Soil Organic Matter and Biochar Effects on Soil Water: Measurements, Pedotransfer Functions and APSIM Simulations

Abstract

The ability of soils to store and regulate water release to plants is critical for crop production; hence, the ability to estimate soil water parameters is critical. This study aimed to (1) determine biochar effects on the relationship between soil organic matter (SOM) and soil water/physical parameters including field capacity (FC), wilting point (WP), saturation (SAT), water holding capacity (WHC) and bulk density (BD); and (2) evaluate four pedotransfer functions (PTFs) and the APSIM-biochar model for their ability to estimate soil water/physical parameters. Forty-eight deep soil cores were collected from fields with and without biochar applications in Iowa, USA, sectioned into five increments (0–5, 5–15, 15–30, 30–50 and 50–90 cm, n = 226 replicated samples), and analysed for texture, SOM, FC, WP, SAT and BD. For every 1% increase in SOM, the 0–30 cm WHC increased by 5 mm (or 12%). The rate of increase was similar (p = 0.427) for soils with and without biochar, indicating that biochar and biogenic SOM have a similar influence on WHC. Across all data sets, the tested PTFs captured observed data variability, with the Saxton and Rawls PTF having the lowest relative root mean square error (RRMSE). The APSIM-biochar model dynamically simulated changes in soil water parameters and BD response to biochar application with an average RRMSE of 8.3% (uncalibrated) to 6.1% (calibrated). We concluded that the Saxton and Rawls PTF is effective for predicting WHC of soils with and without biochar amendments and that the accuracy of WHC predictions can be improved with location/biochar type-specific calibrations.