Effects of five different biochars on aggregation, water retention and mechanical properties of paddy soil: A field experiment of three-season crops

Abstract

Biochar can improve a number of soil physical properties but few studies focused on the impacts of different biochars on soil Atterberg limits and mechanical strength in paddy fields. A field experiment was performed to evaluate the influences of five different biochars on soil physical properties of a paddy soil (Stagnic Anthosol). Biochars produced from rice straw (RB), maize straw (MB), wheat straw (WB), rice husk (HB) and bamboo (BB) were applied to the paddy field with the rate of 0, 11.25, and 22.5 t ha−1, respectively. After three consecutive crops (rice-rape-maize) plantation, the biochar-amended soils were collected to determine a number of soil physical parameters. Compared with control treatment, the application of five biochars at the rate of 22.5 t ha−1 significantly (p < 0.05) increased the content of macroaggregates larger than 5 mm in diameter and aggregate stability evaluated by Le Bissonnais’s methodology. Biochar treatments significantly (p < 0.05) increased soil water retention capacity at matric potential of -0.033 and -1.5 MPa, and consequently, enhanced available water content (AWC) in soil. Biochars at the rate of 22.5 t ha-1 treatments increased the AWC by 15.5, 17.1, 26.4, 42.4, and 15.2 % for RB, MB, WB, HB, and BB, respectively. Biochar application could significantly increase the plastic index (PI) of soil, thus effectively extend soil cultivable water range. The application of biochar significantly reduced the tensile strength and cohesion, but no significant effect on internal frication angle of soil was observed. The application of biochars at the rate of 22.5 t ha−1, on average, increased the yield of first season crop (rice) by 6.3∼13.3 %, but only RB and HB treatments significantly increased the yield of second season crop (rape), and RB, WB, and HB significantly increased the yield of third season crop (maize). The scanning electron microscope (SEM) image and elemental mapping of biochars indicated that biochar could alter pore characteristics (size distribution, shape, connectivity, and porosity) of soil, consequently affecting water retention and mechanical strength of soil. Overall, the HB exhibited the largest effect on the AWC, Atterberg limits, and crop yield. The biochar amendment increased the crop yield by improving soil workability for tillage operations and reducing mechanical resistance to crop root growth.