Soil Physical Properties in Biochar Amended Soils

Abstract

Current concerns about global food security combined with the need to develop more sustainable agricultural systems and reduced greenhouse gas emissions necessitate major changes in agricultural management. Biochar is increasingly being recognized for its potential as a soil amendment to improve soil resources. Although biochar effects on soil chemicals and microbiological properties are already becoming well known, its impact on soil physical properties and their agronomic implications have yet to be better assessed. Therefore, the objective of this chapter is to provide balanced perspective on biochar properties in the context of biochar production and feedstock types and how it could affect soil physical properties in biochar amended soils and their agronomic implications. It was revealed that biochars produced from various feedstocks at temperatures from 300°C to 600°C decreased soil bulk density in most of laboratory, green house and field based studies. Direct dilution effect and soil aggregation attributed to the low bulk densities in short term and long-term studies respectively. No consistence results were found on soil penetration resistance in biochar amended soils, and it was not correlated to decreases in bulk densities in most of studies. Studies also suggested that soil aggregation in biochar amended soils are dependent on the rate of biochar application and size of biochar particles. Laboratory and green house studies (>90 days) and long-term field studies (2–3 years) evidenced improvement in soil aggregation, and this has also led to an increase in porosity, which decreased the bulk density in the long term. Most of the studies have found the changes in total porosity and pore size distribution in biochar amended soils. Biochar is likely to increase the water retention in coarse textured soils (sandy and loamy sand). However, feedstocks and the production conditions could affect pore size distribution, which in turn impacts the water retention properties. Results on hydrophobicity of biochar amended soils showed no consistency, while few studies suggested no change in hydrophobicity while some studies observed a decrease in hydrophobicity over time. The biochar-mediated changes in soil physical properties appeared to be a function of soil texture and biochar type based on its feedstock and production temperature, which determines key biochar characteristics such as surface area and porosity. However, the long-term effects of amending agricultural soils with biochar are difficult to predict, because the mechanisms behind the increase in productivity of biochar amended soils are not yet fully understood. Therefore, it is necessary to evaluate the role of the size, porosity and surface area of biochar in influencing soil physical properties such as soil pore size distribution, soil water retention and water holding capacity, hydrophobicity and soil aggregation. The long-term mechanisms which influence of biochar on these physical properties is unlikely to be like the short-term mechanisms. Therefore, more long-term field based studies are recommended to understand the way biochar alters soil physical properties and how the mechanisms are interrelated.