**4. Discussion**

The above- and below-ground carbon sequestration of productive eucalypts worldwide depends on site conditions and management options, such as genotype, cultural intensity, planting density, and rotation length (**Table 14**). Several types of *Eucalyptus* have promise as SRWCs in Florida [39, 40], including cultivars, such as *E. grandis* G3 and *E. grandis* x *E. urophylla* EH1. EH1 on former citrus beds and managed at relatively low intensity, for example, could sequester over 20 Mg of C/ha/year. The Florida WB and dendroremediation estimates are influenced by their assumed planting densities. Plantations, though, have well-defined planting densities that offer more reliable carbon sequestration values. As the other Florida examples demonstrate, sequestration estimates vary due to tree age, size, management, and genotype. Longer first and coppice rotations may maximize sequestration [3].

Our carbon sequestration estimates for *E. grandis* and *E. grandis* x *E. urophylla* in Florida approximated their potential, as several assumptions were involved. Green weights for *E. grandis* x *E. urophylla* were derived from Florida field data by a speciesspecific equation from Swaziland [19]. Stem wood carbon content was an assumed percentage of green weight. Above- and below-ground sequestration proportions


#### **Table 14.**

*Comparison of estimated above-ground carbon sequestration (C, Mg/ha) by Eucalyptus species in Florida with and without BC to sequestration elsewhere under varied managements and ages (years).*

*Carbon Sequestration by Eucalypts in Florida, USA: Management Options Including Biochar… DOI: http://dx.doi.org/10.5772/intechopen.104923*

were based on *E. grandis* in Brazil [16]. Below-ground sequestration estimates assumed no soil C flux. Similar assumptions were used for sequestration estimates in South Africa [35] and China [36, 37].

In combination with the carbon sequestered in trees, cost estimates of sequestration in *Eucalyptus* plantations by using wood BC as a soil amendment were previously estimated at ~\$5/Mg of BC added per ha [7]. Using the intensively managed *E. grandis* plantation with 4305 trees/ha (**Table 4**), a single planting cycle, and three coppices, the estimated cost for using wood BC at \$750/ton as a soil amendment to accelerate sequestration is ~\$4/Mg of C sequestered. If a second planting cycle is included, the with and without BC cost comparisons are very similar. In a scenario with a minimum of two planting cycles and BC less than \$650/Mg, there is an economic incentive to use BC as a soil amendment to accelerate and increase carbon sequestration. These costs are less than the \$30–50/ton estimated in 2005 for US forestry sequestering up to 500 million tons of C/year [41]. In 2015, the California Air Resources Board listed C sequestration credits at \$12–13/ton [42].

Converting woody biomass into long-term forest products, such as BC, can be a critical component of carbon sequestration. BC produced from hardwoods has a soil residence time exceeding 1000 years [43]. In South Africa, carbon sequestration by *Eucalyptus* and their long-lived forest products may equally result in offsetting some 2% of the country's carbon emissions [35].

Because BC quality influences BC impact on soil properties and plant productivity, Study 5E used GCS' premium BC, which was produced from roundwood, was highly porous, and had high carbon content (93–95% fixed carbon on a dry weight (DW) basis), low ash content (2–3% DW), and high surface area (585–630 m<sup>2</sup> /g).

BC enhances the nutrient properties of Florida's sandy soils as well as the nutrient status of *E. grandis*, especially when applied together with organic amendments, such as GE and/or chemical fertilizers. However, because soil C may decrease as *Eucalyptus* plantations mature [35], BC incorporation into plantation soil can be beneficial. BC application to the soil in Poland is viewed as an important component of the region's circular economy and means of counteracting climate change [44].

The relatively low levels of BC in Studies 7F and 7G had minimal impact on yield. Because both compost and BC improve soil physical properties (waterholding capacity, soil structure, and bulk density), soil chemical properties (cation exchange capacity and plant nutrient availability), and soil biological properties (microbial activity), they could, at higher levels, potentially mitigate symptoms of citrus greening, such as asymmetrical chlorosis of the leaves, foliar micronutrient deficiencies, root degeneration, leaf, and fruit drop and eventually dieback and sometimes death [45].

BC has benefited many crops. BC produced from *E. camaldulensis* increased critical soil properties and groundnut yield in Senegal [46]. BC applications have increased the yields of corn [47, 48], safflower [49], rice [50], cypress [51], and rubber [52]. BC-blended compost significantly improved crop quantity and quality in Europe [53]. In Florida, oak-derived BC as a soil amendment combined with standard fertilizers enhanced lettuce (*Lactuca sativa*) productivity in a greenhouse study [7], and Studies 7A-7E suggest that plant and soil nutrients may be enhanced by GE, BC, and/or BC + GE applications.

