Introduction and Background
As the world’s population rises, people will continue to put more pressure on their terrestrial landscapes in order to extract food, fiber, and fuel to meet their growing needs. At the same time that populations drive a need to find methods of more efficient agricultural land use the climate is changing and land degradation is an increasing problem. In the industrialized world, modern agriculture relies on heavy chemical inputs and creates pollution problems in our waterways and our air. Carbon dioxide, methane, and nitrous oxide are byproducts of modern agriculture that exacerbate climate change. In the developing world farmers often do not have access to the expensive chemical inputs of modern agriculture and thus rely on ‘slash and burn’ techniques. These practices volatilize most of the nutrients accumulated in biomass and cause air quality concerns. Alternatively, farmers may incorporate organic wastes that have short residence times in soil, thereby releasing large amounts of methane and other greenhouse gasses as they decompose. In both systems of agricultural production, nutrient cycles are very leaky with nutrients entering the system and then quickly leaving through leaching, volatilization, erosion, and crop removal.
Agricultural systems are thus faced with two problems. They are in constant need of nutrient additions, while simultaneously leaky nutrient cycles cause nutrient overabundance elsewhere. In the case of carbon, agricultural soils no longer store nearly as much carbon as undisturbed soils. After conversion to agriculture, soils loose 89% of their stored carbon. Land converted to agriculture thus acts as a carbon source, both directly by releasing stored soil carbon and indirectly by requiring heavy nutrient inputs that release carbon during their production.
In natural ecosystems 90% of soil organic matter turns over on decade timescales, much of it being very labile.
There are numerous sites, however, among the old and highly weathered tropical Amazon-basin soils where Amazonian Dark Earth, or Terra Preta, is found (see map on page B-10). These Terra Preta soils contain highly stable organic black carbon waste, or biochar, that was added to the soils by people during the pre-Columbian period, 500-6,000 years ago. These dark colored soils have 70% more carbon in them than surrounding soils, and demonstrate unexpectedly low nutrient leaching and high primary productivity, even with extensive agricultural use.
These unusual Amazonian Dark Earth soils often originate in ancient human middens, or trash pits, where the charred organic waste was mixed with other organic soil amendments such as bones and manures. These mixed-waste Terra Mulata soils are also stable on millennial time scales, suggesting that ancient people used biochar along with other organic wastes to convert poor-quality soil into agriculturally productive soil that could maintain long-term productivity. There is a clear historic use and benefit of biochar addition to agricultural soils, but the applicability of biochar addition to today’s agricultural systems, especially in temperate regions, is largely unknown.
Biochar is produced by baking organic materials in the absence of oxygen, a process known as pyrolysis. The resulting biochar is a polycyclic aromatic hydrocarbon and is very stable due to its structure. Pyrolysis of agricultural waste products such as crop residues and manures can offer a better way to dispose of these wastes, avoiding the air pollution associated with burning them and the green house gas emissions of methane, carbon dioxide, and nitrous oxide associated with leaving them to decompose.
Biochar is a term used to describe any organic waste that has undergone the pyrolysis process. The specific physical and chemical composition of biochar depends on the starting material it was made from. All biochars are carbon-rich, aromatic rings that have high surface areas and porous structures. The addition of biochar affects physical properties of soil texture, structure, porosity, particle size, distribution and density that alter the movement of air, water, microorganisms, and roots through the soil. Additions of biochar increase the soil’s cation exchange capacity and provide protection for many living organisms in the soil, such as mycorrhizal fungi that can intercept leachable nutrients. Thus, biochar can act to hold nutrients in soil chemically, physically, and biologically.
Biochar thus offers three attractive benefits. First, it can sequester carbon in agricultural systems better than no-till agriculture because the recalcitrant form of carbon remains in soils for thousands of years. Second, energy can be harvested during the pyrolysis process. Third, biochar can improve agricultural soil through its beneficial chemical and physical properties. This article will focus on the benefits that biochar can provide to agricultural soils, specifically how it affects the availability of nitrogen in temperate agricultural systems.
