Biochar produced from straw has been shown to improve soil physicochemical properties. This review introduces the fundamental concepts, the broad applications, and underlying theory of straw biochar returning. Current developments in biochar industry and the production practices prevalent among enterprises in China are critiques. This review analyzes current knowledge gaps, challenges, and opportunities in the industrial application of straw biochar returning. Biochar standards, the quantitative and qualitative analysis methods for biochar, and high-value-added products that are based on biochar are critically examined with goal of providing recommendations for future studies. We propose production and modification of biochar that is application oriented to enhance its fitness for purpose as well as long-term and large-space–scale field study to better understand its impact on soil properties and ecotoxicology. Finally, we make prospects for the future development of SBR, including constructing a standard system about straw biochar returning and promoting self-discipline of biochar industry and the establishment of a biochar-based agricultural production model.
In this study, we report on the extraction, characterization, and potential applications of colloidal biochar derived from pyrolyzed wood—an untapped source of carbonaceous particles. A series of characterizations was performed on biochar colloids to unravel their colloidal properties and surface chemistry through which it was found that they have a net negative charge and are stable between pH 3 and 10. Moreover, our initial toxicity tests showed that biochar colloids themselves are not toxic and they can be used in remediation applications, which led us to investigate (1) their copper sorption, a model inorganic contaminant, in a scenario that biochar colloids are released into the environment and (2) their potential use in organic pollutants adsorption and degradation. Copper sorption studies showed that biochar colloids have a copper sorption capacity as high as 22 mg $\hbox {g}^{-1}$ in sub-ppm copper solutions. This increased the acute 48 h lethal concentration ($\hbox {LC}_{50}$) of copper for Daphnia magna by 21 ppb, which is comparable to the previously reported effect by dissolved organic matter. Adsorption and degradation of methylene blue (MB), an often-used proxy for organic contaminants in water, were studied by coupling the biochar colloids to positively charged $\hbox {TiO}_{2}$ nanoparticles and using it as a photocatalyst. The hybrid MB photodegradation efficiency was $21\%$ higher than that of $\hbox {TiO}_{2}$ nanoparticles alone. Enhancement of demethylation is proposed as the main degradation mechanism of MB, as confirmed by liquid chromatography–mass spectroscopy (LC/MS), and the positive impact of biochar colloids is ascribed to their abundant adsorption sites, which may facilitate MB adsorption and its photocatalytic degradation.
Field studies were conducted over 2 years to determine the response of soil microbial biomass pool to biochar and N fertilizer combinations in a rain-fed rice cropping system. Biochar was applied at four doses: 0 t ha−1, 3 t ha−1, 6 t ha−1 and 12 t ha−1 in combination with N fertilizer at four rates: 0 kg ha−1, 30 kg ha−1, 60 kg ha−1 and 90 kg ha−1 to a Typic Paleustalf Alfisol. Soil samples from two depths (0–10 and 10–20 cm) were collected to determine microbial biomass C (MBC), N (MBN), P (MBP), MBC/N ratio, MBC/P ratio, soil CO2 flux, microbial qCO2, cultivable bacterial and fungal abundance. Biochar and N fertilizer combination effects on MBC, MBN and MBP pools were dependent on biochar doses, N fertilizer rates and soil depth. MBC/N and MBC/P ratios were decreased after 2 years. Soil CO2 flux was maximum at post-seeding stage of rice plant, while decreasing trends occurred at active tillering and harvest stage. Increasing doses of biochar irrespective of its combination with N fertilizer rates decreased CO2 flux and microbial qCO2. Combinations of biochar and N fertilizer increased fungal/bacterial ratio and induced a shift to a more fungal-dominated population after 2 years. Our results suggest that combination of biochar doses (3–12 t ha−1) with N fertilizer rates had stimulatory effects on microbial biomass pools and activity with positive implications for organic carbon accumulation, nitrogen (N) and phosphorus (P) retention in tropical soils.
The impacts of biochar addition with nitrogen fertilizer (Urea-N) on greenhouse gas (GHG) fluxes and grain yields are not comprehensively understood. Therefore, we designed a field experiment in an intensive rice–wheat cropping system located in the Taihu Lake region of China and measured CH4 and N2O emissions for 2 consecutive years to examine the impacts of biochar combined with N-fertilizer on rice production and GHG flux. Three field experimental treatments were designed: (1) no N-fertilizer application (N0); (2) 270 kg N ha−1 application (N270); and (3) 270 kg N-fertilizer ha−1 plus 25 t ha−1 biochar application (N270 + C). We found that, compared with urea application alone, biochar applied with Urea-N fertilizer increased N use efficiency (NUE) and resulted in more stable growth of rice yield. In addition, biochar addition increased CH4 emissions by 0.5–37.5% on average during the two consecutive rice-growing seasons, and decreased N2O–N loss by ~ 16.7%. During the first growing season, biochar addition did not significantly affect the global warming potential (GWPt) or the greenhouse gas intensity (GHGI) of rice production (p > 0.05). By contrast, during the second rice-growing season, biochar application significantly increased GWPt and GHGI by 28.9% and 18.8%, respectively, mainly because of increased CH4 emissions. Our results suggest that biochar amendment could improve grain yields and NUE, and increased soil GWPt, resulting in a higher potential environmental cost, but that biochar additions enhance exogenous carbon sequestration by the soil, which could offset the increases in GHG emissions.
