Effects of biochar on water infiltration, evaporation and nitrate leaching in semi-arid loess area

Water scarcity and low use efficiency of nitrogen are the major limiting factors for agriculture sustainable development in dryland areas of the Loess Plateau. Biochar has been widely proposed as a promising novel alternative of soil amendment to improve soil quality and increase crop productivity, but limited quantitative work has been addressed on the soil water infiltration process, evaporation characteristics and nitrate leaching. A better understanding of these characteristics can provide the solid basis for the evaluation of the effect of biochar amendment on soil hydrology and nitrogen retention in arid and semi-arid regions. In the present study, by using the soil column simulation investigation, biochar derived from maize stover (pyrolysis temperature of 400℃) was applied to 3 different types of soil samples (aeolian soil, cultivated loessial soil and dark loessial soil) collected from the Loess Plateau at 6 rates of 0, 0.5%, 1%, 2%, 3% and 5% (w/w) with triplicate. The wetting process, the cumulative infiltration amount, the permeability and the stable infiltration rate were determined. The water infiltration analysis showed that the advancement of wetting front and the cumulative infiltration amount changed with the biochar addition amount and soil texture. The migration rate of water was the fastest in aeolian soil and the slowest in cultivated loessial soil due to the soil texture and structure. The migration rate of wetting front in aeolian soil and dark loessial soil was decreased after the biochar addition. As the rate of biochar addition increased, the cumulative infiltration amount of aeolian soil and dark loessial soil gradually declined. For cultivated loessial soil, the higher biochar addition rates (3% and 5%) increased the time of the wetting front to the bottom of the column, while the lower rates (0.5%, 1% and 2%) increased the wetting front migration rate; the cumulative infiltration amount was not decreased at the rate of 0.5%, 1% and 2%, but reduced significantly at the rates of 3% and 5%. Overall, biochar addition reduced the water infiltration capacity and increased the water holding capacity for aeolian soil and dark loessial soil; the lower rates of biochar addition in cultivated loessial soil were not conducive to the water retention, while the higher rates of biochar addition favored. There were extremely significant power function relationships between wetting front distance and infiltration time under different rates of biochar treatments with the determination coefficient of greater than 0.99. Philip infiltration model was suitable for the simulation of soil water infiltration process under biochar treatments with the determination coefficient ranging from 0.991 to 0.999. During the successive evaporation of 30 days, biochar addition had no significant effects on the cumulative evaporation in cultivated loessial soil and dark loessial soil. However, biochar amendment changed the evaporation characteristics of aeolian soil: the evaporation was inhibited in the early stage; due to the continuous supply of water, the evaporation gradually increased in the later stage, but not significantly. The nitrate leaching was determined by the volume and the concentration of the leaching solution. The 2% addition rate of biochar in aeolian soil and the 1% addition rate of biochar in loessial soil had no significant effects on the total amount of nitrate leaching (P>0.05), when compared with the levels of the control (0%). Biochar addition reduced nitrate leaching by 18.1%-37.2% in aeolian soil, 33.8%-87.7% in cultivated loessial soil and 13.8%-80.8% in dark loessial soil, which may lead to increase the capacity of nitrogen holding in soil, reduce the nitrogen leaching risk in environment. Our results suggest that the use of biochar as soil amendment in agricultural soils plays an important role in increasing soil water holding capacity, improving available nitrogen and helping crop development in dryland areas of the Loess Plateau. However, more studies are needed to understand the mechanisms through which these benefits are achieved.