Often, PGPR must be repeatedly added to a soil to maintain high population densities. One of the greatest research gaps limiting this field is inconsistent results when cultures are taken from lab to field. To better manage the ecosystem function of PGPR it is critical to study the survival and activity of both inoculated and native soil microorganisms in non-sterile soil.Carrier materials can influence inoculum success by providing protective habitats and also by affecting soil aggregate formation, which provide protection from predation . Soil inoculants are commonly prepared in formulations of powders, granules, and liquids . Previous research has evaluated the use of carrier materials to improve survival and distribution of inocula, much of which was conducted for soil inoculation with rhizobia . Peat moss is commonly used a microbial carrier and its benefits increase markedly if sterilized prior to inoculation . Vermiculite, lignite, and sodium alginate encapsulation have all been studied as alternative carriers to peat. Also,hydroponic gutter charcoal maintains high populations of inoculum suitable for use after 280 days of storage .
While this information has existed for years, very little work has been continued to examine the suitability of charcoal as a carrier for microorganisms.There is a variety of well-studied carrier materials, all of which have limitations that restrict their widespread use. For example, peat has had success in the rhizobia industry, but it is a highly variable material and the extraction of peat from bogs is unsustainable . The process of obtaining vermiculite also requires mining, which is associated with negative environmental impacts. As a result of limited sources, both peat and vermiculite are sometimes unavailable in regions where they are not naturally present . Alginate encapsulation of inoculum preparations offers a promising alternative to liquid inoculum, clay, and peat formulations. However, production of alginate beads is more expensive than the alternatives . Although the technology behind bioencapsulation has been reported since the 1980’s, the c rrent devices that prod ce the beads are still predominantly limited to lab-scale .Although countless studies have been performed on plant responses to soil inoculants, far less research has focused on inoculum preparation. A literature survey by Xavier et al.found that less than 0.5% of publications that cover rhizobia research discuss inoculum formulation. Many factors will differentially influence the survival of inoculum when applied to soils with diverse physical and chemical characteristics. For example, several abiotic soil parameters affected the survival of Azospirillum brasilense in bulk soil, including soil texture, water holding capacity, soil nitrogen, and organic matter . Accordingly, for PGPR to be considered a successful means to address major agricultural challenges, several things must be considered.
Carrier materials must be affordable, sustainable, and widely available. Also, the formulation of the inoculum must ensure that high PGPR populations can be stored and distributed into soils, and that once applied to soil, the PGPR can thrive, colonize roots, and commence plant-beneficial activities.Although biochar is a relatively new term to the scientific community, it is rooted in an ancient tradition of native Amazonians. Their method for disposing organic wastes involved heating it in deep earth pits, under low levels of oxygen. This left behind carbon rich “Terra Preta”, or black earth, which is still stable hundreds of years later and has proven to be an excellent soil amendment . Terra Preta soils contain high concentrations of nitrogen, phosphorus, potassium, calcium, and stable organic matter . These highly fertile pockets of soil are a stark contrast to the acidic indigenous soils, which are low in nutrients and organic matter and considered to be incapable of supporting agriculture . Interestingly, similar soil patches can be found throughout the world, and their high fertility is consistently associated with an abundance of black carbon . The use of charcoal also has a long history in agriculture and has been used to promote agronomic productivity for centuries . This tradition can be modernized by way of pyrolysis machinery specially engineered to heat solid or liquid biomass at designated temperatures in zero to low levels of oxygen. The resulting product has been termed biochar and biochar materials are receiving a lot of attention from scientists, engineers, farmers, and entrepreneurs .Given the current annual increase in atmospheric carbon dioxide of 4.1 tons year-1 , reduction of anthropogenic greenhouse gas emissions is critical .
Products of pyrolysis are carbonaceous and recalcitrant and can be incorporated into soils thereby serving as a stable carbon sink and climate change mitigation strategy . Biochars have been estimated to have mean residence times in soils of temperate climates of about 2000 years whereas fresh organic matter may be degraded in less than a decade . Pyrolyzing waste materials typically sequesters 50% of the source carbon as compared to traditional slash-and-burn techniques, which sequester only 3%, and natural decomposition, which retains 10– 15% . Biochar production has been credited as a tool that could offset 12% of anthropogenic carbon dioxide carbon equivalents annually if implemented on a global scale . Therefore, biochar production from all bio-wastes offers a sustainable mechanism for land and waste management while providing a carbon negative system.The process of pyrolysis generates porous, charred particles that structurally resemble the parent material but have carboxylated aromatic cores with slight negative charge . During pyrolysis a lack of oxygen available to the system results in a residual material rich in carbon. As pyrolysis temperatures are increased the resulting biochars become increasingly aromatic as oxygen-bound functional groups escape. NMR- based diagrams of slow and fast pyrolysis chars prepared with pyrolysis temperatures of 500°C, and gasification produced char generated at 750°C, display typical aromatic clusters that comprise biochar materials . Chen et al.used elemental analysis and FTIR to examine the sorption behavior of eight pine needle biochars produced at pyrolysis temperatures ranging from 100°C to 700°C. Pyrolysis temperatures up to 300°C mark the initial removal of OH, aliphatic C-O, and ester C=O groups from outer surfaces of such structures. At 400 ° C there is complete destruction of aliphatic alkyl and ester C=O groups that shield the aromatic core. At temperatures above 500 °C, there is further removal of aromatic COand phenolic –OH groups. The removal of these outer groups and exposure of the aromatic core is a key determinant of the sorption behavior and cation exchange capacity of the biochar . In general,u planting gutter for lignocellulosic materials, carbonization increases as combustion temperatures rise to 500°C and the materials approach full carbonization as temperatures reach 1000°C .The exposure of aromatic structures lends these biochars the property of having many fine pores. The abundance of nano to meso-sized pores contributes to the large surface areas associated with higher-temperature chars as well as to their enhanced ability to adsorb non-polar compounds . Biochar materials often retain the cellular structure of the feedstock, which can provide and intricate network of pores on the order of tens of micrometers in diameter . Hence, factors such as feedstock and pyrolysis conditions, the highest treatment temperature achieved during pyrolysis in particular, will affect the porosity, specific surface area, cation exchange capacity , and adsorptivity of the resulting biochar. This allows the opportunity to derive biochar materials with optimized properties for specific uses, such as agronomy, soil remediation, water filtration, or soil inoculum delivery. Meta-analyses of publications containing biochar field trials and greenhouse studies reveal that biochar application resulted in average increased above ground biomass ranging from a conservative 10% to 30% and a recent meta-analysis, which incorporated publications up to April, 2013 demonstrated an overall mean increase in crop productivity of approximately 11% with biochar application . From each of these meta-analyses it becomes apparent that biochar will have different effects on plant biomass and yield that is influenced by soil and biochar characteristics, application rate, crop variety, and time post amendment.
