Biochar is being used as a soil amendment to improve soil hydraulic properties. It has been reported that biochar increases plant-available water, particularly in coarse-textured soils; however, effects on fine-textured soils are less pronounced. Here, we quantified the effects of biochar particle size on plant-available water in sand, silt loam, and clay soils. We amended these soils with a hydrophilic biochar, and packed soil-biochar mixtures into columns of 5.35-cm diameter and 3.0-cm height. Biochar was produced by gasification of woody forest residues. We applied the biochar at two different rates (0.5% and 2% by weight of C) and with four different biochar particle-size fractions (<4.75, <2.0, <1.0 and <0.5 mm). Our results show that biochar amendments increased the plant-available water (when expressed gravimetrically) of all three soils, particularly at the highest biochar application rate. Although the absolute increase in plant-available water was largest for the silt loam soil, the relative increase of plant-available water was greater for the sand than for the silt loam and clay. When expressed in volumetric terms, however, the effects on water retention were less pronounced: the sand showed a small increase in plant-available water with biochar amendment, and no change was observed with the silt loam and clay. Particle size of biochar had little effect on plant-available water (when expressed in gravimetric terms) in the three soils, but silt loam and clay showed increased plant-available water (when expressed in volumetric terms) with decreasing biochar particle size, while no effect was observed for the sand. While our findings corroborate many of the previously reported effects of biochar on soil-water retention, we show that the effects and their magnitude depend on whether the results are reported in terms of gravimetric or volumetric water contents. Because bulk density tends to decrease with increasing biochar application, so will the volumetric water content, and thus the magnitude of the effect on plant-available water will be smaller when expressed in volumetric water content.

生物炭被用作土壤改良剂以改善土壤水力特性。据报道，生物炭增加了植物可用水，特别是在质地粗糙的土壤中；然而，对细质地土壤的影响不那么明显。在这里，我们量化了生物炭粒径对沙子、粉砂壤土和粘土中植物可利用水的影响。我们用亲水性生物炭修正这些土壤，并将土壤-生物炭混合物填充成直径为 5.35 厘米、高度为 3.0 厘米的柱子。生物炭是通过木本森林残留物的气化生产的。我们以两种不同的比率（C 重量的 0.5% 和 2%）和四种不同的生物炭粒度级分（<4.75、<2.0、<1.0 和 <0.5 mm）应用生物炭。我们的结果表明，生物炭改良剂增加了所有三种土壤的植物可用水（以重量分析表示），尤其是在生物炭施用率最高的情况下。尽管粉砂壤土的植物可利用水的绝对增加最大，但沙土的植物可利用水的相对增加大于粉砂壤土和粘土。然而，当以体积表示时，对保水率的影响不那么明显：沙子显示出使用生物炭改良剂的植物可利用水略有增加，而粉砂壤土和粘土则没有观察到变化。生物炭的粒径对三种土壤中植物可利用水（以重量计）几乎没有影响，但是粉砂壤土和粘土显示出随着生物炭颗粒尺寸的减小，植物可用水增加（以体积表示），而对沙子没有观察到影响。尽管我们的发现证实了生物炭对土壤保水的许多先前报道的影响，但我们表明，这种影响及其影响程度取决于是否以重量或体积水含量报告结果。由于体积密度随着生物炭施用量的增加而趋于降低，因此体积含水量也会降低，因此当以体积含水量表示时，对植物可利用水的影响程度会更小。我们表明，影响及其大小取决于结果是以重量含水量还是体积含水量报告的。由于体积密度随着生物炭施用量的增加而趋于降低，因此体积含水量也会降低，因此当以体积含水量表示时，对植物可利用水的影响程度会更小。我们表明，影响及其大小取决于结果是以重量含水量还是体积含水量报告的。由于体积密度随着生物炭施用量的增加而趋于降低，因此体积含水量也会降低，因此当以体积含水量表示时，对植物可利用水的影响程度会更小。