Impact of Biochar Amendment on the Hydraulic and Nutrient Retention Properties of a Sandy Soil.

Sustainable agriculture in sandy soil generally faces large constraints due to low water holding and nutrient retention capacity, and accelerated mineralization of soil organic matter. Recent studies have shown that biochar can improve hydraulic properties and nutrient retention in soils. However, no study has been conducted to examine the impact of biochar on sandy soil.  In this study, the effect of biochar on sandy soil hydraulic properties and nutrient (nitrate and phosphorus) retention was investigated. Biochar was generated in the laboratory at three different pyrolysis temperatures (300, 400 and 500^oC) from black locust (Robinia pseudoacacia L.) collected from Tottori sand dune forest. Two mixing rates of biochar (10 and 20 Mg/ha) were used in the experiment. Biochar amendment improved sandy soil hydraulic properties and nutrient retention. The effect of temperature in function of mixing rate had impact on soil hydraulic properties. It was found that biochar produced at 500^oC with 20 Mg/ha rate exhibited the highest impact. Application of biochar increased the available water capacity by 97%, saturation water content by 56%, and reduced both saturated and unsaturated hydraulic conductivity by more than 100 times. Nitrate retention was significantly affected by both pyrolysis temperature and mixing rate while phosphorus retention was affected by only mixing rate. The control retained only 1% of the applied nitrate whereas, biochar application increased the soil retention of nitrate up to 23% with 500^oC at 10 Mg/ha and 37% with 300^oC at 20 Mg/ha mixing ratio. Biochar produced at 500^oC and mixed at 20 Mg/ha rate had the highest retention of phosphorus while that produced at 400^oC and mixed at 10 Mg/ha rate retained the least amount of phosphorus. This study highlights the importance of pyrolysis temperature and mixing rate of biochar on its effect on sandy soil hydraulic properties and nutrient retention and hence, the amendment values of biochar in a sandy soil.