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ISHS Acta Horticulturae 1146: III International Symposium on Organic Matter Management and Compost Use in Horticulture

Biomass or biochar – which is better at improving soil hydraulic properties?

Authors:   K.A. Spokas, R. Weis, G. Feyereisen, D.W. Watts, J.M. Novak, T.J. Lim, J.A. Ippolito
Keywords:   hydraulic conductivity, particle size, pore size, soil amendment
DOI:   10.17660/ActaHortic.2016.1146.31
When amended to soils, both biochar and biomass impact soil hydraulic properties. However, the exact comparison between these two material forms is not known. The objective of this research was to evaluate and compare the impacts of raw biomass chips with biochar produced from the same feedstock. Both raw biomass (pine chips; Pinus taeda) and a corresponding pine-chip biochar (slow pyrolysis; 2 h, 500C) were added to a sandy-textured Ultisol at three different rates and four incremental particle-size fractions (1-2, 0.5-1, 0.2-0.5, and <0.2 mm). Results demonstrated that the immediate impact on hydraulic conductivity (Ksat) of the amended soil was influenced by application rate and particle size, with remarkable similarity between the two amendments. All additions significantly reduced both the soil bulk density and Ksat (P<0.05). These alterations in the hydraulic properties were postulated to be due to alterations in soil particle packing (i.e., tortuosity). Alterations in pore geometry, with blocking of larger macropores by the amendment, could explain this behavior, and this was supported by the similar behavior between raw feedstock and biochar of equal particle sizes. Thus, the immediate alterations in the hydraulic properties of an amended soil were primarily a function of the particle size of the material, regardless of whether or not the raw feedstock had been converted to biochar. With decreasing particle size, both additions increased water-holding capacity at saturation. This suggests that small-particle-size additions to a sandy-textured soil would reduce infiltration rates and net water gained per precipitation event due to the reduced soil moisture potential gradient and Ksat. However, the effects are a function of the amendment particle size distribution and the original soil texture.

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