Encyclopedia
Please note this is a comparison between Version 1 by Amit Dhingra and Version 2 by Peter Tang.
Biochar (BC) is produced through the thermochemical decomposition of organic matter in a process known as pyrolysis. Importantly, the source of organic material, or ‘feedstock’, used in this process and different parameters of pyrolysis determine the chemical and physical properties of biochar. The incorporation of BC impacts soil–water relations and soil health, and it has been shown to have an overall positive impact on crop yield.
- agronomy
- sustainability
- organic fertilizer
- crop productivity
- soil acidification
- soil organic matter
- pyrolysis
- microbial activity
1. Introduction
What was prevalent in the 1960s holds true yet again—the world stands at a threshold where the availability of food is threatened, albeit for reasons different than six decades ago. The changing climate, deteriorating land and water conditions, and loss of biodiversity present unprecedented challenges for humankind [1]. At present, greenhouse gas (GHG) emissions are increasing rapidly, with carbon dioxide (CO2) levels rising more than 3% annually since the 2000s. These GHG discharges have a drastic impact on the climate, despite global efforts to reduce the emissions over the last few decades [2]. As a step toward reducing GHG emissions, more than 100 countries signed and ratified the Paris Climate Agreement, aiming to limit the increase in global temperature to 1.5–2 °C over the next 30 years [3]. The achievement of this target requires the swift adoption of carbon-neutral and carbon-negative technologies to limit the global GHG emissions to approximately 9.8 gigatons of carbon [4][5][4,5]. Several approaches are being considered for CO2 removal from the atmosphere, such as the adoption of bioenergy, direct carbon capture, afforestation and reforestation, the modification of agricultural practices, the use of bioenergy, and the direct infusion of recalcitrant carbon into the soil using biochar (BC) [6][7][8][9][6,7,8,9]. The longer-term sequestration of carbon into the soil using biochar is one of the potential carbon-negative approaches. As soils store twice as much carbon compared to atmospheric reserves and for longer periods, it has been hypothesized that increasing global soil organic matter stocks by 4 per 1000 (or 0.4%) per year in agricultural land can offset 30% of global greenhouse emission [4].
The agricultural and industrial revolutions, combined with unsustainable farming practices, have significantly affected global soil health. This is mainly a consequence of the type of fertilizer used in crop production. Earlier practices of using manure and compost replenished the soil organic matter (SOM) on a regular basis. However, the use of petroleum-derived chemical fertilizers is detrimental to SOM as they enhance the accumulation of salt and reduce microbial diversity. Fertilizers derived from the Haber–Bosch process contribute to more than 1% of total global CO2 emissions [10][11][10,11]. Soil health has further declined with the gradual acidification of arable lands and continual soil erosion negatively affecting crop yields throughout the world. While the use of compost and manure to enhance and maintain SOM is an option, it presents limitations due to the accumulation of organic pollutants, increased pathogen pressure, and leaching of excess nutrients into waterways, leading to eutrophication [12][13][12,13].
There is a long history of enriching soils with recalcitrant carbon practiced by indigenous farmers in different parts of the world. Black-earth-like anthropogenic soils known as ‘Terra Preta’ have been discovered in several regions of South America and Japan. These dark soils were amended with charcoal-like substances, generally referred to as biochar (BC), and possibly other amendments such as manure, which conferred enhanced fertility to the soil [14][15][14,15]. Chemical analysis revealed that the BC-treated areas contained 70 times more carbon than the surrounding soils, demonstrating its long half-life [16]. The enhanced fertility of these soils most likely resulted from increased SOM, higher pH, higher water-holding capacity, and high nutrient-holding capacity [16][17][18][16,17,18]. Due to the potential advantages of ‘Terra Preta’, several global efforts are afoot to recreate such soils. Biochar represents an organic soil amendment that improves soil quality for agricultural production [19].
