ARS | Publication request: Augmenting soil water storage using uncharred switchgrass and pyrolyzed biochars

Submitted to: Soil Use and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 7, 2012
Publication Date: January 15, 2013
Citation: Novak, J.M., Watts, D.W. 2013. Augmenting soil water storage using uncharred switchgrass and pyrolyzed biochars. Soil Use and Management. DOI: 10.1111/sum.12026.

Interpretive Summary: Biochar is a solid product obtained through the thermal pyrolysis of raw crop residues. Biochar is a viable amendment because it can increase the soil’s ability to retain water. Biochar is expensive to use at the field scale, so we conducted research to find a less-expensive organic amendment that could deliver similar soil water retention improvements. In our evaluation, we incubated raw switchgrass in a sandy soil and compared the soil’s ability to retain water to biochar made from switchgrass. This was accomplished by repeatedly irrigating treated and untreated soils with water. After drainage had ceased, the mass of collected water was determined and the amount of water stored by each treated and untreated soil was determined. We found that all three amendments increased soil water storage relative to the control, but the raw switchgrass provided slightly higher improvements in water retention. Our results show that raw switchgrass applied to sandy soils are also effective at improving water retention as applied biochar. This finding could be appealing to include in soil water management programs for crop production in drier climates especially with agricultural fields containing degraded sandy soils.

Technical Abstract: Biochar is an amendment that can augment soil water storage; however, its projected cost per ton could be financially limiting at field application scales. It may be more monetarily convenient if an alternate amendment were available that could deliver similar soil enhancements. We compared two switchgrass biochars pyrolyzed at 250 and 500 degree Celsius (°C) with raw switchgrass (uncharred) on moisture storage and bulk density changes in a Norfolk loamy sand (Fine-loamy, kaolinitic, thermic Typic Kandiudult). Amendments were mixed into triplicate pots at 2% by weight along with untreated controls. Soils were laboratory incubated at 10% moisture content by weight for 118 days and the pots were irrigated three times with 1.3 pore volumes of deionized water every 30 days. Soil bulk densities were recorded before each irrigation event. Assessment of alterations in soil water storage was examined through cumulative water evaporative losses from incubation day 0 to day 33 and by monitoring soil water contents for 13 consecutive days past each irrigation event. Ranking of soil water evaporative losses were: uncharred switchgrass = switchgrass (500°C) = switchgrass (250°C) < control. After the first irrigation event, adding uncharred switchgrass significantly elevated moisture storage compared to soil treated with biochar and the control. While all amendments increased water storage relative to the control, uncharred switchgrass delivered equivalent, if not slightly better, moisture storage improvements compared with the two switchgrass biochars. Uncharred switchgrass would likely not be as effective over the long-term (years to decades) as pyrolyzed biochars due to greater degradation of uncharred material.