Effects of biochar application on crop water use efficiency depend on experimental conditions: A meta-analysis

It is widely reported that biochar has the benefit of improving water use efficiency (WUE). However, a lot of researches indicated that the percentage changes in WUE to biochar application were variable (from + 312.5 % to -35.9 %). A total of 43 studies with 284 data pairs were collected and divided into two categories: plant water use efficiency (PWUE) and leaf water use efficiency (LWUE), and then a meta-analysis was conducted with the aim to 1) estimate the effects of adding biochar to soils on WUE; and 2) identify the experimental conditions that benefit WUE compared to the control. Results showed that there was a statistically significant positive effect of biochar on WUE with a grand mean increase of + 18.8 % in PWUE and + 20.0 % in LWUE. However, the effectiveness was limited to specific soil properties, biochar parameters, and management factors. In soil analyses, no significant difference was observed between soil texture categories, while with regard to soil pH there were significant differences in WUE responses and evident contrasting effects of biochar application on PWUE and LWUE. Greatest increases in PWUE were found in soils with pH > 8 (33.4 % on average, 95 % CI: 28.7 %-38.3 %), while negative effects on LWUE were obtained (-8.9 % on average, 95 % CI: -22.5 %-7.0 %). This suggests that one of the main mechanisms for WUE responses to biochar application may be 'the addition of alkaline biochar to alkaline soils', stressing the matching of biochar pH and soil pH. Through the analyses of biochar carbon (C) content, we concluded that the increase in WUE depressed with increasing C content of biochar and proposed another mechanism 'excess carbon released from biochar'. It illustrates why wood biochar showed a significantly small increase in WUE compared to biochar made from herbaceous material. The finding that the highest positive effect on PWUE (32.4 % on average, 95 % CI: 23.3 %-42.2 %) was observed at less than 20 t ha(-1) biochar application rate agrees with and further validates this mechanism. In addition, biochar K content and N application rate could be used as an effective predictor of WUE responses to biochar application according to significant differences between such categories. Our findings can provide suggestions for rational biochar application to improve WUE in crop production, but it is limited by the lack of the longevity of the effects, which is important for developing sustainable biochar policy.