2. Biochar and Soil
2.1. Biochar
Biochar (BC) is a carbon (C)-rich, porous material produced during the process of pyrolysis, which involves the thermochemical decomposition of organic matter in an oxygen-limited environment. Any feedstock, such as forest residue, agricultural by-products, and waste biomass can be converted into liquid fuels, gasses, and BC. The properties and yields of BC are highly variable depending on the rate of pyrolysis (fast/slow), feedstock, pyrolysis temperature, and retention time. Generally, slow pyrolysis with a heating rate of 5–20 °C per minute with higher residence time results in higher BC yield [20][21][22,23]. Fast pyrolysis with a higher heating rate (>100 °C/min) and lower residence time results in a higher yield of liquid fuel and reduced BC output [22][24]. Due to the complex nature of pyrolysis and diversity of feedstock, the final chemical and physical properties of BC vary. For example, a recent meta-analysis concluded that BC produced at higher temperature (600–699 °C) had a higher pH of approximately 9 compared to BC produced at lower temperatures (300–399 °C) with an approximate pH of 5 [23][25]. This observation was supported by another recent meta-analysis [24][26]. The higher reaction temperatures reduce the amount of aliphatic carbons, oxygenated functional groups, cation exchange capacity (CEC), and total content of N, H, and O. However, a higher temperature of production resulted in increased pH, amount of C fixed, total ash content, total C, and surface area of BC [23][25][26][25,27,28]. Ultimately, the bulk property and surface characteristics of any BC is determined by the feedstock source along with the pyrolysis parameters [21][27][23,29]. There remains a critical need to understand the characteristics of BCs produced from different feedstock, pyrolysis parameters, and the resulting relative impact on soil. In the following sections, recent research on BCs has been collated, and the effects of various BC regimens on soil physical properties, soil–water relations, soil organic matter, microbial activity, soil tilth and nutrient status, pH, crop productivity, biotic stresses, and abiotic stresses have been discussed.
2.2. Impact of Biochar on Soil
Physical Properties
Physical properties of soil, such as bulk density, porosity, and water retention are important variables that impact plant growth and development. Human intervention in agricultural practices causes soil compaction, which is one of the key factors affecting plant growth [28][30]. Soil texture also plays a key role. Soil compaction above 1.7 g cm−3 results in restricted root growth and limits access to water and nutrients [29][31]. As a consequence, the yields of many crops such as soybean and corn have been shown to be negatively impacted [30][31][32,33]. The threshold bulk density for impact on root growth varies, with clayey soils having a lower bulk density threshold.
Amending soils with biochar increases soil porosity while decreasing soil bulk density, which aids in the transport of water, nutrients, and gases. These alterations encourage root formation and increased microbial respiration [24][26].
2.3. Soil–Water Relations
Accessible fresh water supplies are becoming increasingly limited, and 70% of available fresh water supports crop irrigation [32][63]. Although biochar holds promise for improved hydrological functions, there are differing schools of thought regarding the role of BC in improving the long- term water-holding capacity of soil [33][64]. BC amendment has been reported to increase rainfall absorption and soil water-holding capacity, particularly in non-irrigated production regions [34][35][36][65,66,67]. However, the pre-existing physical and biochemical characteristics of the soil and the wide array of BC production parameters (feedstock inputs, pyrolysis temperatures, application methods, and geographical variables) ultimately determine the BC’s impact on water-holding capacity. In order to probe the influence of BC on water dynamics, initial experiments were performed with soil columns in greenhouses with the addition of farm or potting soils. Field studies are now becoming prevalent in peer-reviewed literature, particularly within the last 10 years.
The identification of key features that contribute to improved water retention could lead to an expanded role for BC in crop production. Overall, it was determined that feedstock selection and pyrolysis temperature, the most predictive variables impacting water status, impact BC surface chemistry and porosity, the latter of which is a major contributor to the water-holding capacity of BC [37][38][68,69]. Pore saturation is highly dependent on BC surface chemistry, which is affected by pyrolysis temperature. An increase in pyrolysis temperature volatilizes organic elements and thermally cracks the biomass, thereby rendering hydrophobic compounds more hydrophilic and increasing the overall BC porosity [39][70]. Conversely, BCs produced via low-temperature pyrolysis exhibit negative capillary pressure, inhibiting the hydration of the pore space [40][71].
BC was reported to increase the water-holding capacity in coarse and medium textured soils by an average of 51% and 13%, respectively [41][78]. This was attributed to a higher abundance of soil micropores resulting from the intrinsic microporosity of BC. However, a reduction in water-holding capacity was reported in fine-textured soils, which was possibly due to the overall decrease in micropores or occlusion of existing pores. Field studies of high-porosity BCs derived from softwood (600–700 °C) and walnut shell (900 °C) reported a temporary improvement of water-holding capacity; however, no long-term improvement was seen in BC-amended silty clay loam soils subjected to a corn–tomato rotation with conventional or organic production regimes [42][55]. Plant-available water in fine-textured soils could be enhanced through the management or manipulation of hydrophobic properties of BC, thereby improving BC–soil interactions [41][78]. For example, it has been reported that grapevine feedstocks subjected to low pyrolysis temperatures (approximately 400 °C) yield BC with a 23% higher available water content in clay soils [43][79].
