Effect of Biochar on Soil Health and Crop Productivity: A Review

Soil health is the foundation of a vigorous and sustainable food system. As the land is farmed, the agricultural process disturbs the natural soil systems including nutrient cycling and the release and uptake of nutrients (Skjemstad et al., 2002; Lal, 2004). Efficient use of biomass, available as crop residues and other farm wastes, by converting it to a useful source of soil amendment/nutrients is one way to manage soil health and fertility (DeLuca et al., 2006; Glaser et al., 2001). Biochar is a potential soil amendment and carbon sequestration medium. It also reduces farm waste and improves the soil quality.

The key role of agriculture now and in future is to provide safe and quality food for ever growing population. But agriculture is of more significance to global climate change and its effects on soil health and crop productivity. High yields often come from the use of improved crop varieties, fertilizers, pest control measures and irrigation, which have resulted in food and nutritional security (Lehmann et al., 2011). Despite high productivity, farmers see various problems associated with our intensive agricultural systems. It emphasizes the integration of biological, chemical and physical measures of soil quality that affect farmers profit and the environment. Efficient use of crop residue based amendment in soil is an important strategy to improve the soil fertility and productivity in rainfed areas.

Annually 500 Mt crop residues are generated in India, out of which 141 Mt is surplus. These residues are either partially utilized or un-utilized due to various constraints. Surplus and unused crop residues when left unattended, often disrupt land preparation, crop establishment and early crop growth and therefore are typically burnt on farm which causes environmental problems and substantial nutrient losses (Kannan Pandian et al., 2016). Efficient use of huge amount of biomasses, available as crop and agro forestry residues and other farm wastes by converting it in to a useful source of soil amendment. In this concern, biochar is an organic soil amendment, has emerged as a potential strategy to mitigate climate change, to maintain soil health and ensure the sustainable food production at the global scale.
 
Biochar
 
Biochar is a solid material obtained from thermo-chemical conservation of biomass in an oxygen limited environment (IBI, 2015). Biochar is a fine-grained, carbon-rich, porous product remaining after plant biomass has been subjected to thermo-chemical conversion process (pyrolysis) at temperatures (~350-600°C) in an environment with little or no oxygen. (Ameloot et al., 2015). The word Biochar is derived from Greek word. Bios means life; char means charcoal (product of carbonization of biomass). The term biochar was invented by Peter Read (lobbyists for biochar plantations). Biochar is a carbon material made by a process called pyrolysis and it is used as a soil amendment (Krull et al., 2006). It is the black carbon rich material derived by heating biomass with limited supply of oxygen. Unlike the original biomass, it contributes to long term removal of CO₂ from atmosphere, since it is chemically and biologically more stable.
 
Need of biochar research in India
 
a. Reduce the crop residue burning in the field
 
Open field burning of crop residues is an age old practice to boost soil fertility in terms of P and K, but it often leads to a loss of other nutrients such as N and S, organic matter and microbial activity required for maintaining better soil health (IARI, 2012). But maintenance of threshold level of organic matter in rainfed soil is crucial to sustain soil physical, chemical and biological health. For more effective management and disposal of the crop and agro forestry residues, their conversion into biochar through thermo-chemical process (slow pyrolysis) is an alternative way of managing unusable and excess crop residues, examples of crop residues are given in Table 1.


 
b. To improve organic carbon in soil
 
Biochar contains organic matter and nutrients, its addition increased soil pH, EC, Organic carbon in soil and thereby improves the soil fertility.
 
c. To reduce CO2  rise in atmosphere
 
The burning and natural decomposition of biomass and in particular agricultural waste adds large amounts of CO₂ to the atmosphere. Biochar is a stable way of storing carbon in the ground for centuries, potentially reducing or stalling the growth in atmospheric green house gas levels.
 
d. Environmental degradation
 
Biochar helps in improving environmental quality by reducing soil nutrient leaching losses, reducing bioavailability of environmental contaminants, sequestering carbon, reducing GHG emissions and enhancing crop productivity in highly weathered or degraded soil.
 
