Current World Environment

Introduction

Carbonis stored within soil mainly in two forms, soil inorganic carbon and soil organic carbon.The source of inorganic carbon (IC) in the soil iseither parent rock material or the dissolution and precipitation of carbonate.SIC has relatively a longer turnover time because of lower activity,but redistribute rapidly in a short time period under high microbial activity^1,2.Organic carbon forms of soil are the residues of plants and animals. Soil carbon ranges from freshly deposited plant debris to highly resistant forms such as humus. Soil carbonparticipatesin nutrient cycling, grain yield and various other soil functioning^3,4,5,6,7,8. The turnover rate of SOM influences the biogeochemical transformation of nutrients and stability of soil structure⁹. SOM is categorise into diverse groups by their turn over rates and their regulatory elements¹⁰. Carbon fractions with higher turnover rate such as particulate organic carbon, potentially mineralizable carbon, microbial biomass carbon, and KMnO4 oxidizable carbonare more superficial to forecast changes in SOC dynamics than the total SOC^11,12,13. Soil organic carbon was classified into different fractions by their protection covering into protected (physical and biochemical protection) and unprotected organic carbon¹⁴. SOM is a mixture of substance that diverges in nutrient content and also reflects dynamicity because of its continuous changes in turnover time¹⁵. SOM is a collection of a number of carbon pools whose decomposition rate is ranging from a year (very active) to thousands of years (stable)^16,17. Labile soil organic carbonfractions, decompose relatively easily and have a greater turnover rate as compared with recalcitrant organic C fractions¹⁸. They are not only involved in the biogeochemical transformation of nutrients such as N, P and S, but also contribute to soil structure and stabilization¹⁹. Labile organic C fractions react more rapidly to changes in the environment or pollution than recalcitrant organic carbon, and are thus actin gas the primary indicator of soil quality^20,21,22.

Soil Carbon Fractionation

Soil carbon fractionation is separation of soil organic matter into diverse pools based on their inherent properties^1,23. Carbon fraction with relatively larger turnover rate acts as quick and sensitive indicator of soil quality dynamics^24,21,18. The main drawbacks of organic carbon as an sensitive indicator of changes in soil quality is its testing ability, i.e. change in SOC in response to fluctuations in controlling factors, due to the longer turn over time and the large amount of stable organic carbon already present in soil¹¹. Use of subparts, or fractions, of carbon as sensitivity indicator, makes soil carbon pool, very useful for estimating the impact of different management practices on soil carbon^25,26,27.

Each fraction of SOC may differ in size, composition, Physico-chemical properties and rate of decomposition. Microclimate of soil such as soil type, water availability and management practices can influence the amount, relative stability and biological availability of carbon stored in the different fractions. Soil carbon fractionation provides an overview of plant function with a property of controlling overall quality and quantity of litter. Soil carbon composition and turnover is influenced by land management, relations between living and non-living elements, and associated rates of biogeochemical cycles. Intrinsic properties of discrete fractions, their degree of stability and location in the soil matrix also influences composition of a particular SOM fraction. Soil carbon fractionation is helpful for evaluating carbon dioxidereleases, the capability of soils for carbon storage in response to climate change. Different soil carbon fractions with an ability to rapidly change in response to different management practices proves as an efficient tool to identify optimized agricultural management practices that results in the increase in quality and quantity of soil carbon.

Carbon Pools in Different Size Aggregates

Different carbon pools influence aggregation differently and also SOM is influenced by aggregate turn over^28,29,30,31. Thus C content in aggregates and their size are interdependent. C pools in soil aggregates provide substantial view on decomposition and the storage capacity of carbon in different aggregate size fractions. Suitable management practices help in protection of carbon pools in soil aggregates. The distribution of the C fraction among soil aggregate size fractions is variable^32,33. On the basis of turn over time C pool is classified as Active, Slow and Intermediate pools. Active pools change seasonally and influence aggregation^34,35. Soil carbon concentrations alter gradually over time because of their bulk quantity³⁶. Particulate organic carbon isreflected as transitional pools in soil carbon dynamics and acts as a substrate for microbial activity^28,36,29.

