Biochar particle size, shape, and porosity act together to influence soil water properties

Abstract

Many studies report that, under some circumstances, amending soil with biochar can improve field capacity and plant-available water. However, little is known about the mechanisms that control these improvements, making it challenging to predict when biochar will improve soil water properties. To develop a conceptual model explaining biochar’s effects on soil hydrologic processes, we conducted a series of well constrained laboratory experiments using a sand matrix to test the effects of biochar particle size and porosity on soil water retention curves. We showed that biochar particle size affects soil water storage through changing pore space between particles (interpores) and by adding pores that are part of the biochar (intrapores). We used these experimental results to better understand how biochar intrapores and biochar particle shape control the observed changes in water retention when capillary pressure is the main component of soil water potential. We propose that biochar’s intrapores increase water content of biochar-sand mixtures when soils are drier. When biochar-sand mixtures are wetter, biochar particles’ elongated shape disrupts the packing of grains in the sandy matrix, increasing the volume between grains (interpores) available for water storage. These results imply that biochars with a high intraporosity and irregular shapes will most effectively increase water storage in coarse soils.

Dynamic effects of biochar concentration and particle size on hydraulic properties of sand

“…The total pore volume of the mixture increased with increase in biochar fractions, as indicated by the higher water content at saturation (Table ). The result is consistent with the results of previous studies (Abel et al, ; Blanco‐Canqui, ; Liu, Dugan, Masiello, & Gonnermann, ; Omondi et al, ; Ulyett, Sakrabani, Kibblewhite, & Hann, ). The increase in pore volume is mainly attributed to the increase in macroporosity caused by the dilution effect of adding larger diameter and low‐density biochar amendment to sand (Herath, Camps‐Arbestain, & Hedley, ; Lim et al, ).…”

“…The control sand and 5% unsieved biochar treatment showed only a small difference in saturated and residual soil moisture content, indicating 5% unsieved biochar is not sufficient to improve water retention. We attributed the increase in water retention to the presence of interparticle and intraparticle pores of the added biochar particles and their effect in altering the pore‐size distribution of amended sand (Liu et al, ). The creation of large macropores in the soil surrounding the biochar particles could increase total porosity of biochar‐amended soil (Hardie, Clothier, Bound, Oliver, & Close, ).…”

Surface oxygenation of biochar through ozonization for dramatically enhancing cation exchange capacity

“…Recently, it was reported that biochar particle size, shape, and porosity act together to influence soil water properties (Liu et al 2017). Therefore, our observation here indicated that the ozone treatment may have certain relatively minor effects on these physical and morphological properties in relation to the water-retention field capacity.…”

Biochar Phosphorus Release Is Limited by High pH and Excess Calcium

“…To assess the time‐dependent release of P from the rice husk biochar, the RH 700 material was reduced to smaller minimum dimension (0.5–1 mm) using a sieve and extracted using unbuffered DI water. This reflects biochar that has been physically fragmented through exposure to physical, biological, and chemical processes (Liu et al, 2017). A nonionic surfactant, 2‐octanol (98% for synthesis, Merck) was included, to address any short‐term effects of hydrophobicity that rapidly diminish in soil, and consequently to reflect long‐term P leaching conditions (Ojeda et al, 2015).…”