Introduction

Several soil classification studies conducted by soil science research groups in Nigerian universities, have classified most of the soils in southeastern Nigeria as Ultisols and Oxisol under the United States Department of Agriculture (USDA) soil classification system.

These soils are characterised by poor physical and chemical properties due to a high degree of weathering and multiple nutrient deficiencies. These limitations are further compounded by unsustainable agricultural practises, which are prevalent in this agro-ecological zone. As a result of land scarcity and rural poverty, subsistence farming has led to intensive cultivation on available farm lands, which translates to soil degradation and poor soil quality.

Over the years, subsistence farmers in these regions have been dependent on traditional methods of soil conservation, such as manure application and rotational cropping, as their only means of sustaining soil productivity. These methods, however, are proving inefficient, due to ensuing socio-economic factors. This is prompting calls for the adoption of more efficient and sustainable means of improving soil quality, especially under climate change scenarios.

The discovery of and subsequent intensive research into a carbon rich dark soil known as terra preta in the Amazonian agro-ecological zone led to the development of biochar technology. Biochar is a solid carbon rich material produced from the carbonisation (thermochemical pyrolysis) of biomass materials. This fine grained, highly porous charcoal can be added to the soil to help improve its physico-chemical and biological properties.

Various studies have indicated that most of the problems associated with the poor quality of degraded tropical soils can be salvaged by biochar application, but surprisingly, biochar remains unknown in some parts of sub-Saharan Africa, especially in southeastern Nigeria where biochar application can help remediate most of the soil quality problems. This article presents the major soil quality issues and soil improvement potentials of biochar in southeastern Nigeria. It further suggests how subsistence farmers can benefit from biochar technology especially under climate change scenarios, which threaten food security and are expected to be very severe in this region.

The highly weathered tropical soils of southeastern Nigeria

The rainforest agro-ecological zone of southeastern Nigeria falls within a tropical climate which is characterised by a bimodal rainfall pattern that peaks in the months of June and September, with a short dry spell in the month of August. The mean annual rainfall in this region ranges between 1,750 – 2,000mm, the mean annual temperature is 26.5 – 27.5° C and the mean relative humidity varies between 71.6 – 85.6%.

The combined effects of temperature, humidity and rainfall govern the nature and properties of soils in this region. Like most tropical rainforest systems, the major soil types found in this area are Oxisols and UItisol (USDA soil classification) derived from coastal plain sand (Enwezor et al, 1990). These soils are the most highly weathered soil orders and always form in hot tropical climates. The mineralogical constituent of the underlying clay materials is known as the kaolinites. The continuous weathering of this silicate mineral leads to the leaching of silica and production of aluminum and iron hydrous oxides.

The organic matter content of these highly weathered soils is usually very low, due to constant warm temperatures that accelerate biological activity and rapid mineralisation of plant detritus, preventing an accumulation of organic substances. The mineralised nutrients are rapidly washed out through water seepage before reaching plant roots.

The physical properties of highly weathered tropical soils combine with high rainfall intensity to exacerbate nutrient leaching. This is because these soils are unconsolidated and very porous, allowing the free movement and drainage of water through the soil profile, thus leaching out all the basic cations that are loosely held in the low activity clay. For this reason, most of the soils that occur in the tropical rainforest agro-ecological zone are known for their strong acidity, low clay activity and poor fertility status, thus will require the addition of nutrient cations such as Ca, Mg, NO3- and K for it to be suitable for crop production.

P is also a limiting nutrient because of its tendency to form strong bonds with the oxides that occur in the weathered soil profile. Application of manure and fertiliser has served as the major means of nutrient replenishment in the rainforest agro-ecological zone of southeastern Nigeria, but the incessant rise in the price of fertiliser makes it unaffordable to most farmers – who depend on subsistence farming as their only means of livelihood.

The conversion of tropical rainforest to continuously cultivated land poses negative effects on soil quality. The tight coupling between carbon and nutrient cycles is interrupted by deforestation, because less plant litter is delivered to the soils for mineralisation, leading to a rapid depletion of nutrient stocks.

