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Biochar, a co-product of a controlled pyrolysis process, can be used as a tool for sequestering C in soil to offset greenhouse gas (GHG) emissions, and as a soil amendment. Whereas the impacts of biochar application on soil chemical properties are widely known, the research information on soil physical properties is scarce. The objectives of this review are to (i) synthesize available data on soil physical properties and GHG emissions, (ii) offer possible mechanisms related to the biochar-amended soil processes, and (iii) identify researchable priorities. Application rates of 1%–2% (w/w) of biochar can significantly improve soil physical quality in terms of bulk density (BD), and water holding capacity (WHC). However, little data are available on surface area (SA), aggregation stability, and penetration resistance (PR) of biochar-amended soil. While biochar amendment can initially accentuate the flux of carbon dioxide (CO2), the emission of GHGs may be suppressed over time. A 2-phase complexation hypothesis is proposed regarding the mechanisms of the interaction between soil and biochar

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Assessment of management-induced changes in soil quality is important to sustaining high crop yield. A large diversity of cultivated soils necessitate identification development of an appropriate soil quality index (SQI) based on relative soil properties and crop yield. Whereas numerous attempts have been made to estimate SQI for major soils across the World, there is no standard method established and thus, a strong need exists for developing a user-friendly and credible SQI through comparison of various available methods. Therefore, the objective of this article is to compare three widely used methods to estimate SQI using the data collected from 72 soil samples from three on-farm study sites in Ohio. Additionally, challenge lies in establishing a correlation between crop yield versus SQI calculated either depth wise or in combination of soil layers as standard methodology is not yet available and was not given much attention to date. Predominant soils of the study included one organic (Mc), and two mineral (CrB, Ko) soils. Three methods used to estimate SQI were: (i) simple additive SQI (SQI-1), (ii) weighted additive SQI (SQI-2), and (iii) statistically modeled SQI (SQI-3) based on principal component analysis (PCA). The SQI varied between treatments and soil types and ranged between 0–0.9 (1 being the maximum SQI). In general, SQIs did not significantly differ at depths under any method suggesting that soil quality did not significantly differ for different depths at the studied sites. Additionally, data indicate that SQI-3 was most strongly correlated with crop yield, the correlation coefficient ranged between 0.74–0.78. All three SQIs were significantly correlated (r = 0.92–0.97) to each other and with crop yield (r = 0.65–0.79). Separate analyses by crop variety revealed that correlation was low indicating that some key aspects of soil quality related to crop response are important requirements for estimating SQI.

Any strategy towards widespread adoption of biochar as a soil amendment is constrained by the scarcity of field-scale data on crop response, soil quality and environmental footprint. Impacts of biochar as a soil amendment over a short period based on laboratory and greenhouse studies are often inconclusive and contradictory. Yet biochar is widely advocated as a promising tool to improve soil quality, enhance C sequestration, and increase agronomic yield. While substantial reviews exist on positive aspects of biochar research, almost no review to date has compiled negative aspects of it. Although biochar science is advancing, available data indicate several areas of uncertainty. This article reviews a range of negative impacts of biochar on soil quality, crop yield, and associated financial risk. This review is important because advances in biochar research demand identification of the risks (if any) of using biochar as a soil amendment before any large-scale field application is recommended. It is the first attempt to acknowledge such issues with biochar application in soil. Thus, the aims of this review are to assess the uncertainties of using biochar as a soil amendment, and to clarify ambiguity regarding interpretation of research results. Along with several unfavourable changes in soil chemical, physical and biological properties, reduction in crop yield has been reported. Relative to controls, the yield for biochar-amended soil (application rate 0.2–20% w/w) has been reduced by 27, 11, 36, 74, and 2% for rice (Oryza sativa L.) (control 3.0 Mg ha–1), wheat (Triticum spp. L.) (control 4.6 Mg ha–1), maize (Zea mays L.) (control 4.7 Mg ha–1), lettuce (Lactuca sativa L.) (control 5.4 Mg ha–1), and tomato (Solanum lycopersicum L.) (control 265 Mg ha–1), respectively. Additionally, compared with unamended soils, gaseous emissions from biochar-amended soils (application rate 0.005–10% w/w) have been enhanced up to 61, 152 and 14% for CO2 (control 9.7 Mg ha–1 year–1), CH4 (control 222 kg ha–1 year–1), and N2O (control 4.3 kg ha–1 year–1), respectively. Although biochar has the potential to mitigate several environmental problems, the data collated herein indicate that a systematic road-map for manufacturing classification of biochars, and cost–benefit analysis, must be developed before implementation of field-scale application.