Soil Organic Carbon as an Indicator of Soil Quality

Physical, chemical, and biological fractions of SOC pools provide a sensitive indication of changes in the SOC dynamics relative to total soil carbon alone. Soil biological quality can be used as a sensitive indicator of soil quality evaluation, including soil carbon sequestration in response to sustainable management practices³⁷. The carbon dynamics also depends on the microbial population, the intrinsic properties of plants and the availability of nutrients³⁸. Soil microbes tangiblyorganise soil particles together and enrich soil clump which protects C in macro aggregates^39,40. Soil organic matter is physically protected within aggregates⁴¹.Aggregates have a major impact on microbial community structure, gaseous exchange, water maintenance and nutrient cycling⁴². Soil structure consisted of free primary particles (sand, silt, and clay), micro aggregates, and macro aggregates as a physical unit of aggregates. Binding forces responsible for their aggregation are mainly microbial and plant-derived carbohydrates in association with different metals and compounds⁴³. Increase in organic carbon increases soil aggregate stability. Fresh residue is a base of soil aggregation by providing C source for microbial activity and the creation of clumping agents as a by-product. Microscopic populations and their functions fluctuatein aggregate fractions of different sizes⁴⁴.Soil aggregates have a high variation in microbial communities and activities at a microbial scale and binding of organic matter within soil aggregates also report long-lasting variations in C pools in soil aggregates⁴⁵. Soil aggregation influences conservation and mineralization of carbon⁴³. Macroaggregates are fractions of larger than 250 micro msize and are composed of organic residues and coarse sand coated with a clay matrix. These macro-aggregates were colonized by numerous microorganisms⁴⁶. Macro aggregate organic matter content decreased with cultivation⁴⁷.Long-term cropping decreases the soil organic matter and caused a reduction in macro aggregation, but micro aggregation was relatively unaffected⁴⁸. Macro aggregates have more organic C, N, and P, than microaggregates. Microaggregates (20 to 250 micro m size fractions) are the building blocks of soil structure and become united to form macroaggregates. Organic matter concentration of microaggregates was considerably less than that of macroaggregate in soils. Microaggregates have more N and P than C. Microaggregates are rich in organic matter which are highly processed, humified, old, passive and recalcitrant^49,50,51.

Effect of Management Practices on SOC Pool

The soilproperties vary widely in differently managed ecosystems results in variations in soil functioning^52,53,54,55. Inappropriate management of soil leads to soil erosion, exhaustion of organic matter and other nutrients which results in permanent soil mortification and efficiency losses^56,57.

For the sustainable growth of both productivity and soil fertility, a good management practices are essential⁴⁰. A variation in management practices results in alterations in quality and quantity of soil carbon^58,59. Conventional management practices include plowing, chisel plowing and multiple tillage trips. Conventional tillage incorporates residues into soil, thereby increases soil-residue contact, favouring rapid decomposition of soil organic matter⁶⁰. Incorporation of soil residues results in an optimum level of moisture and temperature with proficient microbial population, healthier soil structure and significantenhancement in soil qualities and thus decreases the degradation of carbon⁶¹. Soil biological, chemical and physical quality,enhanced in No-tillage and degraded under conventional tillage under a certain period of time^{62,63,64,65,66,67}. According to Lee et al., tillage can physically disrupt the soil aggregates and decreases its stability⁶⁸. Conservation management practices include putting offthe remains of plant biomass on an uninterrupted surface which greatly enhances C accumulation within soil aggregates. Increased tillage intensity in many conventional tillage systems decreases total carbon, particularly active carbon and increases catabolism of carbon by disrupting soil aggregates and exposure of aggregate protected C to microbial attack⁶⁹. The adoption of conservation tillage practices offers soil carbon sequestration opportunity, more favourable plant growth environment and soil health improvement relative to conventional tillage⁷⁰. Conservation tillage practices   prevent the loss of SOC^71,72.Among the conservation and conventional management practices, the conservation management practices implemented better to develop soil quality properties over time. No-till greatly enhances long term carbon conservation within different sizes of soils^73,74.