Nitrogen is one of the nutrients that most often limits primary production in ecosystems. Recently disturbed sites, such as those cleared for agriculture or subject to burning, are especially nitrogen limited. Biochar, however, has great potential to be used as a soil amendment in temperate agriculture and of particular interest are the effects biochar addition has had on nitrogen cycling. From the limited scientific research that has been conducted on the effects of biochar on nitrogen there is general agreement that it offers a three-pronged benefit to soil nitrogen.
Biochar additions 1) increase plant-use efficiency of nitrogen, 2) decrease leaching of nitrogen, and 3) decrease nitrous oxide emissions from soil. In addition, biochar appears to alter the microbial community, favoring fungi, and thus providing opportunities to manipulate nutrient cycling through altering the microbial mediators of those nutrients.
Biochar’s three observed benefits to soil nitrogen may be explained by its ability to increase cation exchange capacity in soil, enabling soil to hold on to nutrients for long-term plant growth. By increasing the soil’s ability to hold on to available nitrogen, biochar can reduce nitrogen losses from the system from ammonium leaching and nitrous oxide emissions. Biochar’s physical structure, rich in microsites, coupled with its cation exchange benefits that help hold nutrients in the soil for future plant use favor a microbial community dominated by fungi and rhizobial bacteria that capture and store more carbon and nitrogen. Thus biochar simultaneously increases carbon storage and nitrogen directly available for plant use.
Structure of Biochar
Biochar’s physical structure increases the ability of soil to retain moisture, while simultaneously increasing porosity and permeability that allows excess water to move through the system. This favors aerobic microorganisms as opposed to denitrifying bacteria that can produce nitrous oxide. Biochar porosity also offers habitat for microbes that aid in making nutrients available to plants. Thus, a unified understanding of how biochar benefits soil nitrogen chemically, physically, and biologically can help us understand the interactions between these factors and mechanisms.
Some studies find that biochar additions alone can increase production, likely due to its ability to absorb and retain water. Biochar additions alone to temperate rice fields in China showed yield increases of 14%, likely due to its water-holding ability. The greatest benefit, however, is in the addition of biochar alongside fertilizers. One study examined the effects of biochar additions on barley yield alongside different amounts of nitrogen fertilizer. Researchers found that additions of biochar with nitrogen fertilizer could increase barley yields by 30% above nitrogen fertilizer alone. Another field study highlighted the role of biochar in increasing nitrogen fertilizer use efficiency. This field study found that coupled biochar and fertilizer additions produced higher yields of radishes than nitrogen fertilization alone.
One laboratory study measured nutrient losses in three different soils from the US Midwest amended with poultry manure with varying amounts of biochar. As biochar rates increased, the amount of N, P, Mg, and Si leached from the soil decreased, even though the biochar itself contained ‘substantial’ additions of these elements. Over their 45 week trial, even at the low biochar addition rate of 20 g/kg soil, total nitrogen and phosphorus leaching decreased by 11% and 69% respectively. These results suggest that biochar additions to productive Midwest soils could help manage nutrient leaching problems.
These studies provide encouraging evidence that biochar can be used as an additive in current conventional and organic agricultural practices in order to increase the use-efficiency of nutrient addition. Some of these studies were short-term experiments, however, conducted in laboratory settings, and thus do not include all of the varied conditions that can affect processes in the field.
Microbes and Biochar
The increased surface area, microsites, and soil structure that biochar provides create environments from which microorganisms can benefit. Surprisingly, the initial addition of biochar can reduce microbial activity, at least in laboratory experiments, while in general biochar additions increase long-term microbial activity. The initial decline in microbial activity followed by a long-term increase may be explained by shifting community dynamics. The original microbial community in highly fertilized agricultural systems is likely dominated by copiotrophs (organisms which predominate in nutrient-rich environments), which may decline when biochar is added to the soil, binding available nutrients in cation exchange and changing the physical properties of the soil. The replacement community may be more oligotrophic (capable of surviving where there is little to sustain life) and competitive when leachable nutrients are less readily available.