The depletion of soil organic matter subsequently leads to a destabilisation and loss of soils’ structure in the surface horizon, inducing erosion, soil compaction and higher surface runoff. The conservation of organic matter is critical to preserving soil fertility and quality.

This central role of soil organic matter content in the conservation of soil quality was first observed in the Amazonian forest, which shares similar agro-ecological characteristics with southeastern Nigeria. Patches of terra preta were found adjacent to the infertile ferralsol, which is dominant in the Amazonian basin. These soils can be classified as Anthrosols because a rapidly expanding body of scientific literature has reached the consensus that these soils were created under the anthropogenic influence of indigenous people who settled in the region more than 2,000 years ago. The occurrence of ceramic fragments and lithic artefacts, combined with carbon dating of charcoal from terra preta sites support this theory.

A comparative analysis by Lima et al (2002) on the physicochemical properties of terra preta and ferralsols showed that total organic carbon (TOC) content in terra preta was twice of those adjacent to highly weathered ferralsols. The occurrence of pyrogenic carbon throughout the terra preta profile is an indication that the carbon contained in the soil is a residue of incomplete combustion, which is highly stable and recalcitrant to microbial degradation.

Over time, the gradual oxidation of this recalcitrant carbon will produce negatively charged carboxylic groups at the edges of the aromatic backbone, increasing the nutrient retention capacity of the soil. As a result, terra preta exhibit elevated concentrations of plant macro and micronutrients such as Ca, Mg, P, Zn, and Mn. It was based on this principle that biochar technology was developed and has been used as a cheap and practical means of improving soil physico-chemical and biological properties in areas with highly weathered soils, scarce organic resources, and inadequate water and chemical fertiliser supplies.

Biochar production and application

Biochar is produced by the incomplete combustion of biomass materials in the absence of oxygen, a process referred to as pyrolysis. In practise, it is not possible to create a completely oxygen free environment and as such, a small amount of oxidation will always occur. The degree of oxidation of the organic matter, however, is relatively small when compared to combustion, where almost complete oxidation of organic matter occurs. The biomass material that is pyrolysed and turned into biochar is known as feedstock.

The physicochemical properties of biochar are determined mainly by feedstock and the pyrolysis temperature. Many biomass materials have been proposed as biochar feedstock, including wood straw, grain husks, nut shells, and crop residues. Other organic waste materials such as sewage sludge, municipal waste, livestock manure and compost can also serve as feedstock, but they are undermined by the occurrence of impurities such as organic pollutants and heavy metals.

Selecting the most suitable biochar feedstock for any soil type will require an understanding of the major edaphic limitations that are intended to be improved through biochar application. Considering the acidic and nutrient poor conditions of the highly weathered soils of southeastern Nigeria, a suitable biochar feedstock for this region will combine biomass materials that contain nutrient cations, such as Ca, Mg, NO3- and K (higher cation exchange capacity) with high liming capacity and possibly a higher mechanical strength.

The latter will help to improve the weathered soil’s physical strength and can be obtained by woody feedstock, while the former can be supplied by biochar from crop residue feedstock. Also, high concentrations of calcium carbonate (CaCO2) can be found in pulp and paper sludge and are retained in the ash fraction of the biochar, making them suitable feedstocks for biochar production in southeastern Nigeria.

Complementing these prescribed feedstocks with low carbon:nitrogen (CN) ratio feedstock (e.g. treated livestock manure) will be necessary when producing biochar for the improvement of soil quality in allotments used for annual vegetable crop production. Auspiciously, the recommended feedstocks are locally available in abundance and are even considered as waste materials in most parts of southeastern Nigeria.

The combination of feedstock heterogeneity and the wide range of chemical reactions that occur during the pyrolysis process, give rise to biochar products with a unique set of structural and chemical characteristics. The yield of products from pyrolysis varies much with temperature.

