Crop residues are invaluable source of nutrients. However, their production in huge amount needs some effective disposal solutions. Burning of crop residues in the open field is one of the commonly adopted methods by farmers to get rid of this waste. This practice not only causes environment pollution due to emission of green house gases and serious fine dust problem but also cause changes in air circulation and monsoon pattern. Alternatively, these crop residues can be used to produce compost and can also be processed to produce biochar whose application to infertile soil may benefit the agriculture production. Recently it has been proposed as one of the effective waste management strategy to improve crop productivity.

What is biochar

Biochar is a porous, dark colored, stable carbon rich form of charcoal that can be produced from wide range of biomass sources including animal manure, green waste and agricultural residues. It is produced by pyrolysis (Pyro means fire and lysis means decomposition) where biomass is heated with little or no oxygen. Biomass heats up to the point at which the pyrolysis starts (350oC). The reaction becomes exothermic and the molecules of biomass are reorganized to form biochar along with gases and volatiles. Type of feed stock and production conditions affect the quality of the produced biochar. Thus, production conditions need to be optimized for each feed stock to ensure the desired benefits of added product in the soil. Incorrect production of biochar and its subsequent application to soil could be detrimental to both agricultural production and environment. Pyrolysis of biomass into biochar creates three primary benefits of waste management, soil improvement and mitigation of climate change (Fig. 1).

Fig. 1

Biochar can be applied in soil as
(i) Deep banding with compost/ manures
(ii) Through liquid slurries
(iii) Spreading by hand /machine and its incorporation in soil through tillage

Ideal biochar application rates are not known. However, its application rate may vary depending upon the soil type. A knowledge of appropriate application rates and biochar types that should be applied to different soil types under different climatic conditions is essential to prevent its adverse impact on agricultural production.

What is in biochar?

Biochar is not a fertilizer itself though; it contains stable and unstable matter, moisture content and ash. The ash content of biochar can vary. The animal manure biochar contains high amount of ash compared to those prepared from plant residues. Ash has nutrients such as calcium, magnesium and inorganic carbonates left after the volatilization of carbon, hydrogen and nitrogen. Stable matter of biochar helps in nutrient retention. Incorporation of biochar in soil can exert both short and long term effect on its health. Short term benefits include a liming effect for soils with low pH and long term effects include increased nutrients and water retention capacity of soil that can affect the crop productivity.

Impact on soil properties

1. Water retention

Addition of biochar to infertile land reduces bulk density and increases its porosity due to its internal porous structure, nature of its particle size and shape. Increased soil porosity increases surface area of soil and water can penetrate easily.

2. Ion exchange capacity

The nutrient retention capacities of both biochars and soils depend on their cation and anion exchange capacity. Freshly produced biochars and those produced at temperatures > 600oC have little or no cation exchange capacity (CEC) , while their anion exchange capacity is substantial. However, the biochar produced at low temperatures have a high cation exchange capacity. As biochar ages or matures in the soil, its cation exchange capacity increases. High cation exchange capacity biochars have the ability to adsorb heavy metals and organic contaminants such as pesticides and herbicides from the environment. Therefore, the addition of biochar with high CEC can be used to alleviate the contaminant level in soil. Biochars to be used as soil amendment should not be produced at high temperatures. Soil with high CEC can hold the nutrients and increases their availability for plant uptake

3. Effect on soil pH

pH is one of the most important characteristics of soil In terms of nutrient availability to plants. Most of the biochars are alkaline in reaction (> 9.0) and can exert a liming effect in soil where pH is lower than optimal for intended use. Therefore, their effectiveness is more under acidic soil. A small increment of pH has been recorded with all types of biochar applied to soil at different application rate. The increase in pH is due to high initial cation exchange capacity of biochar.

4. Nutrient availability

Type of feed stock, duration of pyrolysis and temperature during pyrolysis affect the composition and structure of biochar resulting in significant differences in nutritional content of biochar produced. Biochar produced from animal product feed stock, and manures is relatively rich in nutrients compared with plant and wood derived biochars. Biochar derived from crop residues have the high carbon exchange and water holding capacity. Biochar application results in reduced leaching of calcium, potassium, magnesium and nitrogen. This means higher availability of nutrients for plant uptake and low requirement for chemical fertilizer. Amendment of soil with biochar also reduces the runoff of phosphorus (P) into surface water and leaching of nitrate into ground water. Thus, it helps in reducing the pollution caused by chemical P fertilizer runoff. Biochars are not primary source of nutrients rather they are more important for their use as a soil amendment and driver of nutrient transformation.

Microbial decomposition of organic matter regulate P (an important plant nutrient) mineralisation and hence its availability to plants. Several studies have demonstrated enhanced P uptake by plants in the presence of biochar. The possible mechanisms may include biochar (i) itself acts as a source of P (ii) store of phosphorus bound to surface sites through its anion exchange capacity (iii) modifier of soil pH, thereby modifying the pH-dependent solubility characteristics of P compounds and (iv) promoter of microbial activity and P mineralization. Biochar has also been reported to improve the availability of sulphur. The sorption capacity of some biochars for NH4+ ions is much higher than activated carbon.

5. Effect on soil biological activity

Due to its high porous nature, high surface area, ability to absorb soluble organic matter and inorganic nutrients, biochar serves as a suitable habitat for millions of microorganisms. Small pores of biochar may act as niche for microbial flora and protect them from external competition and predation. Microbial diversity and abundance is strongly affected by pH of the biochar. Biochar application also increases soil microbial biomass carbon significantly compared with chemical fertilizers.

6. Biochar and soil interaction

Stability of biochar depends upon its specific properties and mineralogical composition of soil and soil properties. The organic carbon content and total nitrogen content of soil has been reported to increase with reduction in N2O emission, with no change in CO2 emission in paddy. Integrated use of biochar and chemical fertilizer to acidic and nutrient poor soils can produce yields greater than either fertilizer or biochar alone. However, the effect of biochar on crop growth depends on application rates and the soil type to which it is applied. The key feature of biochar addition to soils is increased nitrogen use efficiency by plants. This suggests biochar application can reduce the nitrogen fertilizer input without reducing the crop yield.

Currently there is high interest in biochar but the extensive risk analyses regarding the uncertainties associated with its application needs to be done. The positive effects of biochar application in soil are not universal as fertile soils have also recorded some negative impacts on plant growth. Research is needed to ensure that addition of biochar will not affect the microbial abundance, their diversity and reduce soil fertility in the long run.

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