2.4. Soil Tilth and Nutrient Status
Defining management approaches to increase the productivity of agricultural soils remains a priority as food demand increases and arable farmland decreases [44][80]. As a mineral-rich organic material, BC can be incorporated into agricultural soils, potentially serving as a slow-releasing fertilizer, positively affecting soil tilth and enhancing the nutrient status of agricultural soils [45][46][47][81,82,83]. The basis for this potential use lies in the unique porosity of BC, its facilitation of chemical and physical interactions between nutrients and the carbon material, and its strong intrinsic sorption properties. Due to the large surface area, porous microstructures, and negative surface charge, BC enhances nutrient retention in the soil. Furthermore, the nutrient retention properties of BC may significantly reduce irrigation or the rainfall-induced leaching of water-soluble minerals [35][48][66,84]. The slow desorption of the BC-sequestered nutrient elements may supply a steady rate of nutrient delivery, thereby alleviating the need for excessive fertilizer use. Together, these agronomic benefits to soil health may also mitigate freshwater eutrophication that results from fertilizer runoff, prevent pesticide contamination, and reduce the risk of environmental damage [49][50][51][85,86,87].
While composition varies based on feedstock and pyrolysis parameters, a universal characteristic of BC is that it is carbon-dense, which facilitates the retention of necessary plant nutrients such as N, P, K, Mg, Fe, and Ca [52][53][54][55][88,89,90,91]. Depending on the soil status and existing nutrient deficiencies, BCs can be custom-manufactured to replenish depleted nutrients. It has been demonstrated that BCs derived from different feedstocks possess variable amounts of beneficial plant nutrients [56][35][55][57][58][59][47,66,91,92,93,94]. The general characteristics of three major BC feedstock sources are as follows:
-
Organic waste feedstocks, such as animal manure and sewage sludge-derived BC, are rich in potassium and phosphorus, low in C levels, and low in surface area; additionally, eggshell-derived BC is elevated in calcium levels
-
Wood-based BC is high in organic matter and surface area, while low in CEC and N, P, and K levels
-
Crop residue-derived BC properties reside somewhere in between those of the two previous categories, with specific crops producing BC with different properties (e.g., wheat and rice BC is high in silicon content; soybean BC is high in N).
In addition to improving mineral nutrient retention, BC has a role in the amelioration of soil erosion and the improvement of overall soil structure [60][61][98,99]. A study utilizing hardwood (600 °C) BC at 15 and 30 t/ ha concentrations to amend clay-rich soils in incubation containers demonstrated improved soil aggregate structure and soil stabilization [62][100]. This is likely due to the interaction of carboxylic and phenolic functional groups on the BC surface, resulting in the formation of cation bridges and consequent BC–mineral complexes [63][101]. For example, microaggregates observed to form upon the incorporation of hardwood-derived BC (700 °C) into soil with application rates of 2.5% or 5% correlated with a 50–64% decrease in soil loss, respectively [64][102]. An additional study with oak wood-derived BC applied at a rate of 10 Mg/ha provided further evidence for the stabilizing effects of BC, with significant decreases in soil loss of almost 20% observed in a simulated rainfall experiment. In addition to improving soil retention, BC appeared to reduce the impact force from rainfall, thereby facilitating the reduction of particle detachment [65][103].
2.5. Soil Acidification
The expanding global incidence of soil acidification is concerning, with acidic soils (pH < 5.5) currently accounting for approximately 50% of arable land [66][67][104,105]. The excessively low pH of acidic soil results in reduced productivity and decreased crop fertility. The main causes of soil acidification include the use of ammonia-based fertilizers and low nitrogen-use efficiency. In soil, ammonia fertilizers are converted to nitrates and hydrogen ions. The hydrogen ions that are left over following the uptake of nitrates by crops or after nitrate leaching increase the soil acidity [68][106]. The removal of crop residue also accelerates soil acidification. An excessive reduction of pH leads to the increased solubility of soil-bound aluminum; thus, soil acidification generally leads to aluminum (Al) toxicity [67][105]. Aluminum toxicity, in turn, leads to deficiencies in phosphorus, calcium, magnesium, and potassium cations and contributes to impaired root growth.
