Abstract 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.

中文翻译：

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生物炭施用对作物水分利用效率的影响取决于实验条件：荟萃分析

摘要 生物炭具有提高水资源利用效率（WUE）的益处，已被广泛报道。然而，大量研究表明，WUE 对生物炭应用的百分比变化是可变的（从 +312.5% 到 -35.9%）。共收集了 43 项研究，284 个数据对，分为植物水分利用效率 (PWUE) 和叶片水分利用效率 (LWUE) 两类，然后进行了荟萃分析，目的是 1) 估计影响在 WUE 上向土壤中添加生物炭；2) 确定与对照相比有利于 WUE 的实验条件。结果表明，生物炭对 WUE 具有统计学上显着的积极影响，PWUE 的总体平均增加 +18.8%，LWUE 的总体平均增加 +20.0%。然而，有效性仅限于特定的土壤特性、生物炭参数、和管理因素。在土壤分析中，土壤质地类别之间没有观察到显着差异，而在土壤 pH 值方面，WUE 响应存在显着差异，并且生物炭施用对 PWUE 和 LWUE 的明显对比影响。在 pH > 8 的土壤中发现 PWUE 增幅最大（平均 33.4 %，95 % CI：28.7 %–38.3 %），而对 LWUE 产生负面影响（平均 -8.9 %，95 % CI：-22.5 %） –7.0 %)。这表明 WUE 响应生物炭应用的主要机制之一可能是“将碱性生物炭添加到碱性土壤中”，强调生物炭 pH 值与土壤 pH 值的匹配。通过生物炭碳（C）含量的分析，我们得出结论，WUE 的增加随着生物炭中 C 含量的增加而降低，并提出了另一种机制“生物炭释放过量碳”。它说明了为什么与草本材料制成的生物炭相比，木质生物炭的 WUE 增加幅度很小。在低于 20 t ha-1 生物炭施用率时观察到对 PWUE 的最高积极影响（平均 32.4 %，95 % CI：23.3 %–42.2 %）的发现同意并进一步验证了这一机制。此外，根据这些类别之间的显着差异，生物炭 K 含量和 N 施用率可用作 WUE 对生物炭施用响应的有效预测指标。我们的研究结果可以为合理应用生物炭以提高作物生产中的 WUE 提供建议，但由于缺乏持续时间而受到限制，