(120b) Evaluation of Designer Biochars to Ameliorate Select Chemical and Physical Characteristics of Degraded Soils

In the southeastern USA Coastal Plain region, soils have low fertility and poor physical characteristics. Poor soil quality is partly due to their age and environment. The soils are classified as Ultisols, 0.5 to 5 million years old, and their environment leads to rapid degradation because of high annual rainfall (1200 mm) and warm soil temperatures (annual mean 17ºC). Ultisols typically exhibit low soil organic carbon contents (SOC, 0.2 to 1.0%), acidic pH values (4.5 to 5.9), and low cation exchange capacities (CEC, 2 to 8 cmolc kg-1). Loss of SOC has also resulted in these soils exhibiting poor structure and high penetration resistances both of which contribute to low water holding capacities and poor rooting environments. These soil characteristics severely limit agricultural management and productivity.

Fertility problems associated with southeastern Coastal Plain Ultisols are similar to Oxisols, so often associated with tropical rainforests. The Oxisols poor soil fertility raises concerns about the sustainability of agriculture in the tropics and has spurred the development of management practices to restore or improve their fertility status (Glaser et al., 2002). Applications of compost/mulches to Oxisols can improve fertility, but the effects are short-lived because high rainfall and temperatures hastens decomposition (Tissen et al., 1994). Applications of biochar (charcoal produced by pyrolysis of biomass feedstock) to infertile Oxisols in the Amazonian basin has been shown to provide longer-lasting improvements in the fertility of Terra Preta soils (Glaser et al., 2002; Steiner et al., 2007). Some biochars can last for millennia in Terra Preta Oxisols because they are composed of condensed poly-aromatic C structures; characteristics that cause resistance to microbial mineralization. In contrast, biochars composed of single ring aromatic structures and aliphatic C will mineralize more rapidly (Lehman, 2007). Overall, repeated biochar addition to the Oxisols by pre-Amazonian inhabitants acted as a soil conditioner, improving soil physical properties and nutrient use efficiency, thereby increasing plant growth and productivity (Lehmann, 2007). In fact, today the Terra Preta soils have become a valuable commodity; they are sold as potting mixtures to greenhouses and as soil conditioners to regions containing infertile agricultural soils.

Biochars are known to improve the fertility and physical problems of South American Oxisols; however, adaption of this technology to southeastern Coastal Plain Ultisols requires an understanding of the characteristics of both the biochars and the soil to be improved. When biomass is pyrolyzed, it produces biochar, bio-oil, and gasses. Biomass pyrolysis at high temperatures (500 to 700ºC) causes breakdown and removal of plant material like lignin, cellulose, hemi-cellulose, and oils, which can be later recaptured as bio-oil and gases. Under these harsh conditions, the residual biochar is often composed of C (58%) in polymeric aromatic structures that are recalcitrant to degradation by soil microbial communities (Novak et al., 2009). The recalcitrant nature of biochar is an important characteristic if the key goal is to sequester C for millennia in the highly stable SOC pool.

On the other hand, if the goals are to improve soil fertility and also increase C sequestration, then pyrolytic conditions could be altered to produce biochars with more aliphatic and carboxylic acid character; both of which contain surface functional groups. Rutherford et al. (2004) reported that low pyrolysis temperatures (250 to 350ºC) of organic feedstock resulted in more acidic functional groups than at higher temperatures. This is an important concept when using biochars to improve soil fertility because the functional groups remaining on the biochar provide additional cation exchange sites which can retain nutrients while the biochar itself provides a readily oxidizable C source for microbial communities. Microbial communities are known to be involved in nutrient cycling processes as well as improving soil aggregation (Stevenson, 1994).

When we make biochars from organic feedstock, we propose that the pyrolytic conditions can be selected to design biochars with targeted characteristics either as a C sequestration amendment, as a soil fertility correction, or both. We have chemically manufactured eight biochars from four different organic feedstocks (pecan shells, switchgrass, peanut hulls, and poultry litter). All feedstocks were pyrolyzed at low (250 to 350ºC) and high (500 to 700ºC) temperatures. Initial chemical characterization revealed that biochars pyrolyzed at low temperatures had less ash contents and %C, and much higher %O and H contents than biochars produced at the higher temperatures. High temperature biochars were more alkaline because of the concentration of salts. The biochars structural and functional group composition are yet to be determined using a combination of 13C nuclear magnetic resonance, infrared spectroscopy, and titrations.

The eight biochars were mixed into pots containing a Norfolk loamy sand soil at 2 % (w w-1) and are being laboratory incubated at 10 % (w w-1) soil moisture content for up to 120 days. The CO2 flux from each pot is being measured (3x per week) and the pots will be leached every 30 days using deionized H2O. The leachates chemical composition, pH and electrical conductivity will be analyzed. The amount of water retained by each pot and soil strength will be measured to determine if improvements in water holding capacity and penetration resistance occurred. At termination, soil fertility characteristics along with shifts in soil microbial communities will be determined and statistically compared to initial conditions. Preliminary CO2 flux values after 18 days of incubation have shown distinct trends between treatments suggesting that biochar feedstock and pyrolysis temperature influence its mineralization. It is anticipated that results of this project will identify which pyrolytic conditions and feedstock type can be manipulated to design biochars that target specific soil chemical and physical issues in the Norfolk loamy sand.