In soil, H+ is produced through the aerobic conversion of ammonia to nitrate. Experimental results have demonstrated that BC amendment leads to decreased soil nitrification through the adsorption of NH3 and NH4 onto the BC surface. Soil amendment with wheat straw-derived BC (500 °C) led to reduced nitrification in cadmium-contaminated Ferralsol soil by decreasing soil acidity [69][119]. Similarly, amendment with pig manure-derived BC (300 °C) resulted in decreased soil acidification and increased cation exchange capacity [70][71][120,121], and crop residue-derived BC (500 °C) led to improved rice growth, yield, and soil nutrient availability in acidified soil [72][122]. Collectively, information from the literature has established that carbon content, nutrient availability, and alkalinity are highest when BC is generated from manure feedstock, intermediate when generated from crop residue feedstock, and lowest when generated from woody plants-based feedstock. Finally, biochar produced at higher temperature has higher pH and might be more suitable for countering soil acidity.
3. Biochemical Properties
3.1. Soil Organic Matter (SOM)
Soil organic matter comprises the total organic carbon in a soil and is the main determinant of overall soil fertility. SOM components consist of plant residue, animal waste, microbial populations, and active and stable organic matter in soil. SOM contributes to soil fertility by serving as a nutrient source for crops and microbes, causes soil aggregation, and improves water retention and nutrient exchange. It also helps to reduce soil compaction and surface crusting. It has been reported that the impact of biochar on SOM depends on the following variables [73][74][75][127,128,129]:
-
Type of biomass used for production of BC
-
Pyrolysis temperature
-
Pre-existing SOM levels in the soil
Amending soils with biochar often results in alterations in C cycling and mineralization, and this effect is known as ‘priming’. Previous studies have reported both positive and negative effects of priming. Grass-derived BC produced at lower temperatures (250 °C and 400 °C) resulted in positive priming resulting in increased C mineralization. However, BC produced at higher temperatures (525 °C and 600 °C) from hardwood resulted in negative priming [74][128]. It was hypothesized that negative priming resulted from the organic matter binding to the biochar and thereby becoming unavailable to microbial and enzymatic action.
3.2. Microbial Activity
Considerable emphasis has been placed on the topic of microbial dynamics in agricultural systems and their role in crop productivity. The health and diversity of soil microbial populations as a function of agro-ecosystem well-being has diverse implications for water-use efficiency, soil structure and stability, nutrient cycling, disease resistance, and eventual crop productivity [76][77][144,145]. While other organic amendments are only stable for relatively short periods in the soil environment, BC is more stable and remains in the soil for hundreds to thousands of years, as it is not easily degraded, and it could support soil microbial communities for an extended period of time with reduced inputs [35][66]. The diverse and specific physiochemical characteristics of BC that influence soil microbial composition are increased labile carbon, pH, surface area for colonization, and water content in amended soils. BC addition induces remodeling of the microbial diversity and community structure of the soil; however, the changes are highly variable and dependent on the individual soil properties [78][79][146,147]. It was reported that low pyrolysis temperature BCs (>350 °C) harbor a greater number of organic residues and are commonly characterized by lower pH. In contrast, at high temperatures (<600 °C), the abundance of organic moieties contributes to the production of a higher pH BC. It was concluded that pyrolysis temperature (and the BC-related characteristics associated with temperature) is the single most important factor that determines how the microbial communities are influenced [80][148]. Overall, there is a consensus that BCs foster the growth and maintenance of soil microbial communities [81][82][83][84][95,149,150,151].3.3. Abiotic and Biotic Stressors
3.3.1. Heavy Metals
Soil contamination with organic and inorganic toxins increases environmental and agricultural risks and poses a threat to both plants and humans. Efforts to develop remediation processes that bind the contaminants, limit their mobility and bioavailability, and foster improved soil health are ongoing. Currently, organic materials such as charcoal, soot, kerogen and activated carbon are used as amendments for limiting and reducing the bioavailability of multiple soil contaminants [85][86][172,173]. The organic contaminants have been shown to sorb preferentially to the carbonaceous fractions present in soil, limiting their bioavailability [87][174]. BC has also been shown to reduce the bioavailability of heavy metal contaminants. Several studies analyzed the effect of BC amendment on soils contaminated with heavy metals such as arsenic, cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) [88][89][90][175,176,177]. The high surface area of BCs results in more effective contaminant binding; however, one of the recent meta-analyses pointed out that the pyrolysis temperature at which a given BC is produced influences its remediation efficiency and type of contaminant that can be removed [88][175]. A majority of studies tested the effect of BC on Cd pollution and concluded that the higher BC surface area had a smaller effect on Cd bioavailability [88][175]. The BCs produced at lower temperatures (300–500 °C) have a higher density of functional groups, while BC produced at higher temperatures results in a larger surface area and lower density of functional groups. Another study revealed that BC produced from wheat straw at 450 °C, with a higher density of functional groups, was more effective in treating Cd and Pd-contaminated soil [91][178]. However, BC produced at higher temperature is also more alkaline and results in the immobilization of heavy metals in acidic soil via the liming effect. The addition of rice and wheat straw-derived BC in soils contaminated with Pb, Cu, Cd, and Zn led to a reduced mobility and bioavailability of the heavy metals, resulting in increased yields and a decreased enrichment of heavy metals in the tested plants [89][176]. Biochar is also used for the remediation of soil from contaminated sites due to rapid industrialization. It has been recently demonstrated that BC derived from pine wood was able to reduce the bioavailability of Cd, Pb, and Zn in metalloid-contaminated soils at a smelting site and promoted plant growth [92][179]. Biochar derived from hardwood (600 °C) was shown to be effective in reducing Ni and Zn by 83–93% in a historically polluted site in the United Kingdom [92][179].
