased soil pH and SOC concentrations. • Combined amendment altered soil bacterial community composition. • Combined slag and biochar amendment can maintain the stability of soil C pool. • Soil properties and bacterial community influenced soil C sequestration. [ABSTRACT FROM AUTHOR]

Nitrogen (N)-enriched biochar is a novel type of biochar prepared by mixing biomass with N-rich sources at a higher temperature via pyrolysis. Although the agronomic benefits of N-enriched biochar are reportedly positive, its effects on GHG emissions, crop yields, and soil bacterial communities in rice paddy fields remain undefined. This study examines the short-time effects of adding different rates of N-enriched biochar (0, 4, and 8 t ha−1) to subtropical early and late rice cropping soils on GHG emissions – namely carbon dioxide (CO 2) and methane (CH 4), rice yields, and soil bacteria communities. Across the two seasons, cumulative CO 2 emissions increased under both biochar treatments while cumulative CH 4 emissions declined with the application of biochar at 8 t ha−1. Early rice's global warming potential increased by 86 and 21% following application of biochar at 4 and 8 t ha−1, respectively, and by 43 and 41%, respectively, in late rice. Rice yields significantly (P < 0.05) increased with the application of 4 t N-enriched biochar ha−1 while rates at 8 t ha−1 had no effect. Application of N-enriched biochar altered the relative abundance of dominant bacterial taxa related to N and C cycling, particularly of denitrifying Rhodanobacter and Sphingomona in early and late rice soils, respectively. Furthermore, both rates of N-enriched biochar raised the relative abundance of Thiobacillus and Lysobacter in early and late rice-the latter associated with rice yields-while reducing Streptomyces and H16 taxa. Overall, findings suggest that the application of 4t ha−1 nitrogen-enriched biochar can increase rice yield and reduce CH 4 emissions while increased soil cumulative C emissions GWP caused by CO 2 emission increase, in part via biochar-induced changes on the plant growth characteristics and soil's bacterial community. This study reflects that the application of moderate doses of biochar can become an important sustainable soil management method for rice fields in subtropical regions, and the application of exogenous organic nutrients can promote crop yields and reduce GHC emissions. Now, a long-term experiment would be required to assess what is the optimal yearly frequency of application of these moderate doses of biochar. [Display omitted] • N-enriched biochar improves sustainability of double rice cropping systems. • N-enriched biochar raises soil cumulative CO 2 but reduced CH 4 emissions. • Use of N-enriched biochar in cropping soils increases global warming potential and rice yields. • N-enriched biochar alters dominant bacterial taxa involved in N and C cycling. • Greenhouse gas emissions were linked to changes in soil properties and bacterial diversity. [ABSTRACT FROM AUTHOR]

Biochar can be a soil amendment that increases nutrient retention and carbon (C) sequestration in rice paddy systems. However, biochar can lose nutrient during its production processes so modifications such as coating with the nitrogen (N) that can slowly release nutrients to soil following application are necessary. Dynamic changes in paddy soil microbial communities affect the biogeochemical cycling of soil nutrients; however, the effects of addition of N-enriched biochar on paddy soil microorganisms and nutrient stoichiometry are unclear. Here, we investigated the effects of N-enriched biochar on soil bacteria and fungi community structure and on carbon (C), N, phosphorous (P), and iron (Fe) stoichiometry in subtropical paddy soils. The soil concentrations of TC, TN and TP increased by 0.27–18.55%, 1.31–18.15% and 10.35–54.24% respectively under the nitrogen rich biochar treatment. Under N-enriched biochar application, ratios of N/P and C/P decreased, while P-fixation capacity increased; the decrease in N/P ratio indicated that rice productivity was N-limited. The quantity of soil fungi increased by 42.07% and 10.89% in early and late rice, respectively in the treatment group applying 4t ha−1 of N-enriched biochar. Relative abundance of the bacteria Bacillus , Geobacter, and Sideroxydans decreased with N-enriched biochar, whereas that of Thiobacillus and Thermomonas increased; relative abundance of the fungi Spicellomyces and Crustoderma increased with biochar. Relative abundance of the bacteria Bacteroides was negatively correlated with TC and Fe3+ (P < 0.05) and Sideroxydans and Geoyhrx was negatively correlated with soil TP (P < 0.05). Relative abundance of the fungi genera Westerdykella , Synchytrium , Russula , and Orbilia was positively correlated with soil TC (P < 0.05), Trapelia , Leptosphaerulina, and Tremella was positively correlated with soil TN (P < 0.05), and relative abundance of Leucanium , Monoblepharis was positively correlated with soil TP (P < 0.05). Overall, application of N-enriched biochar to paddy soils affected the microbial community composition through changes in soil physicochemical properties that led to shifts in microbial community function and associated improvements in soil nutrient enrichment. [Display omitted] • N-enriched biochar increased TC, TN, TP and available P content in paddy soil. • N-enriched biochar decreased N/P and C/P ratios in paddy soil. • N-enriched biochar increased abundance and diversity of soil microbial community. • Fungi genus Westerdykella , Synchytrium , Russula , Orbilia positively correlated with TC. • N-enriched biochar could be used to manage soil nutrients and microbial function. [ABSTRACT FROM AUTHOR]