Yes, biochar can reduce carbon dioxide levels by sequestering carbon in the soil and enhance agricultural sustainability by improving soil health and fertility.

First of we should know about what are the definition for key word that are in the question.

01. Biochar is a stable solid, Rich in carbon that is made from organic waste material (wood chips, leaf litter, dead plants.) or biomass that is partially combusted in the presence of limited oxygen condition. And this process is also called aspyrolysis.

02. Sustainable agriculture is defined as the integration of biological, chemical, physical, ecological, economic and farming and social sciences in a comprehensive way to develop new farming practices that are safe and do not degrade our environment. (Lichtfouse et al., 2009)

First question is can biochar reduce carbon dioxide levels. Cannot give exact answer for this.In more situation biochar can reduce the carbon dioxide levels. But in some condition and situation biochar can enhance the carbon dioxide levels. Therefore, some situation biochar can reduce CO2 levels and in some situation, biochar can't reduce CO2 levels. (These answers are based on some researches that are related to the biochar.)

In order to research there are some factors affected to the above answers.

A. Soil type.

B. site condition.

C. Feedstock used to make biochar.

D. Methods used to combustion.

E. Type of microorganisms in the soil.

Normally when we consider the burning organic waste material or biomass we do it in a open field and more oxygen condition. Therefore, burning of organic matter, carbon dioxide, Methane, Nitrous oxide like gases are emission in to the atmosphere. These gases are we can call as a Green House Gases. It caused increase the global warming.

When we consider the making of biochar is a process do in the lack of oxygen condition and the controlled field condition. Organic plant material has the unstable carbon and the process of pyrolysis convert it to a more stable form of carbon and it can call as the biochar. When biochar applied to the soil it stores the carbon in a secure place for hundreds or thousands of years. So, when we consider the burning of organic waste under process of pyrolysis it always reduces the carbon dioxide amount that are releasing to the atmosphere than normal condition. But when we add biochar in to the soil, situation vary with factors that I have mention.

Some researchers are shown, when biochar add to the soil levels of carbon dioxide that are emission in to the atmosphere can be reduce or increase. The reference is,

• In study with paddy soil, application of biochar from bamboo and rice straw decreased rates of carbon dioxide emission from the amended soil.
• In a study conducted in Colombia reported increased CO2 emissions when soil amended with biochar from mango trees.
• In a field study of biochar application to an arable soil in England, reported a significantly higher CO2 emissions from unplanted plots amended with biochar but no significant increase was observed from vegetated plots with biochar.
• In a field study with cattle waste biochar in Australia, reported no significant differences in GHGs emissions from control when compared to biochar-amended red Ferrosol soil.
• Application of biochar led to a reduction in CO2 emissions from SOC-(Soil Organic Carbon )poor soil . Whereas in the same study, biochar application led to increased CO2 emissions from SOC-rich soil.

How it happens I think you have some of confusion about that. So, when we consider biochar mainly it can sequestration the carbon dioxide as a more stable carbon. But it is not only function of biochar it has more functions. I talk about what are these functions later.

In relation to explain for I provide one of main function. It is biochar can increase the soil microbial activities and their metabolism. So that microorganisms can breakdown the Soil Organic Carbon more rapidly. Therefore, it causes the increase of carbon dioxide emission.

• So, some soil has high SOC adding biochar can may be increase the emission of the carbon dioxide and when we consider the low SOC has some soil adding biochar can maybe reduce the emission of CO2 than when soil has high SOC. This can also vary from some factors. ( Eg; Types of microorganisms, Soil physical and chemical properties etc.)

Soil organic carbon, this carbon constitutes the largest terrestrial carbon pool and makes soils integral to our planet's carbon cycling. On a global scale, the top 30 cm of soil contains more carbon than the atmosphere and vegetation combined. (https://www.fao.org/home/en/)

• In conclusion of first part of the question is, when we simply thinking only about combustion of organic material or biomass and form of biochar definitely it helps to reduce CO2 emission and biochar can sequestration CO2 as a more stable carbon.

But when we consideradding biochar into soil situation is somewhat different. Maybe it can help to increase the emission of CO2 or can reduce the emission of CO2 or cannot see significant different of whether increase or decrease.

So, when we add biochar in to the soil we can't simply adding it. We have to consider more factors that are affecting for it. Otherwise it causes increase the Green House Gas (GHG) emission.

Second part of the question is roles of the biochar, Biochar has more function one of major function is carbon sequestration. Other than that, there are more functions. It can increase the soil physical and chemical and biological properties. Biochar can influence the soil surface area, soil porosity, Pore size distribution, bulk density, soil water holding capacity, soil color like soil physical properties.

When we consider biochar, it has higher surface area and more soil pores therefore it helps to retain more water in soil. It helps to grow plant under water shortage, it is one of advantage and the function of the biochar. increasing pore size help toreduce the bulk density of the soil bulk density mainly affect to the root growth of the plant. If there is low bulk density cause the easy and preferable root growth. And also, biochar can improve the soil color.

And also, biochar can improve the soil nutrient retention, reduce nutrient leaching, can adjust the soil pH (It can use as a soil amendment to the acidic soil), Increase the microbial activity and increase the Cation Exchange Capacity (CEC). When we consider biochar, it has negative sign therefore it can retain the positive ion which are important to the plant growth. It caused the reduce of fertilizer requirement.

Biochar can provide suitable habitats for microorganisms and help to increase their metabolic activities. it is one of the major functions which i had already discuss earlier. CEC is a measure of a soil's ability to hold and exchange cations. It also related to the nutrient retention.

When adding biochar in to the soil can improve the crop yield because of enhance of physical, chemicals and biological prosperities. But there is one thing only add biochar in to soil can improve crop yield but it is limiting value. For prevent that kind of limitation we have to have add the inorganic fertilizers. We have to use the integrated fertilizer application system, then we can get the appropriate harvest. Researches provide evident for that,

Maize yield was increased by 15.8% and 7.3% without N fertilization, and by 8.8% and 12.1% with N fertilization under biochar amendment at 20 t ha−1 and 40 t ha−1.

application of biochar to calcareous and infertile dry croplands poor in soil organic carbon will enhance crop productivity and reduce GHGs emissions.(Zhang et al. 2012b)

Therefore, in conclusion biochar help to increase soil health and improve the sustainability of agriculture.

Dr Vidoosha Sandeepa thank you for your contribution to the discussion

Effectivement, le biochar peut réduire le CO2 grâce à sa richesse en carbone stable dans le sol.

De nos jours, le biochar est une technologie adaptée surtout aux conditions locales pour développer l'agriculture durable

Dr Jeancy Ntuka thank you for your contribution to the discussion

Je vous en prie Dr Neresha.

Je serai de partager davantage avec vous sur la question.

J'y travaille là dessus depuis 2019.

Mon email est [email protected]

Dr Jeancy Ntuka thank you for your contribution to the discussion