3.3.2. Salt
Salt stress is known to negatively affect soil properties, plant development, and crop productivity due to disturbed soil structure, soil organic matter, microbial activity, and C:N ratio. Salt stress causes oxidative stress in plants, down-regulating antioxidant enzyme activity [93][192]. Due to excessive salinization, the sodium ions bind to cation exchange sites in soil, causing poor crop growth and yields. Although saline soil can be reclaimed by washing or excessively irrigating with water to remove excessive salts, it is neither economically nor physically feasible for large fields [94][193]. On the other hand, sodic soils require treatment with other cations such as calcium to remove excess sodium from cation exchange sites followed by the leaching of sodium [94][193]. The application of organic amendments such as manure or compost has been shown to improve soil fertility by reducing salt stress. In saline soil, the organic amendment improves soil porosity, leading to the leaching of excess salt. In sodic soil, organic amendments might help by improving the physical characteristics of soil, such as triggering cation exchange with the calcium present in organic amendment and Na present in soil.
3.3.3. Biotic Stress
Several recent reports have emerged showing BC to aid plants in countering biotic stresses. It has been suggested that BC-mediated nutrient retention, adsorption, pH adjustment, and increased water holding provides plants with the capacity to respond to pathogens and to counter the effect of toxic metabolites generated by plants [95][205].4. Impact of Biochar on Crop Production
Increasing crop yields to feed a burgeoning population is a daunting task in the face of a myriad abiotic and biotic challenges, including the reduction of arable farmlands and increased plant stressors due to the changing climate [96][97][98][212,213,214]. These issues are especially important in organic production systems where the average crop yields are 5–34% lower compared to conventional farming [99][100][101][102][215,216,217,218]. The use of biochar in soil remediation can be a useful strategy, especially in degraded soils [103][104][219,220]. Furthermore, the potential to significantly reduce the organic yield gap through better fertilization regimes has been proposed, suggesting an expanded role for nutrient-rich biochars [105][221].
A meta-analysis of BC effects on plant productivity concluded that BC use holds promise as a method to increase crop yields and could further promote ecosystem services and carbon storage [55][106][107][108][91,222,223,224]. It was noted that increased soil N, P, K, the reduction of soil acidity due to the liming effect of BC, and improved water relations contributed to various soil and crop responses. In Hereinthis review, a comprehensive literature search was performed in the Google Scholar search engine with the search terms “biochar crop productivity yield” for the years 2017–2019, which yielded 330 entries. These entries were further parsed using minimal criteria terms—BC feedstock source, pyrolysis temp, retention time, and soil type. The second round reduced the number of entries to 18. The results of the literature search are summarized in Table 1.
Table 1. Impact of BC on crop productivity summarized from a comprehensive literature search. Soil types listed in the table correspond to the types reported in the original studies. CEC: cation exchange capacity.
