Table 1.
Descriptions of silica sand and biochar used in this study.
Table 2.
Particle size, skeletal density (ρs), and envelope density (ρe) of sand and biochar used in this study.
We report average and standard deviation of at least three measurements.
Table 3.
Bulk density (ρb), total porosity (ϕT), and saturated water content (θs) of samples for measuring water retention curves by the Hyprop device.
We report average and standard deviation of at least three measurements.
Table 4.
Bimodal van Genuchten model parameters and goodness of fit include: the weighting factors of soil water retention curve for interpores (w1) and intrapores (w2), the inverse of the air entry pressure for interpores (α1) and intrapores (α2), the measure of the pore-size distribution for interpores (n1) and intrapores (n2), R-square (R2) and root-mean-square error (RMSE).
Table 5.
Median diameter of particles’ shortest chord (Dmin50) and particles’ aspect ratio (AR defined as Dmin50 divided by Dmax50) of biochar and sand used in this study.
We made measurements through dynamic image analysis (Camsizer, Retsch Technology, Germany).
Fig 1.
Comparisons of water retention curves (water content, θ, versus soil water potential, ψ) between sand and sand plus 2 wt% (a) fine (b) medium and (c) coarse biochar showed that biochar addition increased water content at given soil water potential. Data indicated with the dots were measured by the Hyprop and the WP4C and data indicated by the lines were fitted by bimodal van Genuchten model (VGbi). We report average and standard deviation of at least three measurements.
Fig 2.
Field capacity (θfc), permanent wilting point (θpwp), and plant available water (θpaw) of sand, sand plus 2 wt% fine, medium, and coarse biochar indicated that θfc, θpwp, and θpaw increased with biochar addition as well as biochar particle size.
Values and error bars for θfc were the average and standard deviation of at least three replicates conducted for each treatment. Values for θpwp and θpaw are only one replicate. Error bars of θpaw are the same as error bars of θfc.
Fig 3.
(a). Measured water retention curves (water content, θ, versus soil water potential, ψ, measured by the Hyprop) and (b) bimodal van Genuchten model (VGbi) of data from Fig 3A. Sand, fine sand plus sand (volume of fine sand is equal to volume of fine biochar at 2 wt% biochar rate), and coarse sand plus sand (volume of coarse sand is equal to volume of coarse biochar at 2 wt% biochar rate). These three curves overlapped with each other indicating that addition of small fraction of different sizes of sand did not cause significant change in soil water retention at such low rate.
Fig 4.
Negative Pearson correlation coefficients (R) between bulk density (ρb) and (a) initial water content (θi), (b) field capacity (θfc), (c) permanent wilting point (θpwp), (d) plant available water (θpaw) and positive R between total porosity (ϕT) and (e) initial water content, (f) filed capacity, (g) permanent wilting point, (h) plant available water showed that soil water retention decreased with ρb increase but increased with ϕT increase.
Fig 5.
Schematic of (a) and (b) sand (dark gray); (c) and (d) sand plus medium biochar (black) on a plot of water retention curves for these two samples. Pores inside of biochar particles were filled with water (light gray) thus increased in water content at saturation as well as field capacity.
Fig 6.
Schematic of (a) and (b) sand (dark gray) plus fine sand and (c) and (d) sand plus fine biochar (black) on a plot of water retention curves for these two samples. Biochar particles are more elongated which creates more pore space when packing. This may increase the distance between particles resulting in increased of interporosity. Sand plus fine biochar had a higher water (light gray shade) content than that of sand plus fine sand at higher soil water potential, probably due to its higher interporosity. However, the two water retention curves merged at lower soil water potential values (less than -33kPa) indicating that the intrapores of the fine biochar does not contribute to soil water retention as discussed in section 4.4.