Does biochar improve soil water retention? A systematic review and meta-analysis

Biochar is widely suggested as a soil amendment to improve soil physical properties for crop production. However, the heterogeneity between experiments in terms of biochar characteristics, experimental conditions and soil properties makes it difficult to compare and extrapolate results from different studies. We conducted a statistical meta-analysis of literature published between 2010 and 2019, and quantified biochar's impacts on soil bulk density (BD) and water retention variables, namely, soil water content held at field capacity (FC), wilting point (WP) and plant available water content (AW). Across all soil textural groups, biochar on average decreased BD by 9%. FC and WP significantly increased for the coarse-textured soils (by 51% and 47%, respectively), and moderately for the medium-textured soils (by 13% and 9%, respectively). For the fine-textured soils, FC remained unchanged ( < 1%), but WP marginally decreased by 5%. Biochar significantly increased AW in the coarse-textured soils (by 45%) compared to the medium- and fine-textured soils (by 21% and 14%, respectively) suggesting that biochar may have greater benefit on coarse-textured soils. We introduced a novel parameter called `biochar carbon added', estimated from biochar carbon content and the rate of application, which can be useful to make a sound comparison of biochar impacts on soil physical properties reported in different case studies when variable biochar attributes, experimental conditions and soil types have been used. The meta-analysis highlights again the need to provide adequate information on biochar experiments and soil water retention variables to allow better elucidation of the underlying mechanisms of biochar's impact on soil water retention and more.