|
Crop Productivity |
Soil type, Experiment Type, Length |
Biochar Feedstock |
Pyrolysis Temp °C, Residence Time, Application Rate |
References |
|||
|---|---|---|---|---|---|---|---|
|
Crop Tested |
Productivity |
Beneficial |
Detrimental |
||||
|
Cherry tomato (Solanum lycopersicum) |
Bamboo BC increased tomato yields |
Both BCs improved tomato quality with increased total sugars |
Rice husk BC did not improve total N % |
Clay loamy |
Rice husk and bamboo |
500 |
|
|
Greenhouse |
1 h |
||||||
|
Short-term ≤ 1 year |
2% and 5% (w/w) |
||||||
|
Lettuce (Lactuca sativa) |
For both soils, BC rates of 20 and 30 t/ha−1 significantly increased above-ground biomass |
Effective fertilizer for lettuce production at least for two growing cycles |
Biosolid BC could increase harmful soil elements such as heavy metals |
Silty loam and sandy loam |
Fecal matter |
450 |
|
|
Greenhouse |
1 h |
||||||
|
Short-term ≤ 1 year |
10, 20, and 30 t/ha |
||||||
|
Chrysanthemum (Glebionis coronaria, cv. ‘Crown Daisy’) Leaf lettuce |
3% BC significantly decreased yields No effect |
BC increased WHC(water holding capacity) and SOM |
Higher BC application reduced plant productivity |
Pedocals, silt-clay |
Peanut shells |
350 |
|
|
Greenhouse |
3 h |
||||||
|
Short-term ≤ 1 year |
0%, 1.5%, 3%, and 5% (w/w) = to 0, 37.5, 75, and 125 t/ha in the field |
||||||
|
Beans |
Bean yields were significantly reduced with BC application |
Increased germination rate in BC-amended soils |
Significant decreases in some macro and micronutrients |
Krome loamy |
Melaleuca quinquenervia (Broad-leaved paperbark) hardwood |
350 |
|
|
Greenhouse |
7 h |
||||||
|
Short-term ≤ 1 year |
2% and 5% (w/w) |
||||||
|
Wheat (cv. ‘Yecora Rojo’) |
300 °C BC with NPK increased yields |
Increased soil water retention and decreased bulk density |
BC alone decreased yields with BC produced at higher temp° (400, 500, 600 °C) |
Loamy sand |
Date palm tree residues |
300, 400, 500, and 600 |
|
|
Greenhouse |
4 h |
||||||
|
Short-term ≤ 1 year |
8 t/ha |
||||||
|
Potatoes (Solanum tuberosum L., cv. ‘Russet Burbank’) |
No significant differences in yield |
BC increased soil CEC |
BC had no effect on leaf greenness rate or photosystem activity |
Sandy |
Green plantain peels |
450–500 |
|
|
Field Study |
18–25 min |
||||||
|
Long-term, 2 years |
13.5 t/ha (1% w/w) |
||||||
|
Tomato & Maize (Zea mays) |
BC does not have a significant long-term effect on yield |
Increased K+, Ca2+, and PO4-P in the soil in year 2 |
Delayed nutrient availability from BC and short-lived effects |
Rincon silty clay loam |
Walnut shells |
900 |
|
|
Field Study |
1–2 h |
||||||
|
Long-term, 4 years |
10 t/ha |
||||||
|
Winter wheat (cv. ‘Xiaoyan no. 22’) |
Low levels (1%, 2%) of BC had a positive effect on wheat yields |
Total nitrogen and SOC increased with BC applications |
Under drought conditions, BC addition decreased the availability of nutrients |
Silty-clay |
Apple wood |
450 |
|
|
Outdoor pot study |
8 h |
||||||
|
Short-term ≤ 1 year |
1%, 2%, 4%, and 6% (w/w) |
||||||
|
Maize |
BC and fertilizer led to a significant increase in maize yield |
BC improved soil water-holding capacity |
BC alone had no effect on maize yields |
Sandy clay loam |
Maize cobs |
500 |
|
|
Field Study |
1 h |
||||||
|
Short-term ≤ 1 year |
20 t/ha |
||||||
|
Chinese cabbage (Brassica rapa) |
BC significantly improved crop yields |
BC increased soil pH and CEC |
BC did not affect the soil bulk density and porosity |
Loamy |
Barley straw |
400 |
|
|
Field Study |
1 h |
||||||
|
Short-term ≤ 1 year |
10 t/ha |
||||||
|
Radish (Raphanus sativus L. cv. French Breakfast) |
Increased yields in second year |
Reduced bulk density and increased porosity, moisture content, soil pH |
No effect on first-year growth |
Alfisol or Luvisol |
Local hardwoods (Parkis biglosa, Khaya senegalensis, Prosopis africana and Terminalia glaucescens) |
580 |
|
|
Field Study |
24 h |
||||||
|
Long-term, 2 years |
25 and 50 t/ha |
||||||
|
Rice (cv. ‘Naveen’) |
Increased grain yield up to 24% |
Increased total organic C in soils |
Microbial carbon use efficiency decreased due to BC addition |
Sandy clay loam |
Rice husk |
350 |
|
|
Field Study |
6 h |
||||||
|
Long-term, 3 years |
0.5, 1, 2, 4, 8, 10 t/ha |
||||||
|
Maize (cv. ‘hybrid LG 6030’) |
Increased corn yields |
