Research Progress on the Effect of Biochar on Soil Organic Carbon

Shuhong LIU

Abstract Soil organic carbon has been identified as one of the most important components in soil. Changes in soil organic carbon content will significantly affect atmospheric CO2 concentration， which is related to soil fertility， water holding capacity， nutrient cycling， and changes in microbial communities. In recent years， biochar has been proven to be an effective material for increasing carbon storage， and research on the application of biochar in soil ecosystem carbon sequestration and emission reduction functions has received widespread attention. However， due to differences in the specific methods and research objects used in the research process， the current mechanism of the impact of biochar on soil organic carbon is still controversial. This paper mainly revealed the effects of biochar on soil organic carbon from the aspects of the effects of biochar addition on soil aggregates and microorganisms， and also elaborated on the direction worth exploring and objectively evaluated the impact mechanism of biochar on soil organic carbon.

Key words Biochar; Soil organic carbon; Soil aggregate; Soil microorganisms

Soil is an important component of the terrestrial biological carbon pool， which constitutes the main exchange pool and reservoir pool of carbon in the ecosystem together with the terrestrial biosphere and the atmosphere[1]. Soil organic carbon is one of the most important components in the soil because it has multiple roles in soil fertility， water holding capacity， nutrient cycling， and the development of microbial communities[2]. Changes in soil organic carbon pool will significantly affect atmospheric CO2 concentration. However， due to land-use conversion and degradation globally， soil organic carbon reserves have declined dramatically. Therefore， more and more attention is focused on how to restore degraded soil and increase soil organic carbon reserves. At present， the technology of using biochar to improve the soil environment has received widespread attention. Biochar application to soil is a win-win policy， which can not only follow the principle of resource recycling and solve the problem of resource waste and environmental pollution， but also plays an important role in increasing soil fertility and mitigating climate change[3]. Previous studies have shown that after adding biochar， soil pH， electrical conductivity （EC）， cation exchange capacity （CEC）， water holding capacity， and nutrient retention can be improved， and there are significant effects in increasing soil carbon storage， reducing soil greenhouse gas emissions and improving the retention and utilization efficiency of nutrients[4].

The impact of biochar on soil organic carbon has been expanded to the research of the impact mechanism with the deepening of the research， but most of them focus on a single aspect， and the impact results are different. There are also inconsistent views on the effects of biochar application on soil organic carbon， and there is a lack of induction and unification of the impact mechanism. Therefore， this paper mainly analyzed the two aspects of soil aggregates and microorganisms， summarized the effects of biochar added to the soil on the structure of soil aggregates and microbial communities， and then discussed the impact on soil organic carbon production. Moreover， based on the existing research results， we propose following two mechanisms： the addition of biochar to the soil can promote the formation of large aggregates （>0.25 mm） in the soil， enhance the stability of the aggregates， and strengthen the physical protection on organic carbon， which is the result of the interaction between biochar and soil organic matter; and biochar added to the soil changes the physical and chemical properties of the soil， causing changes in the structure of the microbial community， weakening it to biological activity， reducing the amount of organic carbon degraded by microbes and increasing the content of soil organic carbon.

Biochar promotes carbon sequestration in soil aggregates

Effect of biochar on soil aggregate structure

Soil aggregates are an important part of the soil. They are the material basis for good soil structure， and have high stability. In soil science， they are generally divided into large aggregates （>0.25 mm） and micro-aggregates （<0.25 mm）[5]. The number distribution and spatial arrangement of aggregates of different sizes determine the distribution and continuity of soil pores， and have a very important impact on the structure， quality and fertility of the soil. They not only affect the water holding capacity， porosity and physical properties such as corrosion resistance， but also affect the activities of soil microorganisms and have an impact on the transfer， circulation and savings of nutrients such as carbon and nitrogen in the soil[5].

The porous structure and surface of biochar are rich in a large number of compounds. Through the adsorption capacity and strong stability of biochar， organic matter on the surface of biochar and minerals in the soil form a large number of organic-inorganic mineral complexes， which promote the formation of soil aggregates. Biochar added to the soil， due to its own adsorption capacity， can also enhance the overall soil adsorption capacity and enhance soil microbial activity， thereby generating more secretions， which form the cementing material of soil aggregates and promote the formation of soil aggregates. In addition， soil aggregates will have a certain physical protection effect on biochar， which will not be decomposed by microorganisms and will be retained in the soil for a long time. Some studies have found that the addition of biochar can significantly increase the content of large aggregates， especially aggregates with a diameter larger than 1 mm， and the number of large aggregates increases with the increase of the amount of biochar added[6]. The results of research by Zheng et al.[7] showed that the addition of biochar can significantly increase the content of large aggregates in soil. However， some studies have also shown that biochar has no significant effect on changes in aggregate content， which is mainly related to conditions such as soil type， addition amount， and cultivation period[5-7]. Some experts have suggested that biochar provides a large amount of available organic matter for microorganisms after being added to the soil， and promotes the active decomposition of soil organic matter by microorganisms， making the content of soil aggregates show a decreasing trend.  Therefore， the effect of biochar on the structure of soil aggregates is related to the properties of biochar， the amount of biochar， the physical and chemical properties of the soil itself， and the test cycle.

