**Abstract**

Many tree species worldwide are suitable for making biochar (BC), with planted eucalypts in particular being very productive and extensive. Above- and belowground carbon sequestration by Eucalyptus plantations depends on plantation management options. An intensively managed cultivar could sequester over 100 mt of C/ha at a cost of \$21–40/mt. BC production systems ranging in size from small mobile units to large centralized facilities and many kiln technologies influence the quality and price of the BC produced as well as the ability to control emissions. While BC from wood has many applications, its use as a soil amendment in forest plantations is appealing as a long-term sequestration strategy and opportunity to grow more robust trees and increase survival rates. Research in Florida USA and elsewhere addresses responses of forest and agronomic crops to wood BC soil amendments with and without other fertilizers. In combination with the carbon sequestered through tree growth, sequestration of 2.5 mt/ha of wood BC as a soil amendment in Eucalyptus plantations has estimated costs ranging from \$3.30–5.49/ton of C.

**Keywords:** biochar, trees, Eucalyptus, production systems, carbon sequestration, soil amendment

### **1. Introduction**

BC's multiple uses (www.biochar-international.org) and numerous benefits [1] include soil and crop improvement, carbon sequestration, retention of nutrients and water, reduced leaching, water purification as well as general and specialty industrial applications, and interest in and demand for BC are growing [2]. From a global BC market value of \$1.04 billion in 2016, the market is projected to grow at 13% annually to a value of \$3.2 billion in 2025 [3].

Focusing on Florida USA, we previously published on eucalypts'suitability as a BC feedstock and assessed BC's potential for improving soil properties, tree nutrition, and tree growth [4].

In this chapter, we review (1) the advantages of forest trees for BC by documenting the availability and relative suitability of major tree species, particularly eucalypts (*Eucalyptus*) and related species, (2) carbon sequestration by planted *Eucalyptus*, (3) BC production systems ranging in size and the associated quality of BC, (4) promising BC applications, (5) recent and ongoing BC research, and (6) carbon sequestration potential and associated cost of *Eucalyptus* plantations using wood BC as a soil amendment.

**3. Carbon sequestration by planted** *Eucalyptus*

*DOI: http://dx.doi.org/10.5772/intechopen.92377*

*Forest Trees for Biochar and Carbon Sequestration: Production and Benefits*

and harvest age (**Figure 1**).

**Planting density (trees/ha)**

**Table 2.**

**Figure 1.**

**29**

G3 on clay settling areas

EH1 on former citrus beds with intensive culture

*(trees/ha,THA) scenario by age (without BC application).*

*Eucalyptus* planting density trials have assessed the effect of stand density on biomass production. On former citrus lands and phosphate mined clay settling areas in central and south Florida, *E. grandis* cultivars can have maximum mean annual biomass increments (MAImax) as high as 78.2 green mt/ha/year with associated internal rates of return greater than 10% [13]. Through 81 months, the intensively managed *E. grandis E. urophylla* cultivar EH1 planted on former citrus beds at two planting densities yielded more at 2471 trees/ha than at 1181 trees/ha. Annual yield at 2471 trees/ha was over 58 green mt/ha/year in 3.7 years compared to 44 mt/ha/year at 5.0 years. However, planting density also inversely affected average tree Diameter Breast Height (DBH) as the higher planting density produced smaller trees.

To estimate carbon sequestration over a rotation in Florida, we applied carbon allocations for *E. grandis* in Brazil [14] and *E. grandis E. urophylla* in China [15] to Florida tree data. The resulting total carbon sequestration estimates ranged from 38 to 95 mt/ha at the time of peak annual accumulation (**Table 2**), with longer-term totals over 100 mt/ha in 6 years, again depending on cultivar, site, planting density,

Sequestration estimates by *Eucalyptus* elsewhere vary. *Eucalyptus* plantations in southern China sequestered 100 mt C/ha [16]. *E. urophylla E. grandis* planted in southern China accumulated >70 mt C/ha in 6–8 years [15]. In South Africa, 10 and 25-year-old *E. grandis* plantations may store 47 and 270 mt C/ha [17]. *Eucalyptus*

**(years) Stem (wood + bark) Crown Roots Total**

**Tree component Rotation age at MAImax**

*tereticornis* plantations may accumulate up to 129 mt C/ha in 4 years [18].

2533 61.2 4.3 6.8 72.3 4.3 5066 80.5 5.6 8.9 95.0 4.2 8841 32.3 2.2 3.6 38.1 3.4

1181 63.4 6.7 5.7 75.8 5.5 2471 64.5 6.9 5.8 77.2 4.7

*rotation age for cultivars G3 and EH1 at three and two planting densities, respectively.*

*Predicted carbon sequestration (mt/ha) by tree components at maximum mean annual increment (MAImax)*

*Cumulative total (stem + crown + roots) carbon sequestration (C, mt/ha) for each genotype planting density*

## **2. Forest trees for biochar**

Many tree species worldwide are suitable for making BC, with planted eucalypts in particular being very productive and extensive. Eucalypts are the world's most valuable and widely planted hardwoods (20 million ha in 2018 [5], up to 21.7 million ha in 61 countries by 2030 [6]) and have numerous potential applications [7, 8]. In Florida, several *Eucalyptus* species, including cultivars of *E. grandis* and *E. grandis E. urophylla*, have promise as short rotation woody crops (SRWC, [9, 10]).

BCs from *E. grandis E. urophylla* cultivar EH1, *Corymbia torelliana*, *E. grandis* cultivar G2, *E. amplifolia*, and *Quercus virginiana*, were similar and suitable for commercial BC production ([4], **Table 1**). Compared to high quality European *Quercus* spp., all five Florida trees were similar for recalcitrant carbon but higher in pH and water holding.

Other evaluations of BCs made from various woods and other feedstocks indicate that feedstock and pyrolysis condition influence properties important for using BC as a soil amendment [11, 12]. Since key objectives in BC production include minimizing the combustion of carbon, maximizing carbon content, and minimizing ash, consistency of feedstock and the production operating environment are imperative.


*\*\*Estimated at 80% of fixed carbon on a dry ash-free basis.*

### **Table 1.**

*Properties of BC made from Florida* E. grandis *cultivar G2, C. torelliana (CT),* E. grandis *E. urophylla cultivar EH1,* E. amplifolia *(EA), and* Q. virginiana *(Qv), and European Q. sp. (Qsp) test trees (adapted from [4]).*