Increased P levels during the two years of cultivation |
BC was unable to supply the necessary K for further crop production |
Red-Yellow Latosol with clayey texture |
Sewage sludge |
300 and 500 |
|
|
Field Study |
30 min |
||||||
|
Long-term, 2 years |
15 Mg/ha |
||||||
|
Okra (Abelmoschus esculentus L., cv. ‘OH-397’) |
Increased yields vs. controls |
Significant increase in SOC and microbial activity |
Lower benefit cost ratios for BC compared to controls |
Inceptisol with sandy loam texture |
Mixed local hardwoods |
450 |
|
|
Field Study |
4 h |
||||||
|
Long-term, 2 years |
5 t/ha |
||||||
|
Rice (Oryza sativa L.) & Wheat (Triticum ssp.) |
Not affected |
BC amendment increased the soil water-holding capacity, soil nutrients, and SOC |
Short-term effects and BC alone did not increase yields |
Hydragric Anthrosol, sandy |
Wheat straw |
350–550 |
|
|
Field Study |
2–3 h |
||||||
|
Long-term, 6 years |
20 and 40 t/ha |
||||||
|
Sunflower (Helianthus annuus L., cv. ‘Embrapa 122/V2000’) |
Sunflower seed and oil yield declined |
Increased levels of most soil minerals and total carbon levels |
Nitrogen levels in leaves and the nitrogen uptake of the entire plant decreased with biochar application |
Dark red soil, Typic Hapludalfs |
Sugarcane bagasse and sunflower residues |
500–600 |
|
|
Field Study |
1 h |
||||||
|
Short-term ≤ 1 year |
1% (w/w) |
||||||
|
Spring barley (Hordeum vulgare L.) |
Increased yields with BC + NPK |
Increased soil water status in BC amended soils in the first year; increased soil carbon status |
BC only decreased yields for both crops compared to control NPK plants |
Sandy loamy silt; calcareous Chernozem on loess |
Hardwood |
550 |
|
|
Field Study |
2 h |
||||||
|
Sunflower |
No difference vs. controls |
Long-term, 2 years |
72 t/ha |
||||
|
Rice (Oryza sativa L.) & Wheat (Triticum ssp.) |
Not affected |
BC amendment increased the soil water-holding capacity, soil nutrients, and SOC |
Short-term effects and BC alone did not increase yields |
Hydragric Anthrosol, sandy |
Wheat straw |
350–550 |
|
|
Field Study |
2–3 h |
||||||
|
Long-term, 6 years |
20 and 40 t/ha |
||||||
|
Cauliflower (Brassica oleracea, cv. ‘Desire’) |
No significant improvement in crop yield |
No negative effects to crop productivity or soil quality |
Soil moisture and bulk density not affected by BC additions |
Ferralsol |
Woody Eucalyptus ‘Blue Mallee’ |
550 |
|
|
Pea (Pisum sativum, cv. ‘Ashton’) |
Field Study |
30 min |
|||||
|
Broccoli ‘Ironman’ |
Short-term, 1 year |
10 t/ha |
|||||
In terms of productivity alone, a majority of the studies reported a beneficial impact of BC on crop yields [119][109][110][113][116][117][118][120][121][122][53,225,226,229,232,233,234,235,236,237]. Experimental plants included lettuce, cabbage, radish, tomato, wheat, rice, maize, and okra. Soils were amended with BC derived from major feedstock sources such as hardwood, manure, and crop residues. Positive results from this mixture of plants and biochars indicate a theoretical system to ‘mix and match’ crop with BC for optimal productivity. Interestingly, none of the studies included perennial plant species in the experimental design. That is another area where the impact of BC remains to be assessed.
Due to the range of tested soil conditions, many factors altered by BC amendment were implicated in reported yield gains. For instance, lettuce yields were positively influenced with 20 and 30 t/ha fecal-derived BC (450 °C) [110][226]. Mineral-enriched BC proved to be an effective fertilizer for two growing cycles in the greenhouse pot study. Additional experiments in greenhouses with leafy crops proved that significant yield increases are possible with BC soil amendment [127][128][20,241]. Two studies with wheat showed increased yields as a result of soils amended with 1–2% apple wood-derived BC (450 °C) due to increased nitrogen levels [116][232] and increased soil water retention with 8 t/ha date palm tree residue-derived BC (300–600 °C) [113][229]. The increase in soil organic carbon and the stimulatory effect on microbial communities raised rice yields in soil amended with rice husk BC (350 °C) [120][235] and okra yields amended with hardwood-derived BC (450 °C) [122][237]. In addition to reporting increases in yields, these studies also discussed the limitations of field applications of BC.