Little is known on the effects of biochar on N uptake and amino acid variability in crops such as winter rye and narrow-leafed lupine despite the fact that amino acids are important indicators, for food quality and plant stress. N uptake of both crops showed contrasting results when treated with different biochar fertilizers. Total amino acid contents referred to total nitrogen generally tend to decrease in rye grains in the presence of biochar; whereas lupine seeds were more or less unaffected by biochar combined with mineral fertilizer or compost. In lupine seeds, total amino acid contents significantly increased when biochar was mixed with digestate but decreased when mixed with fermented digestate. Lysine, one of the most limiting amino acids in cereals, reached the recommended value of 4 g kg−1 in rye grain for most biochar fertilizers. In lupine seeds, lysine decreased when biochar had been applied but were still in the recommended range when used as animal feed. Proline, an indicator for plant stress, significantly decreased (− 49%) in rye when 2 Mg biochar ha−1 was added in combination with mineral fertilizer. In contrast, proline increased when biochar was added to organic (digestate and compost) fertilizers (up to 43%). Further biochar research should focus much more on food quality, which is a key challenge for global food production.
In paddy fields, the opposing transformation of arsenic (As) and cadmium (Cd) poses many challenges for their simultaneous remediation. In our previous study, we reported that combined biochar and zero-valent iron (ZVI) amendment had great potential for the simultaneous alleviation of As and Cd bioavailability in contaminated acid paddy soil. In this study, an As- and Cd-contaminated alkaline paddy soil was further studied, and the same ZVI–biochar mixtures amendments were applied to evaluate the impact of the mixtures on As and Cd transformation and translocation in the soil–rice system by performing pot experiments with rice. In line with our previous study, the ZVI–biochar composites significantly reduced As and Cd accumulation in different rice tissues, leading to a 42% and 47% decrease in rice grain As and Cd levels, respectively, compared with the control values. The ZVI–biochar mixtures exhibited synergistic effects of biochar and ZVI by enhancing the transformation of bioavailable As and Cd fractions into less bioavailable fractions, and by increasing iron plaque formation to reduce As and Cd bioavailability. Although the bioaccumulation and translocation factors of As and Cd in alkaline paddy soil were generally lower than those in acid paddy soil, particularly in the presence of the ZVI–biochar mixtures, the grain As and Cd levels did not achieve the desired food safety standard levels, probably related to the high soil As content and the small changes in soil pH. Nevertheless, for treating lightly and moderately contaminated paddy soils, ZVI–biochar mixtures can still be a good choice in the future.
Soil β-glucosidase (BG), the rate-limiting enzyme in the final step of cellulose hydrolysis, plays a key role in microbial metabolism, carbon (C) cycling and sequestration in terrestrial ecosystems. Biochar application is known to affect soil BG activity; however, most of the biochar studies have focused on the potential activity of BG, and it is not clear how biochar influences the kinetic and thermodynamic behavior of BG in the soil. The objective of this study was to investigate the effect of maize residue biochar on soil BG kinetic and thermodynamic parameters. Soil BG kinetic (Vmax and Km) and thermodynamic (Ea, ΔHa and Q10) parameters were determined within soils (clayey and sandy loam soils) amended with either maize residue (as positive control) or its biochar (600 °C) at 0.5 and 1.0% ratios (w/w), and the mixtures were incubated for 90 days. BG showed an increase in potential enzymatic activity (81%), enzyme concentration (higher Vmax value) (25%) and substrate affinity (lower Km value) (32%) in the biochar-amended sandy loam soil only at high addition rates compared with the control, and an increase by about 86% of the catalytic efficiency (Vmax/Km). In the clayey soil, biochar addition decreased potential BG activity (by 10–29%), increased the Vmax value (by 20–25%) and had no impact on enzyme–substrate binding affinity, but still increased the catalytic efficiency by 47–72%. Adsorption of soil BG by biochar particles did not affect the catalytic efficiency in the soil. Generally, application of maize residue biochar to the soil decreased the Ea, ΔHa and Q10 values of BG compared with the negative controls at both biochar rates in the light-textured soil and only at low biochar rate in heavy-textured soil. The direction and magnitude of BG responses (activity, kinetics, and thermodynamics) to biochar were more related to the soil characteristics. Biochar would increase soil BG thermal stability and decrease its sensitivity to increasing temperature and global warming.
Agriculture under changing climate scenario is facing major challenges of water scarcity and resource imbalances. Crop water productivity (WP) may act as an indicator of crop responses to water limitation. Organic amendments such as biochar and manure application to soil are suggested for improving soil quality and reducing water requirements from agricultural sector. However, studies exploring the impact of biochar as sole or in combination with organic and/or chemical fertilizers on WP in dry tropical agro-ecosystems are limited. In this study, we observed the effect of rice-husk ash (RHA, biochar) along with farm-yard manure (FYM) and chemical fertilizers (CF) under varying water conditions on soil hydro-physical properties, yield and WP of wheat crop. Water-filled pore space (WFPS), grain and straw yield, irrigation and total water productivity varied significantly (at P < 0.001) at treatment level. Grain and straw yield were found higher under sole and combined CF applied treatments. Sole and combined RHA and FYM amendment improved water holding capacity (WHC) and WFPS, whereas a decrease in crop yield was observed as compared to the control. Irrigation and total water productivity were found higher under combined RHA + FYM and sole CF treatments with reduced water supply (except sole CF) as compared to control and sole RHA treatments with full water irrigation. Crop water productivity was found positively correlated with grain and straw yields, however, significant correlations were not observed with WHC and WFPS. Results indicate that increasing soil hydro-physical properties in silty-loam soil may hinder crop yield and WP under sole biochar applied soils. Overall, the implications of the study would help in devising agro-management practices based on combined application of RHA and FYM with reduced chemical fertilizer and water inputs to mitigate the impacts of climate change without compromising crop yield in the highly vulnerable dry tropical agro-ecosystem of India. Moreover, long-term studies are needed in these ecosystems to identify the appropriate agricultural package for mitigating the forthcoming water scarcity conditions.