Impact of biochar on soil properties
 
Soil physical and chemical properties
 
The physio-chemical properties are soil pH, bulk density, water holding capacity, or cation exchange capacity of soils amended with biochar are positively increased. Khaled et al., (2019) conducted the field experiment in sandy soils with application of different rates of biochar viz.,10, 15 and 20 t ha^-1 and concluded that the significantly increased water use efficiency (WUF), crop growth and water retention by 50 to 90 per cent, respectively as compared to the control like without biochar applied treatment, because the positive effect of BC treatments on water retention in the soil increased with increasing time of incubation and was greatest for the low pyrolysis temperature BC (BC300). This indicated that, as BC ages in soil, it can be more effective in promoting water retention, especially the low pyrolysis temperature. Patel and Yadav (2018) conducted the experiment in sandy soils in application of RDF along with Maize stover char @ 10 t ha^-1 application recorded the significantly lower bulk density (1.05 g cm^-3) and also increasing the Water holding capacity of the soil all so positively significant of 28.27 per cent, these results are confirmed with the (Castaldi et al., 2011). Besides, the decrease in bulk density of biochar-amended soil could be one of the indicators of enhancement of soil structure or aggregation and aeration and could be soil-specific (Atkinson et al., 2010).

Kannan Pandian et al., (2016) conducted a field experiment in Alfisol of Semi-Arid tropics and the results are reported that soil application of red gram biochar improved the field-saturated hydraulic conductivity of the sandy soil, as result net water use efficiency also increased. At the same time chemical properties of soil also alter like soil pH, total C, total N, Olson P and cation exchange capacity, because the applied biochar have more number of functional groups that are easily adsorbed the cation are strongly and increasing the nutrient availability of soils (Hong et al., 2014). Snekapriya and Jayachandran (2018)  conducted a research trial in sugar cane crop with application sugarcane trash biochar @ 2 t ha^-1 along with grade levels fertilizer application and results are concluded that the long-term effect on biochar, the soil properties are positively increased because the biochar will be take time to mineralization and slowly released the nutrients and then crop will be easily increased uptake and availability of soils also increased the same findings also reported by  Supriya et al., (2019) and Jin-Hua et al. (2011). Some important soil properties changes are given in Table 2.


 
Soil biological properties
 
Biochar pores and its high internal surface area which increased ability to absorb organic matter act as refuge for soil microbiota from predators and desiccation. The population of bacteria, actinomycetes and arbuscular mycorrhizal fungi increased due to application of biochar and this microbiota reduce N loss and increase nutrient availability for plants. The chemical stability of a large fraction of a given biochar material means that microbes will not be able to readily utilize the carbon as an energy source or the nitrogen and possibly other nutrients contained in the carbon structure.

Snekapriya and Jayachandran (2018) conducted a research trial on sugar cane with different grades of NPK (50, 75, 100, 125, 150 per cent along with Biochar @ 2 t ha^-1) and with biofertilizers (PSB and Rhizobium) in sandy soil and the results are reported that maximum microbial population was bactriea (33.16 cuf), fungi (10.25 cuf) and actinomycetes (17.44 cuf) observed in application of 150 per cent dose of NPK along with 2 t of biochar + biofertilizer @ 10 kg ha^-1. Segun et al., (2019) tried with different levels of rice straw biochar application (0, 3, 6 and 12 t ha^-1) in sandy loam soil at Nigeria and observed that  12 t ha^-1 of biochar application treatment significantly increasing higher soil enzyme activities. The increase in soil biological activity has been reported by Rondon et al., (2007) for nitrogen fixation in Phaseolus vulgaris and by Chan et al., (2008) for earthworm and microbial biomass. Biochar application to soil has long tradition provided evidence that it has positive effects on the abundance of mycorrhizal fungi.
 
Impact of biochar on soils dynamics
 
Fertilizer use efficiency (FUE)
 
Biochar function and its interaction with nutrient elements and crop roots may throw light on understanding fertilizer use efficiency. The enhanced nutrient retention capacity of biochar-amended soil not only reduces the total fertilizer requirements but also copes up the climate and environmental impact on crops (Chan and Xu, 2009). Biochar significantly increases the efficiency and reduces the need for traditional chemical fertilizers with sustainable crop yields. Biochar helps to improve soil resources by increasing crop yields and productivity by the way of reducing soil acidity and reducing the need for some chemical and fertilizer inputs (Yeboah et al., 2009). Longer-term benefits of biochar application on nutrient availability mainly due to a greater stabilization of organic matter, concurrent slower nutrient release from added organic matter and better retention of all cations due to a greater cation exchange capacity. High rates of biochar addition in the tropical environment have been associated with increased plant uptake of P, K, Ca, Zn and Cu (Lehmann et al., 2007).

