Six slow pyrolysis biochars viz. farmyard manure (FM), water hyacinth (WH), domestic organic waste (DW), quick compost (QC), corn cob (CC) and rice straw (RS) were analyzed for their physical and chemical properties. Biochar yielding capacity varied from 34 to 51%, depending on the used feedstock. Water hyacinth biochar exhibited the highest water holding capacity (495%), whereas corn cob biochar had the lowest (146%) regardless of its highest pore volume. Brunauer-Emmett-Teller (BET) specific surface area was found maximum among the plant derived biochars except corn cob. Rice straw biochar exhibited the least mean pore diameter while highest in domestic organic waste. All biochars possessed pH values more than 9. CEC of water hyacinth (WH) was highest, while lowest was in quick compost (QC) biochar. Smallest average particle size (0.54 µm 2 ) was exhibited by water hyacinth biochar. Organic carbon content ranged from 33 to 49%. Nutrient (N, P, K and S) status of biochar produced from domestic organic waste (DW) was found the maximum compared to the rest and corn cob (CC) biochar showed the lowest nutritional value.

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Application of biochar has been highly credited for its potential to sequester carbon and GHG mitigation from tropical agro-ecosystems. However, experiments show inconsistent results depending on soil and biochar type, cultivation system, climatic condition and the type of evolved GHGs. This study emphasized on the effect of biochar on carbon emission trends from a sequential dry and wet cultivation system of Bangladesh. An incubation study was conducted with two contrasting soils and eight different treatments viz. control, only fertilizer, three different biochars (10 t·ha −1 ) with and without recommended fertilizer dose. Results revealed the fact that, emission of carbon was substantially higher from Sara soil than Kalma soil. Biochar treatments did not have any easing effect on CO 2 emission at field condition; rather, increased in most of the cases. However, emission was significantly (P < 0.05) suppressed at submerged condition by biochar application. Non-fertilized water hyacinth biochar was most effective in this regard. In general, fertilizer application caused higher emission of CO 2 . Biochar application was ineffective to control CH 4 and CO release to atmosphere and submergence further intensified their emission significantly. The overall results indicate that applied biochars have negligible effect on carbon emission except for reducing CO 2 from submerged soils.

Biochar application is claimed to improve nutrient availability in many problem soils; however, pristine biochars are often reported to produce inconsistent results. Therefore, appropriate biochar modification techniques are required to retain soil nutrients at an optimum level. To increase Nitrogen (N) and Phosphorus (P) availability in coastal saline soil, two slow pyrolyzed biochars viz domestic organic waste (DWB) and farmyard manure (FMB) were modified with MgCl 2 . Ten different treatments comprising the biochars (pristine and modified) with and without the recommended fertilizer were applied (2% w/w) to the soil and incubated for ninety days. The soils were analyzed for pH, EC, available 4 NH + , 3 NO − and different phosphorus fractions sequentially extracted by NH 4 Cl, NaHCO 3 , NaOH, and HCl. During the incubation period, biochar treatments increased all phosphorus and nitrogen fractions than the control and recommended fertilizer treatment. The application of FMB significantly (p < 0.05) increased NH 4 Cl, NaHCO 3 , and NaOH extractable P fractions from DWB, while HCl soluble fraction was enhanced (p > 0.05) by DWB. The increased Al and/or Fe bound phosphate after 60 days of incubation had significant correlations to decreasing soil pH and NaHCO 3 -P, indicating reduced availability with time. Further Mg modification slightly increased P availability only after 60 days of incubation. The modification also improved both nitrogen fractions but significantly (p < 0.05) increased the NO 3 -N content which could be the result of electrostatic attraction between Mg 2+ and 3 NO − ions. Overall, Mg-modified biochar may retain both phosphates and nitrates in soil. However, the magnitude of retention will vary depending on biochar type, nutrient species, and aging in soil.

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