These reviews concur that biochar had the greatest positive influence on crop productivity when incorporated into acidic, clay, or sandy soils with low water holding capacity or low organic matter . The economic cost associated with biochar production and application has become a major limiting factor to its wide-spread use . Estimates by Brown et al.projected that current biochar production will only be profitable if bio-oil is simultaneously generated and carbon offsets have values in the range of 20 USD or more per metric ton. Shackley et al.provided a total assessment of the costs and benefits associated with biochar deployment and determined that biochar production costs are best reduced when feedstocks are waste feedstocks that would otherwise have a gate fee or landfill charge associated with their disposal. The greatest economic benefits associated with biochar use were related to energy production, as agronomic cost benefits can be highly inconsistent . Hence, adjustment to biochar products to ensure greater agronomic benefits could transition biochar use into a profitable sector. It is important to gain a fundamental understanding of how biochar application will affect soil biota before its use can be recommended on a broad scale. When applied to soils, biochar has many effects on soil physical and chemical properties that, in turn,affect the properties of the soil as a habitat for microbial growth. To date several exploratory studies have assessed the response of bacteria, fungi, and enzymes to biochar incorporation in a soil. In Figure 1.5, scanning electron micrographs depict clear images of fungal hyphae extending into biochar pores and bacterial cells located on char surfaces . Furthermore, microscopic, chromatographic, and spectroscopic studies have shown root hairs entering water-filled macropores or bonding to biochar surfaces . At this interface the biochar particles can adsorb organic compounds released from growing roots. Thus, biochar pores may serve as an ideal microenvironment for biological activity. However, contrary to these findings Quilliam et al.report minimal colonization of biochar by native soil microorganisms 3 years post 2– 4% amendment with a wood-derived biochar. In examining the influence of biochar on native soil bacteria many investigations have focused on diversity profiles. In a pioneering study, Kim et al.compared Terra Preta soils to adjacent pristine soils and found the Terra Preta to contain 25% greater bacterial species richness and hundreds of novel bacteria taxa . Also, Kolton et al.found that changes in bacterial community structures were observed after soils were amended with a fresh, citrus wood-derived biochar. Pyrosequencing of 16S rRNA genetic markers revealed a decrease in the total numbers of proteobacteria when biochar was added to a soil and an increase in bacteroidetes, and particularly flavobacteria . These are noted chitin degraders that secrete antifungal compounds. Nielsen et al.utilized an ultra high-throughput sequencing platform to obtain 16S rRNA gene sequences of bacteria with low abundance, as low as 0.01% of the total population. Their results agreed with previous findings, that biochar applications resulted in shifts in abundances of various taxonomic groups and also indicated that taxa correlation patterns are altered with biochar application . A phospholipid fatty acid analysis by Steinbeiss et al.revealed that once incorporated into soils, biochar prepared from a protein rich feedstock selected for fungi while biochar derived from a cellulose-based feedstock selected for bacteria. In agreement with this finding, another recent study demonstrated a positive correlation between the C:N ratio of biochar-amended soils and soil total PL A’s and bacterial PL A’s, in particular . However, Jindo et al.report a negative correlation between C:N ratio and bacterial biomass in biochar-compost mixtures. Additionally, biochar products, particularly those prepared at low pyrolysis temperatures, commonly contain a large number of adsorbed volatile organic compounds that may affect microbial growth and plant responses to biochar . Soil enzyme activities are differentially affected by biochar application . Most notably enzymes with enhanced activities in alkaline conditions showed higher activity post biochar amendment . This is reasonable, as pH’s of many biochars tend to be alkaline, a property that is dependent on pyrolysis temperature. Furthermore, Harter et al.used molecular techniques to assay abundance and expression of bacterial genes involved in nitrogen cycling as affected by biochar application. They found that N2O reductase transcript numbers were increased when soils were amended with 10% biochar . This altered microbial activity is especially important when considering greenhouse gas emissions and reduced N2O emissions from soil, a common phenomenon reported with biochar application .