Wide-suction-range hysteretic water retention behaviour of compacted soils amended by decomposing hydrochar

Hydrochar, which is produced by hydrothermal carbonisation, is an alternative to biochar for the alleviation of plant water stress. However, hydrochar shows vulnerability to decomposition. This study measured the effects of grass-derived hydrochar on hysteretic wide-suction range water retention curves (WRCs) of compacted soils and investigated how hydrochar decomposition affected the WRCs and pore size distributions. Hydrochar addition increased the first air-entry value (AEV) at a relatively low suction range due to the reduction in the macro-pore peak diameter but reduced the second AEV at a higher suction range following the increase in pore volume at diameters larger than 0.3 µm. A strongly positive correlation existed between the AEVs and the peaks of the degree of hysteresis, which suggests that the peak hysteresis is related to the ink-bottle effect despite the modification of soil pore structure due to hydrochar addition. After 120 days of decomposition to an approximately stable state, the hydrochar did not affect all the WRCs at a wide suction range, regardless of the emergence of bacteria and fungi. The test results highlight the potential of hydrochar in promoting plant growth through improvements of the soil’s WRCs and the increase in abundance of functional bacteria and fungi.