Background With the aggravation of global climate change, drought stress has become one of the main limiting factors affecting agricultural production. Reports showed that most crop yield reduction was related to drought, which would result in food shortages especially in arid and semi-arid areas. Therefore, it is critical to improve the drought resistance of crops, and the increase of soil water holding capacity may ensure agricultural production and food security. Biochar soil amendment shows the benefit in promoting crop growth and improving soil structure and fertilizer retaining, and have the function of increasing soil water holding capacity and alleviating crop drought stress.

Methods We reviewed the literatures related to the effect of biochar amendment on soil water retention and on crop drought resistance ability. We conducted literature search via Web of Science, Wiley-Blackwell and CNKI, using the keywords of "biochar" "soil water" and "drought stress". We discussed the effect of biochar soil amendment on soil water retention and crop resistance for drought stress, and the potential mechanisms, and gave a brief overview of the future research direction.

Results Drought stress is the main environmental stress factor affecting agricultural production, which seriously affects crop growth, especially the continuous severe drought may cause the disorder of crop metabolism and even lead to crop death. Under drought stress, crops may adjust biomass allocation, especially promoting root growth to get more water, and crops may adapt to drought stress through a series of physiological and biochemical reactions. Researches demonstrated that biochar improved the drought resistance of crops, including mechanisms of improvement of root architecture, and increased the secretion of root exudate, increased the activity of soil microbe. And biochar may adjust crop osmotic potential and increase the water use efficiency of crops. In addition, biochar soil amendment may increase soil water holding capacity, and the key mechanisms including the large numbers of hydrophilic functional group of biochar, the improvement of soil porosity especially the decrease of soil bulk density increased the field water capacity. Due to the variability of biochar properties and soil conditions, it leads to the variability effect of biochar soil amendment on soil water retention and crop resistance for drought stress, and the mechanisms had insufficient understanding.

Conclusions It is certainly that biochar soil amendment demonstrates the potential to increase soil water retention and increase the drought resistance of crops, and shows a long-term lasting effect. While the mechanisms of biochar on soil water holding capacity and on crop physiological response in different types and textures of soils, and it is urgent to identify the key influencing factors under different. Field experiments are necessary to explore the suitable biochar application method for different soil conditions, and determine the persistence effect, and develop the biochar-based water-retaining products.