BC contains key plant nutrients, although at a low level as demonstrated by several studies, and it may have led to the lack of a complete plant nutrient profile in the soils to obtain a desirable increase in yields [109][225]. Multiple studies reported mixed results in terms of crop production [111][125][227,239] or described no effect [123][114][115][126][166,230,231,240]. Soil nutrient content and CEC were improved with BC amendment but were short-lived and resulted in comparable crop productivity compared to controls in studies with rice and wheat growing in wheat straw BC (350–550 °C) [123][166], potatoes with green plantain peel-derived BC (450–500 °C) [114][230], and tomatoes and maize with walnut shell-derived BC (900 °C) [115][231]. The growth of Spring barley and sunflowers was tested with hardwood-derived BC (550 °C) at 72 t/ha. The treatments increased barley yields but had no effect on sunflower productivity [125][239]. The BC-only amendment did increase soil water status and carbon levels; however, increased barley productivity was noted only when BC was mixed with NPK compared to NPK-only controls. While increased water-holding capacity and soil carbon levels with peanut shell-derived BC (350 °C) were also reported [111][227], these alterations did not lead to any effect on lettuce yields.
Undesirable effects on crop productivity following BC soil amendment were also reported in two of the studies [112][124][228,238]. Although beans demonstrated an increased rate of germination in BC amended soils, their yields were significantly reduced with hardwood BC (350 °C) application at 2% and 5% [112][228]. Other studies with legumes reported a gain in yield when grown in BC-amended soils. The yields of mash bean improved with sugarcane bagasse BC (350 °C), with and without chemical fertilizer, due to the increased SOC, total N, and decreased bulk density. Importantly, nitrogen fixation increased by 83% in the biochar-only treatment due to higher nodule numbers [129][242]. Additionally, fava bean growth with wheat straw-derived BC (500 °C) amendment applied at a 2.5% w/w rate in addition to saltwater irrigation led to significantly increased dry seed yield compared to controls, which was mainly attributed to the high salt sorption capacity of BC [130][243]. The higher nutrient content in the crop residue-derived BCs reported above may have helped elevate yields compared to controls, while the already nutrient poor hardwood-derived BC may have reduced bean yields. While BC can be a source of nutrients, the complex interactions in the soil environment may have reduced the capacity of available nutrients in the soils inflicting significant yield losses [131][244]. Additional studies are required to develop a more comprehensive model of BC effect on legume production.
Other factors potentially responsible for lower productivity include soil nutrient deficiencies found with sugarcane bagasse and sunflower-derived BC-amended (500–600 °C) soils [124][238]. As a result of decreased nitrogen uptake with increasing BC application, sunflower seed and oil yield saw a significant decrease. The 1% field application of the BCs may have increased specific communities of bacteria and enhanced certain enzyme activity such as urease, which is an important enzyme in soil nitrogen status, as reported by a field study with the addition of sugarcane bagasse biochar (SCBC) [132][162]. However, fungal communities suffered due to SCBC addition, and final yields of Brassica chinesis L. (pak choi) were reduced compared to controls. It was found that a 4% application rate of SCBC supported normal plant growth and increased sugar and cane yields [133][245]. The SOC, soil–water related properties, and nutrient levels were enhanced by SCBC, leading to increased plant productivity. Further research is needed to identify BCs appropriate for specific plant species and initial soil characteristics for improved plant growth and development.
Although crop responses were generally positive, the high variability within the listed studies makes it difficult to draw any broad conclusions except that the type and application rate of BC will require customization. The benefits of BC application mainly consist of increased water-holding potential, better nutrient cycling, and increased soil carbon reserves. This may lead to no effect or only minimally increasing yields in the short term, but further testing in the field should illuminate the effects of long-term BC amendment on crop yields [134][246]. Other regions of industrial agriculture and tropical environments may show a more pronounced BC effect and may be better at exploiting the advantages of BC. BC application in marginal soils will likely lead to increased crop productivity by increasing the overall soil fertility through pH and CEC adjustments, better water retention, and increased microbial activity [135][136][247,248]. Nevertheless, considerable caution should be observed when using extremely heavy rates of BC. The elevated risk of heavy metal contamination due to feedstocks rich in accumulated metals or other phytotoxic compounds could decrease crop productivity with increasing BC applications [137][138][249,250].
The overall conclusion is that BC application is favorable for improving crop productivity sustainably. Certain agricultural systems require different inputs to achieve higher crop yields, and designing BC to meet those specific needs could lead to optimized production methods and products.