Biological nitrogen fixation by common beans was increased from 50 to 72% of total nitrogen uptake with increasing rates of biochar additions (0, 31, 62 and 93 t ha^-1) to a low-fertility Oxisol (Rondon et al., 2007). Biochar also adds some macro (P, K, N, Ca and Mg) and micronutrients (Cu, Zn, Fe and Mn) which are needed for sustainable agriculture (Major et al., 2010). It may significantly affect nutrient retention and play a key role in a wide range of bio-geochemical processes in the soil, especially for nutrient cycling. A beneficial impact of biochar on the plant-available phosphorus has been observed in soils enriched with biochar, which in contrast to ammonium, is not a characteristic generally associated with soil organic matter (Lehmann et al., 2007; Steiner et al., 2007).
 
Soil nutient leaching prevention
 
Biochar has been found to decrease nutrient leaching thus enhancing nutrient availability (Chan et al., 2007). Higher nutrient availability for plant is the result of both the direct nutrient addition by the biochar and greater nutrient retention. Biochar may supply a source of plant-available nutrients once applied to the soil. This possibility is suggested by the strong adsorption affinity of biochar for soluble nutrients such as ammonium, nitrate, phosphate and other ionic solutes. Lehmann et al., (2007) found that “cumulative leaching of mineral N, K and Mg in the soil was only 24, 45 and 7%, respectively, to control biochar (Krishna Veni et al., 2017). Results of the column leaching experiment showed that biochar addition significantly influenced Na⁺, K⁺, Ca⁺⁺ and Mg⁺⁺ concentrations in the leachate.  For Na⁺ concentration, no significant difference was found with biochar amendment under non-saline irrigation compared to respective non-biochar control.
 
Modification of soil
 
Biochar is commonly alkaline. The pH values of biochar at different pyrolysis temperature ranged from slightly alkaline (8.2) to highly alkaline (11.5) across a wide variety of feedstocks. Biochars shows positive effect in the case of acidic soils compared to alkaline soils (Biederman and Harpole, 2013). Biochar addition can reduce the bio-availability of toxic forms of Al, Cu and Mn and increase the availability of essential nutrients such as Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺ and Mo, thereby rendering a favourable environment for plant growth (Atkinson et al., 2010, Naresh Kumar et al., 2018). Among different treatments 60 g kg^-1 of biochar application treatment was recorded the lowest exchangeable acidity this was followed by lime application @ 4 g kg^-1 of soil and the control recorded highest exchangeable acidity of without biochar and lime application. This might be due to the soluble and exchangeable Al₃⁺ precipitates as insoluble hydroxyl Al-species at higher pH condition. Apart from increasing the incorporation of biochar to acidic soil can release their base cations which can participate in exchange reactions and replace the exchangeable Fe₃⁺, Al₃⁺ and H⁺ on the soil surface and decrease the soil exchangeable acidity. Biochar can serve as a liming agent resulting in increased pH and nutrient availability for a different soil (Lehmann and Joseph 2007).
 
Biochar application on crop yield
 
Biochar on crop yield depend largely on the amount of biochar application and soil types. It is a positive effect on crop yield in general and it is more effective when applied to low to medium fertile soils (Chen et al., 2019). Liu et al. (2017) reviewed published data from 59 pot experiments and 57 field experiments from 21 countries and found crop productivity was increased by 11% on average. Liu found benefits at field application rates typically below 30 t ha^-1, field application and reported that increases in crop productivity varied with crop type with greater increases for legume crops (30%), vegetables (29%) and grasses (14%) compared to cereal crops corn (8%), wheat (11%) and rice (7%). Combination of higher biochar application rates with NPK fertiliser increased crop yield on tropical Amazonian soils (Steiner et al., 2007) and semi-arid soils in Australia (Ogawa, 1994). Major et al., (2010) conducted a multiyear experiment in a maize - soybean rotation system and found that the maize yield was increased by 28 to 30 per cent in two years continuously cultivation. Several authors are confirmed the biochar application on crop yield will be increased are given in the Table 3.