Effect of biochar on the stability of soil aggregates

The stability of soil aggregates refers to the ability of the aggregates to maintain their original form when they face external forces or when external environmental conditions change. It usually includes water stability， mechanical stability， biological stability， chemical stability， acid-base stability， etc.[5]. The results of research by Li et al.[8] showed that the stability of red soil aggregates in Yunnan tobacco areas was significantly improved after the application of biochar.  The main reason is that the surface of biochar contains a large amount of oxides， and the interaction of carboxyl groups related to the oxides with soil organic matter can form stable organic-inorganic complexes， which effectively promote the formation of soil aggregates， and enhance the stability of soil structure. However， Herath et al.[9] showed that the addition of biochar had no effect on the stability of soil aggregates. The possible reason is that the amount of biochar added to the soil was small， and the amount of decomposition was small， and no cementing agent that promotes agglomerate binding was generated during the decomposition process.

Effect of biochar on soil aggregate organic carbon

Soil aggregates and organic carbon are important indicators for evaluating soil quality， and they are usually closely related. On the one hand， the physical protection of the aggregates can prevent the internal organic carbon from being broken down by microorganisms and increase the stability of soil organic carbon. On the other hand， soil organic carbon is an important cementing material， which can enhance the agglomeration of soil particles and promote the shape of the aggregate structure[3-4]. Adding Biochar to the soil， on the one hand， due to its strong stability and high organic carbon content， can increase the difficulty of mineralizing organic carbon and directly increase the content of soil organic carbon. On the other hand， biochar can adsorb soil organic molecules and promote the polymerization of smaller organic molecules to form larger organic matter through the surface catalytic activity， thereby promoting the formation of large soil aggregates which enclose a large amount of organic carbon. Therefore， the addition of biochar to the soil can promote the formation of large aggregates in the soil， strengthen the physical protection of soil organic carbon from microbial decomposition， improve the stability of the aggregate organic carbon， and then effectively increase the overall organic carbon content of the soil.

Effects of biochar on soil microorganisms and organic carbon

Effect of biochar on soil microbial biomass

Soil microorganisms are the most active component in soil carbon pools and are most sensitive to environmental changes. They play a major role in the nutrient cycling and energy flow of soil ecosystems， and can provide important ecosystem services[10]. Biochar， as a porous carbon-rich substance， can be directly added to the soil to provide nutrients for microbial growth， and its pore structure provides a good living environment for microorganisms[5]. The research results showed that the input of straw charcoal can increase the amount of soil microorganisms and biomass[7]， but the addition of woody biochar will reduce the content of soil microbial biomass[9]， which may be due to the fact that straw charcoal contains richer microbial available components and habitat environment than woody charcoals. Zavalloni[11] found that adding biochar to the soil in a short term had no significant effect on microbial biomass. It may be because there is a large amount of carbon in the biochar that is difficult for microorganisms to degrade and utilize and has low microbial effectiveness. Therefore， the effect of biochar on soil microbial biomass is closely related to the properties of biochar， the cultivation cycle， and the physical and chemical properties of the soil itself.

Effects of biochar on soil microbial structure

The structure and component properties of biochar prepared from different raw materials are different， and will be used by different microbial groups[10]， and the microbial community structure changes caused by it will also be different. It is considered that， due to the limited content of biochar that can be directly used by microorganisms， the change of microbial community structure is mainly achieved through indirect approaches， such as affecting soil nutrient status， chemical properties， and signal substance transmission between microbial cells[10-12]. In addition， the changes in soil microbial activity and community composition of biochar are also affected by the nature of the applied soil.

Effects of biochar on soil microbial biomass carbon and organic carbon

There is a cooperative feedback relationship between soil organic carbon dynamics and microbial community succession. The addition of biochar to the soil mainly affects soil microorganisms by changing the soil environment， and the mechanism of soil carbon fixation is closely related to the evolution of soil properties and soil formation processes. Previous studies have shown that the addition of biochar to the soil can promote the decomposition of soil organic matter by soil microorganisms and reduce the content of soil organic carbon. However， some studies have shown that iron ion-modified biochar reduces soil microbial biomass and effectively increases soil organic carbon content. Therefore， the addition of biochar to the soil can affect the soil organic carbon content by changing the soil microbial community structure.

Shuhong LIU. Research Progress on the Effect of Biochar on Soil Organic Carbon

Prospects

As a new type of carbon-rich organic material， biochar has a huge advantage in improving soil organic carbon content. Biochar added to the soil can affect the soil organic carbon content by changing the structure of soil aggregates and microbial communities. However， the current research on the differences in the properties of biochar on soil microbial properties is still not thorough enough， and the research on the impact mechanism of microorganisms on soil organic carbon is still incomplete， and the internal mechanism needs to be strengthened.

References

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