The SRF GE has also been used in several specialty crops, such as turfgrass, citrus, and landscape plants. Environmental concerns regarding quick release (soluble) fertilizers will continue to increase demand for SRFs like GE, which also add organic matter to the soil.

While BC soil amendments may generally enhance soil health and plant growth in forestry, agriculture, and other applications, responses will vary because BCs differ and are influenced by soil type, climate, vegetation, and management [54]. Agriculture is using BC to improve soil bulk density, root penetration, aggregate stability, water infiltration, water holding capacity or retention, nutrient leaching, pore distribution, organic matter, carbon sequestration, toxins and pollutants, soil disease pathogens, beneficial nematodes, nitrogen-mineralization rate and microbial biomass, respiration rate, and genetic diversity [55].

BC may remediate contaminated soils [56], restore degraded land, and increase agriculture efficiency and carbon fixation [57]. In Brazil, adding 4.2 t/ha/year of sugarcane BC in sugarcane fields could increase soil C by 2.35 t C/ha/year [58]. In European agriculture, BC + low input of nitrogen fertilizer provided the highest C sequestration (61.1 t CO2e/t of biomass) [59]. The renewed interest in biochar was stimulated by the discovery of high organic carbon and remarkably fertile soils in South America, especially Amazonia, that have been called "Amazonian Dark Earths or Terra Preta de Indio" (black Earth of Indians). These soils maintain fertility for years. Remarkably, these areas of the world are often characterized by low fertility and nutrient holding capacity. The fertility of the Amazonian Dark Earths is believed to be largely a consequence of charcoal/biochar applications by the indigenous tribes of the region and the benefits in the soils persisted for thousands of years.

BC is produced via pyrolysis, that is, heating wood in a very low oxygen environment to remove all moisture and volatiles, maximize carbon content, and minimize ash content while increasing porosity and maximizing surface area. BC pyrolysis technologies range from simple batches production techniques, such as open pits, mounds, and kilns, to continuous production systems using rotary kilns and retorts [7].

Given the trends toward sustainable business models and reducing the CO2 footprint of production systems, the type of technology employed is an important consideration in BC production. As one moves up the technology scale, BC producers have the ability to control greater portions of the production process. A simple batch technology has limited ability to control the pyrolysis process compared to continuous production systems. Some of the operating metrics producers may want to control pyrolysis temperature, residence time, combustion of volatiles, and energy capture. To sustainably produce BC, operators will want to control all of these items and more, including, emissions and the source of feedstock.

While there is value in producing BC in remote areas to help support local agriculture or possibly even for export, many of these operations are not sustainable supply chains over the long term. The least sustainable producers are where the virgin forest is harvested to produce BC in open pits, mounds, or kilns. To truly be sustainable, pyrolysis operations should capture all components of value including fully combusting the volatiles inherent in the feedstock, converting this to a usable form of what is bioenergy, and then utilizing that energy in other applications (**Figure 4**). GCS is committed to these goals and the sustainable production of BC.

GCS' operations capture and utilize all components of value in BC production. With a commitment to sustainability and to further improve efficiency, GCS has designed its pyrolysis operations to be continuous, minimize the use of electricity, and capture and convert all volatiles into usable forms of energy for other applications. With a sustainable BC production process, carbon sequestered will have a greater beneficial impact.

Interest in and demand for BC documented in 2020 [7] are still growing due to improved BC production techniques, but BC's multiple applications vary widely in potential market size, timing, competitiveness, and pricing compared to alternative *Carbon Sequestration by Eucalypts in Florida, USA: Management Options Including Biochar… DOI: http://dx.doi.org/10.5772/intechopen.104923*

products (**Table 15**). With the need to replace the substantial loss of soil carbon due to modern agricultural practices [60] and considering the emerging carbon cascades [61], the applications and future potential markets become quite large. There are growing opportunities to utilize BC for (1) soil nutrient and water retention, (2) remediation of contaminated soils and water, (3) filler in concrete, asphalt, and tires, (4) acoustic and thermal insulation in walls, ceilings, and floors, (5) carbon fibers

#### **Figure 4.**

*GCS' pyrolysis process with integrated heat capture and utilization.*


#### **Table 15.**

*Relative market, timing, competition, and pricing for BC applications.*

and polymers, (6) protection against electrosmog, (7) filtration media, and (8) heavy metal adsorption. Growing trends in developing sustainable supply chains and reducing societal carbon footprint will help accelerate the growth of many of these markets.