The microsites of biochar are particularly favored by mycorrhizal fungi, which form symbiotic relationships with plants and help extract nutrients and water for the plant in exchange for photosynthesized carbon compounds. The fungal hyphae of these mycorrhizae intercept leachable nutrients and thus we observe significantly lower nutrient losses when biochar is added to agricultural land. Fungal dominated soils generally store more carbon than bacterial dominated soils. Biochar’s ability to favor fungi may serve as an additional mechanism by which biochar can increase the carbon-storing capacity of soils and thus even offer humans a tool by which to manipulate microbial communities and carbon storage. Additionally, the fact that microorganisms create soil aggregates may provide an explanation of the observed increase in soil structure found in soils amended with biochar applications.
One of the key microorganisms in the soil is the group of Rhizobium spp bacteria that fix atmospheric nitrogen into bioavailable forms and develop symbiotic relationships with plants. One study found that at maximum nodule development alfalfa amended with biochar in field experiments had 227% greater nitrogen than plots without biochar additions. It appears that biochar stimulates biological nitrogen fixation, and can thus directly alter nitrogen addition to soils by altering the microbial community.
Differences in Biochars
Not all biochar is the same. The way it is made can strongly affect the end biochar. The temperature at which pyrolysis occurs also shapes the end product. In general, as the temperature of pyrolysis increases, the yield of biochar decreases. As the organic material is heated to create biochar, many compounds volatilize, leaving only the most recalcitrant material such as aromatic hydrocarbons and basic elements. Nitrogen is one of the first materials to volatilize at about 200˚ K (Kelvin or -73˚ C). Researchers have found that the higher the temperature of pyrolysis, the greater the loss of nitrogen relative to the starting material. It is clear that not all of an element volatilizes at a certain temperature. Biochar made from sewage at 450˚ K (177˚ C) still contained 50% of its original nitrogen and all of its phosphorus, though not in bioavailable forms. A study that examined the amount of the elements P, K, Ca, and Mg in biochars produced at 400˚ C and 500˚ C found that the slightly higher temperature acted to concentrate these elements. Thus, it is clear that the temperature at which pyrolysis occurs has a huge effect on what materials remain in the biochar. Additionally, the form of the nutrients found in the original material affects what elements will remain in the biochar product. In general, biochar produced under low-temperature pyrolysis yields a greater quantity of material that is more labile and has a shorter residence time in soils. Biochar produced at high temperatures, however, is more recalcitrant with a longer resident time in soil.
The temperature at which biochar is formed can also affect its pH. Higher pyrolysis temperatures lead to biochars with higher pH. There is potential for biochar to provide a service to soils by increasing the pH of agricultural soils that have a low pH (often because of long-term inputs of nitrogen fertilizer and acid rain). Changing the buffering effect of biochar through its pyrolysis process is one way that biochar could be created for a specific agricultural use to mitigate a specific agricultural soil problem.
The starting organic material for biochar can also have a large effect on the end product, as well as the conditions under which it is formed. The initial relationships of the starting materials will correlate strongly with the end product. One study measured the element contents of biochars made from different substrates. It found that the final carbon content ranged from 40% in biochar made from poultry litter to 78% in biochar made from pine chips. Interestingly, the amount of nitrogen conserved in the biochar was inversely proportional to the source material nitrogen, with poultry litter and pine chip biochars conserving 24.7% and 89.6% of their starting nitrogen respectively. Additionally, the moisture and lignin content will affect the pyrolysis process and thus the biochar product. For example, due to the high lignin content in nutshells, biochar from nutshell wastes has a higher surface area and more developed micropore structure than many other organic agricultural wastes.
Another way that biochar can aid in nutrient retention is to be added to decomposing materials as a bulking agent before they are composted and applied to fields. In one comparative study scientists mixed poultry manure in a 1:1 ratio with biochar, coffee husk, and sawdust respectively. They found that despite the inert nature of biochar, the biochar manure mixture underwent the highest level of humification of the three trials. The conversion of organic matter to humus (70% in this study) marked the transformation of labile organic matter to more recalcitrant humic acids that have a long-term soil improvement benefit. Thus, biochar appears to have acted as a catalyst by aiding in the transformation of labile organic matter to recalcitrant humus. Other studies have also demonstrated the ability of biochar to enhance the nutrient status of the compost products and reduce nitrogen losses during decomposition of organic matter. While this was a short-term study, it demonstrates how biochar can help enrich the long-term recalcitrance of organic matter in soils.
