Biomass or biochar – Which is better at improving hydraulic properties?

Authors: Spokas, Kurt; Watts, Donald - Don; LEE, TAE-JUN - Rural Development Administration - Korea; Weis, Rena; Novak, Jeffrey; Feyereisen, Gary; Ippolito, James

Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/10/2015
Publication Date: 11/21/2016
Citation: Spokas, K.A., Watts, D.W., Lee, T., Weis, R.D., Novak, J.M., Feyereisen, G.W., Ippolito, J.A. 2016. Biomass or biochar – Which is better at improving hydraulic properties? Acta Horticulturae. 1146:235-242. doi: 10.17660/ActaHortic.2016.1146.31.

Interpretive Summary: Biochar additions have been claimed as an amendment to improve soil’s water holding and water transport abilities. However, compost (raw biomass) has also been observed to improve soil moisture characteristics. Therefore, the question exists which one is better and does the improvement with biochar justify its conversion. To answer this question we evaluated pine chips both raw and those converted to biochar on their impact on both soil moisture retention and hydraulic conductivity changes. What we observed is the initial impacts on soil water moisture properties are determined by the particle size of the amendment and there were no major differences in the behavior observed between the biochar and raw amendments. However, since the biochar is more resistant to soil microbial degradation there is the unresolved question of whether these impacts will last longer in the biochar amended soils. These results are significant to farmers and policy makers and will assist scientists and engineers in developing improved models for predicting the impacts of biochar on soil hydraulic properties. These results are vital in determining the impact of biochar on soil moisture relationships, which are particularly critical for irrigated agricultural soils.

Technical Abstract: Different physical and chemical properties of biochar, which is made out of a variety of biomass materials, can impact hydraulic properties. The objective of this research was to evaluate the impacts of raw biomass compared to biochar additions of the same feedstock and their impact on hydraulic properties in soil mixtures. Both raw biomass (pine chips) and a corresponding pine chip biochar (slow pyrolysis; 2 hr; 500 oC) from the same feedstock were added to soil at four incremental particle size fractions (1-2 mm; 0.5 to 1 mm; 0.2 to 0.5 mm; and <0.2 mm). Furthermore, three other biochars were added to four different textured soils (coarse sand, fine sand, loam, and clay texture) to assess potential biochar specific effects. In addition, inert materials (glass and plastic beads) were also utilized to assess if the alterations in the hydraulic properties were due to the alteration in soil particle packing (tortuosity) or the addition of the intra-particle pore structure. The results showed that the Ksat of the biochar amended soils were significantly influenced by the rate, particle size, and type of biochar. In addition, these correlations were dependent on the original particle size of soil. The Ksat decreased when biochar was added to coarse and fine sands, as a function of the biochar rate. Biochar with larger particles sizes (60%; >1 mm) also decreased Ksat to a larger degree than the smaller particle size biochar (60%; <1 mm) in the two sandy textured soils. Biochar additions universally reduced the bulk density (P<0.05) and simultaneously reduced Ksat for sandy soils (P<0.05). On the other hand, for the clay loam 1% and 2% biochar additions universally increased the Ksat with higher biochar amounts (up to 5%) providing no further alterations. The increasing tortuosity in the amended sandy soil could explain this behavior, and was supported by the similar behavior observed with the inert solid materials. Therefore, the immediate alterations in the hydraulic properties of the amended soil are primarily a function of the particle size of the material, versus the type of material.