 
Carbon sequestration
 
When it is added to soil, biochar has beneficial for growing crops; additionally biochar contains stable carbon (C) and after adding biochar to soil, this carbon remains sequestered for much longer periods than it would in the original biomass that biochar was made from. The biochar can rapidly increase the recalcitrant soil Carbon fraction of soil. Biochar also contains varying concentrations of other elements such as Oxygen (O), Hydrogen (H), Nitrogen (N), Sulfur (S), Phosphors (P), base cations and heavy metals (Segun et al., 2019). The application of biochar had significantly influenced the soil OC content after 3 years of experiment in Alfisol, at the end of the experimentation control soil had only 3.6 g kg^-1 OC, whereas the soils that received different sources of biochar had soil OC content ranged between 4.4 and 4.8 g kg^-1. Increase in the levels of biochar increased the content of OC, WSC (73%) and BMC (37%) in studied soil. (Kannan Pandian et al., 2016).

Biochar addition seems to generally enhance plant growth and soil nutrient status and decrease N₂O emissions. Its amendment reduced CO₂ production for all amendment levels tested (2, 5, 10%, 20, 40 and 60% weight by weight basis; corresponding to 24 to 720 t ha^-1 field application rates). The recalcitrance of the biochar suggested that it could be a viable carbon sequestration strategy and might provide substantial net GHG benefits with long lasting reductions in N₂O production. Biochar has potential to mitigate climate change as maximum of 1.8 mt of CO₂ equivalents per year without affecting food security and ecosystem. This is equivalent to 12% of current anthropogenic CO₂ emissions annually (Woolf et al., 2010). The extent of this stimulation varies according to different estimates, being larger up to 60% in forest and smaller about 14% for pastures and crops. To assess the carbon sequestration potential of adding biochar to soil, we must consider four factors viz., the longevity of char in soil; the avoided rate of GHG emission; how much biochar can be added to soils and how much biochar can be produced by economically and environmentally acceptable means.
 
Effect of biochar application
 
Adverse effect
 
At this high application rate, yields decreased to the level of the unamended control. This is a very large amount that is unlikely to be practically feasible in the field, at least for a one-time amendment. However, Asai et al., (2009) working in Laos reported greater upland rice yields with 4 t ha^-1 biochar, but when 8 or 16 t ha^-1 were applied, yields were not different from the unamended control. A more recent field study on a poor, acidic soil of the USA showed that peanut hull and pinechip biochar applied at 11 and 22 t ha^-1 could reduce corn yields below those obtained in the control plots, under standard fertilizer management (Gaskin et al., 2010).
 
Residual effect
 
The residual effect of biochar and mineral fertilizers was assessed using a mycorrhizal bioassay for soil collected from the field trial 2 years after application of biochar. Biochar and both fertilizers increased mycorrhizal colonisation in clover bioassay plants. Deep-banded biochar provided suitable conditions for mycorrhizal fungi to colonise plant roots (Sdaiman et al., 2010). The application of biochar @ 0, 5, 10 and 20 g kg^-1 soil with and without 5 g kg^-1 of dried swine manure and results show that a significant decrease in the total amount of N, P, Mg and Si that leached from the manure amended columns as biochar rates increased but among columns receiving manure, the 20 kg ha^-1 biochar treatments reduced total N and total dissolved P leaching by 11% and 69%, respectively (Laird et al., 2010).

The soil health concept has increased awareness among agriculturist and horticulturist regarding the importance in maintaining soil fertility, crop productivity and environmental quality over a long term period. Biochar has positive effects on the physico-chemical and biological properties of soil, which means soil health directly and indirectly influenced with the application of biochar. The positive gains of biochar application in the soils include: retaining nutrients and cation exchange capacity, decreasing soil acidity, improving soil structure, nutrient use efficiency, water holding capacity, decreasing release of non-CO₂ green house gases (CH₄, N₂O), increased number of beneficial soil microbes.