Impacts of biochar concentration and particle size on hydraulic conductivity and DOC leaching of biochar–sand mixtures

Abstract

8The amendment of soil with biochar can sequester carbon and alter hydrologic properties by 9 changing physical and chemical characteristics of soil. To understand the effect of biochar 10 amendment on soil hydrology, we measured the hydraulic conductivity (K) of biochar-sand 11 mixtures as well as dissolved organic carbon in leachate. Specifically, we assessed the effects of 12 biochar concentration and particle size on K and amount of DOC in the soil leachate. To better 13 understand how physical properties influenced K, we also measured the skeletal density of 14 biochars and sand, and the bulk density, the water saturation, and the porosity of biochar-sand 15 mixtures. Our model soil was sand (0.251-0.853 mm) with biochar rates from 2-10 wt% (g 16 biochar/g total soil x100%). As biochar (<0.853 mm) concentration increased from 0 to 10 wt%, 17 K decreased by 72 ± 3%. 18 1 Present address: Environmental Program, Colorado College, Colorado Springs, CO 2 Present address: Rm. 3307, French Family Science Center, Duke University, NC 2 When biochar particle size was equal to, greater than, and less than particle size of sand, we 19 found that biochar in different particle sizes have different effects on K. For a 2 wt% biochar 20 rate, K decreased by 72 ± 2% when biochar particles were finer than sand particles, and 21 decreased by 15 ± 2% when biochar particles were coarser than sand particles. When biochar and 22 sand particle size were comparable, we observed no significant effect on K. We propose that the 23 decrease of K through the addition of fine biochar was because finer biochar particles filled 24 spaces between sand particles, which increased tortuosity and reduced pore throat size of the 25 mixture. The decrease of K associated with coarser biochar was caused by the bimodal particle 26 size distribution, resulting in more compact packing and increased tortuosity. 27The loss of biochar C as DOC was related to both biochar rate and particle size. The 28 cumulative DOC loss was 1350% higher from 10 wt% biochar compared to pure sand. This 29 large increase reflected the very small DOC yield from pure sand. In addition, DOC in the 30 leachate decreased as biochar particle size increased. For all treatments, the fraction of carbon 31 lost as DOC ranged from 0.06 to 0.18 wt% of biochar. These experiments suggest that mixing 32 sandy soils with biochar is likely to reduce infiltration rates, holding water near the surface 33 longer with little loss of biochar-derived carbon to groundwater and streams. 34

Enhancement of physical and hydrological properties of a sandy loam soil via application of different biochar particle sizes during incubation period

“…Uzoma et al (2011) concluded that reduction of Ks in sandy soil after application of biochar was a result of increasing water retention capacity. These results together confirm that biochar amendment does influence hydraulic conductivity and the mechanisms that control it are largely physical, including the relative size of biochar and soil particles and their proportions (Liu et al, 2016) and swelling and grain segregation, leading to the clogging of pores, decrease in pore radii, and possibly a variation in bulk density and sample heterogeneity over the experiment (Barnes et al, 2014).…”

“…These fine particles can block pores and decrease Ks. Liu et al (2016) found that Ks decreased significantly (by 67%, p<0.01) when biochar particles (<0.251 mm) are smaller than sand particles (0.251-0.853 mm) and decreased by 15% (p<0.01) when biochar particles (0.853-2.00 mm) are bigger than sand particles. When biochar and sand particles sizes are comparable, no significant changes in hydraulic conductivity were observed.…”

“…2 tion conditions on soil properties; but to the best of our knowledge, very few experiments have carefully examined the effects of different sizes of biochar particles on soil physical and hydrological properties. Liu et al (2016) studied the impact of biochar concentration and particle size on hydraulic conductivity and dissolved organic carbon leaching of biochar-sand mixtures and only used one type of mesquite wood biochar prepared at 400 °C. Therefore, the objective of the present study was to investigate more detailed and comprehensively the effect of different charring conditions, feedstocks and sizes of applied biochar on numerous physical and hydrological properties of sandy loam soil.…”

Biochar and Soil Physical Properties

“…As the amount of biochar increased, bulk density decreased linearly (Głąb et al, 2016;Liu et al, 2016a), but in a few cases, it decreased quadratically (Rogovska et al, 2016). The study by Rogovska et al (2016) found that biochar application rates above 60 Mg ha -1 had a smaller effect on reducing bulk density than application rates below 60 Mg ha -1 .…”

Influence of wood-derived biochar on the compactibility and strength of silt loam soil

“…Soil particle density is 2.6 Mg m -3 regardless of its particle size, while for wood-derived biochar (WBC) with a particle size of less than 70 µm it ranges between 0.6 and 1.6 Mg m -3 , depending on the wood source (Hu et al, 2016;Yargicoglu et al, 2015). Due to the relatively low true density of biochar, its amendment to soils have been reported to decrease soil bulk density (ρ) (Abel et al, 2013;Andrenelli et al, 2016;Bayabil et al, 2015;Castellini et al, 2015;Liu et al, 2016;Obia et al, 2016). For example, Reddy et al (2015) found the specific gravity of silty clay, WBC (particle sizes < 420 µm), and silty clay amended with 5, 10, and 20% WBC to be 2.6, 0.81, 2.1, 2.0, and 1.8, respectively.…”