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While biochar use in agriculture is widely advocated, how the effect of biochar on plant growth varies with biochar forms and crop genotypes is poorly addressed. The role of dissolvable organic matter (DOM) in plant growth has been increasingly addressed for crop production with biochar. In this study, a hydroponic culture of rice seedling growth of two cultivars was treated with bulk mass (DOM-containing), water extract (DOM only), and extracted residue (DOM-free) of maize residue biochar, at a volumetric dosage of 0.01, 0.05, and 0.1%, respectively. On seedling root growth of the two cultivars, bulk biochar exerted a generally negative effect, while the biochar extract had a consistently positive effect across the application dosages. Differently, the extracted biochar showed a contrasting effect between genotypes. In another hydroponic culture with Wuyunjing 7 treated with biochar extract at sequential dosages, seedling growth was promoted by 95% at 0.01% dosage but by 26% at 0.1% dosage, explained with the great promotion of secondary roots rather than of primary roots. Such effects were likely explained by low molecular weight organic acids and nanoparticles contained in the biochar DOM. This study highlights the importance of biochar DOM and crop genotype when evaluating the effect of biochar on plants. The use of low dosage of biochar DOM could help farmers to adopt biochar technology as a solution for agricultural sustainability. [ABSTRACT FROM AUTHOR]

While biochar amendment is widely recommended as an excellent agricultural management practice, the biochar effect on the productivity of legume crops with symbiotic microorganisms has been poorly elucidated. Hence, a pot experiment was conducted by growing soybean with or without commercial rhizobia inoculation in an arbuscular mycorrhizal (AM) fungi rich sandy soil amended with wheat straw biochar produced respectively at 350 °C, 450 °C, and 550 °C. We tested the effect of biochar on soybean–rhizobia symbiosis and its impact on plant growth, leaf gas exchange and mycorrhizal colonisation. In brief, rhizobia inoculation, without biochar, increased net photosynthetic rate by 81% and plant total biomass by 44% compared to no inoculation (the control). Without inoculation, amendment of low-temperature biochar (350 ºC) resulted in higher (by 51%) plant biomass than high-temperature (550 °C) biochar. With rhizobia inoculation, low-temperature biochar decreased plant biomass by 9.3% compared to high-temperature biochar. Further, soybean with rhizobia under high-temperature biochar decreased plant biomass by 17% compared to rhizobia inoculation only but increased by 19% compared to the control. Soybean leaf gas exchange capacity and root morphological traits were consistent with plant biomass but symbiotic activity showed small differences among all the treatments. Overall, N-fixing rhizobia could alter the biochar effect on soybean in soil rich in indigenous AM fungi, depending on the pyrolysis temperature of the biochar used. Low-temperature biochar stimulated soybean growth greater than high-temperature biochar, while rhizobia shifted biochar's positive effect. • Pyrolysis temperature shaped biochar effect on soybean growth with and without rhizobia inoculation in a sandy soil. • Soybean growth without rhizobia inoculation is promoted rather by biochar-350 °C than by biochar-550 °C. • A tradeoff on soybean plant growth between biochar and rhizobia inoculation. • Soil inorganic N correlated with root morphology while leaf gas exchange responded to soil NO 3 -. [ABSTRACT FROM AUTHOR]

Reduced fertilization technology is an eco-friendly strategy to minimize nitrogen (N) and phosphorus (P) surpluses and losses in vegetable production. However, little is known about the performance of chemical fertilizer reduction when supplemented with palm silk biochar (PSB) in subtropical acid soils. A short-term (60 d) field investigation under conditions of in situ incubation was conducted in vegetable farmland in southern China. The treatments included no fertilization (Control), 100% conventional fertilization (CF100), 90% conventional fertilization plus 10% PSB-based fertilization (CF90B10), 85% conventional fertilization plus 15% PSB-based fertilization (CF85B15), and 80% conventional fertilization plus 20% PSB-based fertilization (CF80B20). The CF90B10, CF85B15, and CF80B20 treatments had the same inputs of total N and P as the CF100 treatment. Reduced chemical fertilization generally decreased the soil NH4+-N regardless of the PSB substitution rate (10%, 15%, or 20%), incubation condition (top-covered or top-open: preventing or allowing the leaching process, respectively), and sampling time (1 day or 60 days). Conversely, compared with the CF100 treatment, both the CF85B15 and CF80B20 treatments did not lead to a significant decrease in the NO3−-N concentration in soil under top-open incubation conditions, but significantly (p < 0.05) increased soil NO3−-N under top-covered incubation conditions. The CF80B20 treatment significantly (p < 0.05) decreased soil Olsen-P in comparison with the CF100 treatment, regardless of the incubation condition and sampling time. After applying chemical fertilizer in combination with PSB, soil net ammonification and N mineralization tended to be reduced considerably, with substantial reductions of 39–76% and 24–45%, respectively; reversely, soil net nitrification was stimulated by an increased PSB substitution rate. As the rate of chemical fertilization decreased, the trends in NH4+-N and NO3−-N losses from the soil were similar to the trends observed in soil net ammonification and net nitrification, respectively. Additionally, there were no significant differences in the soil net P mineralization and Olsen-P loss between chemical fertilization alone and in combination with PSB application. Generally, the partial substitution of chemical fertilizer with PSB at a low application rate may not substantially reduce plant-available NO3−-N and Olsen-P. It can also contribute to the sustainable availability of N and P in vegetable farmland soil via a variety of transformation processes, such as mineralization, immobilization, and loss. [ABSTRACT FROM AUTHOR]