The lower the temperature, the more char is produced per unit biomass. High temperature pyrolysis enhances the polymerisation of the molecules within the feedstock, whereby larger molecules are also produced (including both aliphatic and aromatic compounds), as well as the thermal decomposition of some components of the feedstock into smaller molecules. During thermal degradation of the biomass, K, Cl and N constituents will vaporise at relatively low temperatures, while Ca, Mg, P and S will be lost at temperatures that are considerably higher.

Generally, the ideal pyrolysis temperature for the production of more biochar per unit biomass ranges between 400° C to 500° C. Above this temperature, about 85% of the pyrolysis residue will be in the form of syngas, a process known as gasification. With regards to biochar production for the improvement of soil quality in southeastern Nigeria, pyrolysis temperatures between 350° C to 500° C are ideal, since major constituents of the prescribed feedstock, cellulose and lignin, undergo thermal degradation at these temperatures respectively.

Following the production of biochar, the methods used in its application determine the fate of biochar in the soil and its impact on the environment. Currently, three main techniques are used to incorporate biochar into the soil: top dressing, top soil incorporation and deep banding.

Top dressing of biochar entails spreading the biochar on the soil surface and relying on natural processes for the incorporation of the biochar into the topsoil layer. This technique is most suitable in cultivation practises, e.g. the no-till system, where minimal disturbance of soil is required.

With top soil incorporation, the biochar applied on the soil surface is properly mixed with top soil to allow incorporation and minimise the loss of biochar through wind erosion.

Deep banding of biochar involves the placement of biochar materials at depths below the topsoil surface and using a ploughing machine to properly homogenise them with the top soil.

The major problem associated with these methods of biochar application is the release of dust fractions from biochar during spreading and subsequent erosion of some solid fractions from the soil surface. These have raised public health concerns due to their potential to compromise air and water quality in the immediate environment. Generally, the suitable method of application will greatly depend on the land use, crop type and dominant cultivation practises. Due to the subsistent level of crop cultivation in southeastern Nigeria, top dressing and topsoil incorporation are the ideal methods of biochar application in this region.

Questions about the optimum rate of biochar application are frequently asked in any biochar introduction forum. The answer to this question is that the optimum application rate for biochar depends on the specific soil type and crop management practise. Scientific studies aimed at gauging the optimum rate of biochar application for various crop management practises in highly weathered tropical soils are still at the trial stage. Most informal observations of crop growth after biochar applications have been conducted in greenhouse studies where 5-20% biochar application by soil volume has consistently yielded positive and noticeable results.

Some research findings also indicate that a threshold rate of application exists, above which additional biochar will not yield positive results. Given the variability in biochar materials and soils, best management practise will require testing several rates of biochar application on small plots before implementation on a large scale. At present, most experimental results have suggested that application rates 5-50 t/ha (0.5 – 5 kg/m2) will successfully support most crop production practises on degraded tropical soils.

Biochar’s fate in highly weathered tropical soils

The incorporation of biochar into highly weathered and degraded tropical soils is a means of boosting the biological life, physical strength and chemical processes occurring within the soil system. Biochar with a low CN ratio will be the most beneficial to soil microbes in degraded soils, due to the prospects of providing liable carbon and nutrients to the constrained microbial communities.

Furthermore, the poor water retention of these soils alters the food web of the soil organism. This is because many soil organisms, specifically nematodes and protozoa, enter a state of cryptobiosis, whereby all metabolic and physiological processes cease to proceed in the presence of adverse environmental conditions, in order to conserve energy until conditions become favourable.

Application of biochar to such soil leads to an increased water retention of soils and subsequent nutrient flow within the soil ecology. This will have a positive effect on the activity of soil organisms as conditions become suitable for optimum metabolic activities, leading to increased soil functioning and sustainable ecosystem services.

The application of biochar has also been observed to enhance plant-soil organism relations through mycorrhizal interactions with the rhizosphere microflora. Biochar supplies the carbon needed by arbuscular mycorrhizal fungi (AMF) to sustain a symbiotic relationship with plants, which leads to improved crop yield and enhances water use efficiency. There is some evidence that the positive effects of biochar on plants’ productivity may be attributable to increased mycorrhizal associations.

