LAUSR

Abstract Biochar can be potentially used as an amendment to beneficially enhance the physicochemical qualities of agricultural soils. In this study the effects of corn (Zea mays L. cv. Maxima) residue and its biochar on the soil–water characteristic curve (SWCC), the soil penetration resistance curve (SPRC, penetration resistance versus matric suction), plant available water (PAW) content, least limiting water range (LLWR) and integral water capacity (IWC) were examined in greenhouse experiments. Indices of soil physical quality and hard-setting (soil compaction during the drying processes) behavior were also measured. Pots (5 kg) were filled with a calcareous sandy loam mixed with raw corn residue or its biochars at rates of 1, 2, and 4 percent (w/w). Low and high temperature biochars, produced by slow pyrolysis process at 350 and 650 °C were evaluated. Both biochar amendments significantly ameliorated soil physical properties such as bulk density, total porosity, water retention and penetration resistance (Q) compared to the un-amended soil. Pore size distribution was significantly altered in favor of a greater fraction of wider pores (i.e. macro and mesoporosity) in the biochar amended soils. The lowest Q value was observed for the unmodified corn residue at a 4% application rate (CR 4%) in all of the matric suctions. The increase in Q with drying was attributed to Q’s strong relationship with the effective stress which originated from the matric suction. The hard-setting indices inferred from the SWCC and SPRC and shown by the slope of power model b (0.709), ΔQ (3.31 MPa), ΔQ/Q3000 (1.98), hard-setting index related to structural pores, H1 (789 hPa) and hard-setting index proposed by Dexter (2004b), HDexter (1163 hPa), were more pronounced in the untreated compared to the treated soils. Overall the changes in soil available water (SAW) for plants following biochar application was compared using three different indices, PAW, LLWR and IWC. Applying biochar could increase soil water availability such that PAW increased from 0.100 in the control to 0.156 cm3 cm−3 in BC-650 4%, LLWR100 increased from 0.079 in CR1% to 0.122% cm3 cm−3 in BC-350 2% and IWC increased from 0.072 in the control to 0.166 cm3 cm−3 in CR4%. The positive correlations between PAW, LLWR and IWC with soil physical quality indices like S1 and S2 (slopes of SWCC at first and second inflection points, related to structural and matrix pores, respectively) indicated these quantities were a potentially useful tool for evaluating soil physical quality.