Soil Carbon : An Overview on Soil Carbon Function and its Fractionation

The atmospheric carbon reservoir is significantly affected by change in lithogenic carbon reservoir. Carbon reservoir of soil is strongly influenced by the interaction between different biogeochemical cycles and environmental processes. At the local scale land use and soil management have also a significant impact on the soil carbon pool. Soil carbon is the major determinant of soil quality and agronomic viability because of its influence on other soil features. Different fractionation procedure is often described in terms of the soil organic matter (SOM) pool sizes, chemical properties, and turnover rates. Soil carbon separation includes physical and chemical methods and their combinations in a sequence. Separation of soil carbon according to soil aggregate size, density and magnetic properties is termed as physical separation whiles their solubility, oxidizability, hydrolysability is used in chemical fractionation. Carbon fractionation is a useful tool for observing the changes in different labile, recalcitrant and total pool of soil carbon. Thus, helpful in making decision to locals for adopting proper management practices suitable for a particular soil type in response to economic productivity and soil quality.

Carbon is stored within soil mainly in two forms, soil inorganic carbon and soil organic carbon.The source of inorganic carbon (IC) in the soil is either 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 carbon participates in nutrient cycling, grain yield and various other soil functioning 3,4,5,6,7,8 .The turn over rate of SOM influences the biogeochemical transformation of nutrients and stability of soil structure 9 .SOM is categorise into diverse groups by their turn over rates and their regulatory elements 10 .Carbon fractions with higher turn over rate such as particulate organic carbon, potentially mineralizable carbon, microbial biomass carbon, and KMnO4 oxidizable carbon are 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 carbon 14 .SOM is a mixture of substance that diverges in nutrient content and also reflects dynamicity because of its continuous changes in turnover time 15 .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 carbon fractions, decompose relatively easily and have a greater turnover rate as compared with recalcitrant organic C fractions 18 .They are not only involved in the biogeochemical transformation of nutrients such as n, P and S, but also contribute to soil structure and stabilization 19 .Labile organic C fractions react more rapidly to changes in the environment or pollution than recalcitrant organic carbon, and are thus acting as 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 turn over 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 soil 11 .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 di oxide releases, 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
D i f fe r e n t c a r b o n p o o l s i n f l u e n c e 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 quantity 36 .Particulate organic carbon is reflected 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 practices 37 .The carbon dynamics also depends on the microbial population, the intrinsic properties of plants and the availability of nutrients 38 .Soil microbes tangibly organise soil particles together and enrich soil clump which protects C in macro aggregates 39,40 .Soil organic matter is physically protected within aggregates 41 .Aggregates have a major impact on microbial community structure, gaseous exchange, water maintenance and nutrient cycling 42 .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 plantderived carbohydrates in association with different metals and compounds 43 .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 fluctuate in aggregate fractions of different sizes 44 .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 aggregates 45 .Soil aggregation influences conservation and mineralization of carbon 43 .Macroaggregates are fractions of larger than 250 micro m size and are composed of organic residues and coarse sand coated with a clay matrix.These macro-aggregates were colonized by numerous microorganisms 46 .Macro aggregate organic matter content decreased with cultivation 47 .Long-term cropping decreases the soil organic matter and caused a reduction in macro aggregation, but micro aggregation was relatively unaffected 48 .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 soil properties 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 essential 40 .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 matter 60 .Incorporation of soil residues results in an optimum level of moisture and temperature with proficient microbial population, healthier soil structure and significant enhancement in soil qualities and thus decreases the degradation of carbon 61 .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 stability 68 .Conservation management practices include putting off the 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 attack 69 .The adoption of conservation tillage practices offers soil carbon sequestration opportunity, more favourable plant growth environment and soil health improvement relative to conventional tillage 70 .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 an ability 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 capability to 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 time 75 .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 atmosphere 76 .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 practices 77 .Microorganisms presented in the soil produce certain substances that act as binding material for soil particles 78 .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 erosion 79 .Soil organic matter acts as a reservoir of nutrients and is a buffer against harmful substances.Soil organic matter is vital for crop productivity and proliferation of soil microbes, promotes the structure, physical and biological health by providing food and substrate 80 .A direct increase in productivity is observed in response of increasing the carbon content of soil by adopting various management practices 81 .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.