Due to its highly porous nature, biochar has been shown to provide increased levels of micro pore spaces that can harbour a significant number of microorganisms, making them inaccessible to larger prey organisms. This is also another mechanism that explains an observed increase in microbial biomass following biochar application.

It is worthy to note that the pyrolysis temperature can also affect how the biochar will interact with the soil microbial community. This is particularly true for biochar produced from woody feedstock, which at lower pyrolysis temperatures retains an interior layer of bio-oil that will serve as the glucose substrate required for active microbial growth. When pyrolysed at higher temperatures, this internal layer of bio-oil is lost and so it is likely that the biochar will have less impact with regard to promoting soil fertility and increasing microbial biomass when compared to the biochar, which retained the internal layer of bio-oil.

The biological breakdown of biochar by an active soil microbial community translates to a physically strengthened soil condition.

The incorporation of biochar into soil can alter soil’s physical properties such as texture, structure, pore size, distribution and density, with implications for soil aeration, nutrient transport and water holding capacity.

The soil hydrology of highly weathered tropical soil is characterised by a high degree of infiltration due to the large pore spaces dominant in the coarse textured weathered soils. These pore spaces are filled by the small particle size fraction of biochar, thereby decreasing water infiltration rates, nutrient leaching and improving soil water holding capacity. Thus, biochar influences the water and nutrient retention capacity of soil by altering the distribution and connectivity of pores in the soil medium, which is largely regulated by soil particle size (texture), combined with structural characteristics (aggregation) and soil organic matter content. These physical attributes of biochar will resolve the soil quality issues observed in highly weathered soils of southeastern Nigeria.

The effect of biochar application on soil’s chemical properties is related to the large inner surface area of biochar, which helps to increase soil specific surface area and the overall sorption capacity of soils.

Biochar directly contributes to nutrient adsorption through covalent interactions on charged internal reactive surface area of the soil-biochar matrix. Mobile soil nutrients such as nitrates (NO3-) or basic cations, which are susceptible to leaching at low pH, are retained on the large surface area provided by the biochar. This is another mechanism that contributes to the observed decrease in nutrient leaching and increased nutrient use efficiency in biochar amended soils. Also, the liming effect of biochar is the most likely chemical mechanism behind increases in plant productivity after biochar applications.

Lower pH values in soils (greater acidity) often reduce the soil’s cation exchange capacity and thereby the nutrient availability to plants. This implies that the liming effect of biochar will be critical to reducing aluminium toxicity, which forms the major cause of soil acidity in degraded soils of southeastern Nigeria.

Summary

The studies presented in this article contribute to the existing literature on the conservation options for highly weathered tropical soils of southeastern Nigeria. Over the years, farmers in this region have witnessed a decline in soil quality and consequent reduction in agricultural productivity. For this reason, the ensuing food security crisis in some tropical agro-ecological zones has been associated with the degraded state of tropical soils. Although some of the government’s agricultural development schemes have occasionally provided subsidised rates of fertiliser, applying fertiliser alone is not enough as its use efficiency is compromised by the degraded physical conditions of these tropical soils.

Biochar presents an option that addresses both fertility and soil quality limitations in a more economical and sustainable way. Biochar also offers an option for climate change mitigation, due to the carbon sequestration attributes of this practise.

Subsistence farmers in this region are encouraged to adopt biochar techniques, due to the bio-economic potentials, especially the fact that feedstock materials are locally available and affordable. The agricultural departments at various levels of government will need to play a key role in the development and dissemination of biochar technology. This can be achieved by sponsoring agricultural engineering projects that can construct efficient and portable biochar processing plants. The private sector can also seize the opportunity to invest in biochar processing plants. Dissemination of the technology to rural farmers can be achieved through Small Plot Adaptation Technique (SPAT), which can also be used to determine the optimum application rate for different crops and soil types in this agro-ecological zone.

Published: 05th Sep 2013 in AWE International