Biochar applications influence soil physical and chemical properties, microbial diversity, and crop productivity: a meta-analysis.

Authors: SINGH, HARDEEP - Kansas State University; Northup, Brian; PRASAD, VARA - Kansas State University

Submitted to: Biochar
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/17/2022
Publication Date: 2/16/2022
Citation: Singh, H., Northup, B.K., Rice, C.W, Prasad, V. 2022. Biochar applications influence soil physical and chemical properties, microbial diversity, and crop productivity: a meta-analysis. Biochar. 4. Article 8.

Interpretive Summary: The ability of agriculture to produce crops is being limited by slow declines in soil condition and poor efficiencies in use of available nutrients. Over time, these effects can result in food insecurity at regional to national scales. Issues like climate change, increasing human populations, and the urbanizing of croplands place added pressures on agriculture. Therefore, how crop production is undertaken needs to be reconsidered, to resolve the range of problems affecting agriculture. One potential solution is to recycle nutrients back to the soil. This old-school approach would help increase the amounts of organic matter in soils, which typically provides materials that help improve the physical and chemical properties of soils. One source of nutrients that could be recycled is biochar. This product is a soil amendment with well-studied benefits, such as improving soil fertility and trapping heavy metals or waste chemicals that are in soils. Biochar is a solid and stable, carbon-rich material that is formed by heating organic materials in the absence of oxygen in a process called pyrolysis. It can be produced from a range of feedstocks (organic or industrial wastes; plant-based materials like leaves, husks, or cobs; wood products like woodchips or wood pellets) and can be generated with different temperatures and durations of pyrolysis. The combination of feedstock type and pyrolysis conditions allows the production of different biochars with a range of properties, which can lead to difficulties in defining the best amount and type to use. We considered how different biochars, and amounts of biochar, affected soil properties and influenced crop production. Our focus was to apply a technique called meta-analysis, which uses information reported in large numbers of studies from the worldwide pool of scientific papers, to test the effect sizes of different treatments across a broad range of locations and years to identify large-scale trends, or areas requiring future research. The meta-analysis was applied to results from studies conducted in different settings (fields, greenhouses, and laboratories) published during 2012 to 2021, to test how different amounts (1 to 70 tons per acre) and types (manures, wood materials, and crop residues) of biochars prepared at low (below 900 F) and high (above 900 F) temperatures affected properties of clay, loam, and sandy soils. We found that biochar additions had varied effects on soil properties, based on type of feedstock, pyrolysis temperature, amounts added, and soil texture. Soil acidity (pH), cation exchange capacity and carbon contents improved by 46%, 20%, and 27%, respectively. Biochar applications reduced bulk densities by 29%, and increased pore space by 59%. Larger effects occurred in sandy soils, and with biochars prepared at higher temperatures (density decreased 31%; pore space increased 66%). Low temperature biochar had greater effects on the diversity of bacteria and fungi in soils, and diversity of bacteria increased in loam and sandy soils, while diversity of fungi increased in clay soils. Biochar improved crop yields in clay and sandy soils, but only for biochar prepared at low temperatures, and larger effects were noted when larger amounts were applied. We found the effects of biochar applications on crop productivity were largely correlated with responses of bulk density and pore space of soils to biochars. Our analysis noted that there is a need for more long-term field experiments to help describe how biochar affects soil properties, and how the aging of biochar affects its capacity to influence soil properties, and what kind of timing is required for additional applications.

Technical Abstract: Biochar is a widely known soil amendment for improving physical, chemical, and microbial properties of soil, and increasing crop productivity. However, experimental results are mixed due to variations in experimental set-up, biochar properties, and soil properties, making it difficult to assess and generalize results from different studies. We conducted a meta-analysis (MA) of papers published between 2012 and 2021, to quantify impacts of biochar applications on soil soil pH (214 datasets), electrical conductivity (EC, 83 datasets), cation exchange capacity (CEC, 73 datasets), organic carbon (OC, 150 datasets), bulk density (100 datasets), porosity (70 datasets), microbial diversity (bacterial and fungal, 37 datasets), and crop productivity (122 datasets). Overall, application of biochar increased soil pH, CEC, and OC by 46%, 20%, and 27%, respectively, with greater effects recorded in coarse and fine-textured than medium-textured soils. The effects of applications on chemical properties was variable among biochar prepared from different feedstocks. Among physical properties investigated in this MA, biochar application, reduced bulk densities by 29%, and increased porosity by 59%, respectively. Effects of biochar applications was dependent on soil type, with effect sizes of biochar applications on physical properties greater in coarse-textured soils. Additionally, applications of biochar prepared at higher pyrolytic temperatures (>500') improved bulk density and porosity to greater extents (31% and 66%, respectively). Microbial diversity in soils increased in response to biochar application, but had an inverse relationship with rates of application. Biochar prepared at lower pyrolytic temperatures (<500') had greater effect sizes on microbial diversity (both bacterial and fungal). Among soil types, bacterial diversity increased (P<0.05) in medium and coarse textured soils, while fungal diversity increased (P<0.05) in fine textured soils. Biochar applications significantly (P<0.05) increased crop productivity only in fine and coarse textured soil, and only biochar prepared at low pyrolysis temperatures resulted in increased crop productivity; larger effect sizes were noted for higher rates of application. Effect sizes of biochar applications on crop productivity were correlated with responses by physical properties of soils. The MA highlighted the need to conduct longer-term field experiments to provide better explanations for changes in biochar properties as it undergoes aging, its longer-term effects on soil properties, and timing for reapplication of biochar.