Biochar can also aid in the retention of nitrogen in the soil through reducing nitrous oxide emissions. Soil nitrous oxide emissions are primarily a function of moisture content and tillage regime in agricultural soils. Biochar alters the physical location of water within the soil matrix by providing increased surface area, porosity, and a more developed soil structure. Denitrifying bacteria produce nitrous oxide in low oxygen conditions, such as waterlogged soil, as a leaky product along the pathway to forming atmospheric nitrogen (N2). Biochar additions in rice fields in China decreased nitrous oxide emissions in nitrogen fertilized fields by 40-51% and in unfertilized fields by 21-28%. This same study found that biochar significantly increased rice yields and reduced methane emissions by as much as 41%. This is significant, as rice cultivation is notorious for emitting greenhouse gases from the soil and biochar significantly reduced the emission factor of nitrogen fertilizer in this conventional cultivation system. In addition rice is a major food staple for much of the world, especially the developing world where agriculture will need to be intensified and more efficient in the future in order to feed growing populations.
Biochar is expensive to manufacture at large scales such as those needed to sequester considerable amounts of carbon, and requires considerable infrastructure to do so. For the near future, biochar will most likely be created for small-scale agricultural benefits. Even small amounts of biochar additions to soils have been demonstrated to provide real and significant benefits. Small-scale biochar additions incorporated into the rooting zone of plants can help retain phosphate, ammonium, water, and microorganisms that can help rebuild organic matter stocks quickly in denuded landscapes and low-yield agricultural land. Biochar has a particular appeal for small-scale subsistence farmers who can benefit economically and socially from improving their land for the long-term gains that are not often at the forefront of corporate farming interests. Such long-term, small-scale experiments provide the greatest benefits for the lowest cost and are a starting point for biochar use.
In order for broader scale applications of biochar to be feasible, for example on industrial farms, production will have to be cheaper. Currently production costs, infrastructure costs, and transportation costs of such a high bulk-density product, as well as the health hazards that dusty materials pose, make the industrial application of biochar infeasible. Some investigators suggest that biochar could be manufactured into a pelleted product that can be transported easily. They also conclude that if pyrolysis occurs at 550˚ K or less no crystalline materials form and thus there is little health hazard associated with breathing biochar dust during use. Some argue that, given the political and economic will to sequester carbon, biochar may become a cost-effective way to do so. Whether the broader-scale application of biochar to terrestrial ecosystems is driven by the need to sequester carbon or the need to improve agricultural soils, it will provide multiple benefits to soils and communities across the globe.
One chief concern with biofuels and biochar is where the organic substrate material will come from. With growing populations there are increasing pressures on terrestrial ecosystems. Often biofuels are grown on former food cropland, or are made directly from food crops. But biochar can be made from any organic waste material and is a good way to turn crop residues into long-term soil improvements. Crop residues can be charred (pyrolysed) and added back to the soil in agricultural systems. In addition, forestry waste could be charred and added to forest ecosystems, thus providing a ‘slash and char’ rather than a ‘slash and burn’ system for creating agricultural land that has the ability to sustain long-term production. Abandoned and denuded agricultural land also has the possibility to be improved and reclaimed as productive cropland through ‘slash and char’ biochar addition. Though biochar can offer a more sustainable way to clear land for agricultural growth, biochar ought to first and foremost be applied to existing agricultural land for its ability to increase yields and nutrient use efficiency. In addition, there is great potential for biochar to increase the carbon storage ability of any terrestrial ecosystem through its ability to tighten leaky nitrogen cycles in agricultural soils and make nitrogen more available to plants.
One of the great difficulties in better understanding biochar is that there are so many factors that affect its characteristics. Biochar is not one substance, but a family of black carbons that each has its own properties and interactions with the local environment where it is applied. It is this manipulability that makes biochar an appealing additive for agricultural soils that may benefit from its various and diverse characteristics.