Role of Organic Carbon in Productivity of Soil

Soil has anability of renovation and accumulation of nutrients and their kinetics. The efficiency of an Agro-ecosystem depends on its productivity. SOM supports ecosystem function in terrestrial systems.Recalcitrant materials with lengthier residence times observably involve the major pools in soils. SOC has the capabilityto control water and air movement. SOC is closely associated with the soil’s biological and physical status.  Physico-chemical reactivity of soil is also influenced by the SOC. Soil carbon plays a vital role in regulating ecosystem services. Management systems influence all the internal and interrelated properties of soil over time⁷⁵. Soil management practices to obtain various socio-economic and environmental benefits requires decisive action that maintains and enhances soil carbon. Integrated policies and incentives are required to limit soil carbon loss due to bad management practices that results in loss and discharges of greenhouse gases to the atmosphere⁷⁶. Soils are vulnerable to carbon losses and results in the release of carbon dioxide to the atmosphere as a consequence of accelerated degradation due to land use change or unsustainable management practices⁷⁷. Microorganisms presented in the soil produce certain substances that act as binding material for soil particles⁷⁸. These soil aggregates acts as a store house of soil carbon and also helps in the stabilization of soil carbon. Soil carbon is very essential for soil aggregate stability and plant nutrient intake^7,8. If more carbon is stored in the soil as organic matter, it will result in reduction of atmospheric carbon, which means reduction of greenhouse gases and global warming. The storage of carbon in soil as soil organic matter is called ‘soil carbon sequestration’.

Advantages to Increasing or Maintaining a High Level of SOM

Soil organic carbon is the principal associate of soil productiveness. It reserves and discharges nutrients for plant and microbial growth, stimulates the soil structure, healthiness of soil, and acts as a buffer against harmful substances. Carbon present in the organic material expands drought resistance of soil, structural stability and protects it from soil  erosion⁷⁹.Soil organic matter acts as a reservoir of nutrients and is a buffer against harmful substances. Soil organic matter isvital for crop productivity and proliferation of soil microbes, promotes the structure, physical and biological health by providing food and substrate⁸⁰.A direct increase in productivity is observed in response of increasing the carbon content of soil by adopting various management practices⁸¹.There is a limit of storage capability of soil; soil,limited in carbon content has the potential to capture and store carbon. Soil organic carbon is important for the soil health and sustainable agriculture. Organic carbon in soil plays an important role in providing a suitable environment for proper growth of plants like improving resistivity for pests and diseases by making them healthier and acting like a reservoir of water during drought period because of its property of sponginess.

Conclusion

Soil management practices affect soil carbon pools. Various management practices like the addition of appropriate manure, no-tillage and crop rotation can be utilised to increase soil productivity by increasing carbon inputs. Soil carbon fractionation is helpful for predicting trace gas emissions in the atmosphere and ecosystem dynamics in the lithosphere, in response to climate change. The fractionation procedure can be used to determine the state and the rate of change in carbon of different soil systems.  For increasing SOC in soil, certain conservation practices may be adopted. Management and improvement of soil quality are imperative for the vastly growing population who conservatively depends on the soil resources for a constant supply of food and fibre. There is a need for farmers to be made aware them about the facts of soil carbon in agriculture so they can make a proper decision for adopting a suitable management practices for enhancement of productivity and fertility of soil simultaneously.

Acknowledgements

The authors are thankful to the Ministry of Human Resource Development, Government of India for funding the research work. We are also grateful to the Department of Environmental Science and Engineering, Indian School of Mines, Dhanbad for providing the logistic support to carry out field monitoring and laboratory analysis.

References
 

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