Greater, but not necessarily better: The influence of biochar on soil hydraulic properties

Abstract

Biochar is recommended as a soil amendment for its positive influence on soil hydrological properties, which results in improved soil fertility and crop yield. Much research in the last decade has been conducted in field and laboratory conditions on the effect of biochar on the hydraulic properties of soil. However, reported results in the literature are substantially inconsistent. Here we performed a meta‐analysis to capture the variations in change in hydraulic properties of arable soils after application of different rates of biochar. The meta‐analysis revealed that high biochar rates (>50 t ha−1) compared to low rates (<20 t ha−1) significantly improved dry bulk density in sandy and clay soils, in field and laboratory experiments. However, field capacity only improved in laboratory experiments on sandy soils. The plant available water, permanent wilting point and saturated hydraulic conductivity did not significantly increase at high rates of biochar application compared to the low rates when applied to different types of soils in both field and laboratory experiments. We discuss possible reasons for this, including hydrophobicity of the biochar with future research directions. We concluded that the current evidence does not support the notion that the application of biochar improves soils' available water capacity. Highlights Meta‐analysis clarifies the influence of biochar on soil hydraulic properties. Biochar addition at higher rates only improves the water holding capacity of sandy soils. Biochar types and pyrolysis temperatures do not influence soil hydraulic properties. The efficiency of biochar may depend on its pore size distribution and hydrophobicity.

Impact of biochar amendment on soil hydrological properties and crop water use efficiency: A global meta‐analysis and structural equation model

“…For instance, Omondi et al (2016) found that the biochar's effect of medium pyrolysis temperature on total porosity was better than that at low pyrolysis, while Edeh et al (2020) reported that pyrolysis temperature had no significant effect on the response of SP to biochar application when taking 500°C as the boundary between high and low pyrolysis temperature. The disparate database could also be a cause of such difference because experimental conditions had an influence on the results (Edeh et al, 2020; Omondi et al, 2016; Rabbi et al, 2021; Razzaghi et al, 2020), and our database only includes field researches.…”

“…Lots of meta‐analyses have focused on the effects of biochar application on crop yield, soil basic properties, and greenhouse gas emissions (Crane‐Droesch et al, 2013; Jeffery et al, 2011; Liu et al, 2013; Ye et al, 2020). However, there have been limited meta‐analyses to investigate the response of soil hydrological properties to biochar application though inconsistency of results was presented (Edeh et al, 2020; Omondi et al, 2016; Rabbi et al, 2021; Razzaghi et al, 2020). Razzaghi et al (2020) and Omondi et al (2016) reported that biochar could reduce soil bulk density (BD) by around 7%, but Edeh et al (2020) found that biochar had no significant effect on soil BD.…”

“…Besides, the experimental types considered by these studies were also diversified. In addition to the field experiments, pot, greenhouse, or laboratory incubation was also included in some studies, which may not represent the actual field situations (Edeh et al, 2020; Omondi et al, 2016; Rabbi et al, 2021; Razzaghi et al, 2020). Apart from the inconsistent results, previous studies covered incomplete soil hydraulic properties and did not combine soil hydrology with crop WUE or reveal the potential mechanism deeply.…”

“…Apart from the inconsistent results, previous studies covered incomplete soil hydraulic properties and did not combine soil hydrology with crop WUE or reveal the potential mechanism deeply. (Edeh et al, 2020; Omondi et al, 2016; Rabbi et al, 2021; Razzaghi et al, 2020). Besides, it is worth noting that the meta‐analysis on the response of both soil hydrological properties and WUE to biochar application is very limited, and the reasons leading to the variation of biochar performance also need to be further explored.…”

Appraisal for organic amendments and plant growth‐promoting rhizobacteria to enhance crop productivity under drought stress: A review

“…The aged biochars may also have a detrimental effect on the growth of earthworms and/or fungi (Anyanwu et al., 2018), thereby affecting soil health and nutrient availability. Despite the above, a recent meta‐analysis study reported that application of biochar @ >50 t/ha would be required for improvement of hydraulic properties of sandy soils under laboratory conditions (Rabbi et al., 2021). The authors suggested that the application of such a huge amount would involve high costs during biochar production and would require further research I producing effective biochar with optimum porosity, surface area, and hydrophilic properties.…”