Dear Lakshmi Gopakumar

With what specific ecosystem you plan to compare SOC (soil org. carbon)? Is it a natural forest, plantation, pasture, agriculture, grassland, peat/bog, wetland, or else? Each type furnish different level of SOC range.

Beside spatial diversification, SOC may differ with vertical depths/ horizons.

Regards

Siddiq

Here in Brazll most soil reference tables indicates that a content higher than 1,5% of organic matter in thesurface soil is considerated adequate to soil plant nutrition

soil organic matter content depends on environmental conditios like climate (low temparature) x height, water content and alo soil properties (soil texture, structure..)

Organic carbon in soils depends on many factors like, land use, climate, soil substrates etc . The standard methods for determination of Org. C is to be followed and sample should be of 100mesh size. Nevertheless the fact remains there is no optimum limit for organic carbon. In rubber growing areas of Kerala and other part Goa (Typical tropical region), the carbon may be as high as 1.0 to 1.5% at the surface and decrease sharply in subsoils  whereas in some areas it may be as low as 0 .5-0.8%.

Soil organic carbon (SOC) concentration varies depend upon land use types such as Forest, Grassland, agriculture land, plantation and so on. Ultimately in all the land uses, SOC storage is totally depend on litter input, litter decomposition and soil respiration. Litter decomposition and soil respiration is depend on the following main factors such as temperature, soil moisture and soil microbial diversity. Soil microbial diversity is more important for litter decomposition and this totally depend on the quality of litter. Apart from the above you should know the depth from which you have collected soil samples. Generally SOC decrease with increasing depth (exceptions are there). Tropical soils are more dynamic and act as a sink for carbon. Hope its clear. Thanks.     

Soil organic carbon can varyies from 0.1 to more than 0.5 (in mass)  from very sandy to organic soil horizons in the tropical region. What you mean for optimal range ? The natural range will change with soil classes (the main factor ontrolling will be - texture, mineralogy), landscapes position, vegetation and local climate (including soil climate). 

Soil organic C is much more depends on C inputs in a particular ecosystem and in any ecosystem temperature and moisture plays a very key role in SOC decomposition. For example SOC in temperate soils have less decomposition than in tropical soils.

To add my 2c on what Gudigopuram has said, also soil type plays a huge role in determining optimum ranges for SOC for instance, sandy soils are generally known to have lower SOC than clay soils due to their poor water and nutrient holding status, the opposite is true for most clay soils. This property influences microbial activity which also influences your carbon levels in the soil. The norms for the different soil types are available on the Cornell soil health assessment handbook. You should however note as well that the type of OM input differs in area and therefore the norms might slightly differ for tropical areas vs dry areas.

Thanks for the answer and information regarding reference.

Highly appreciative discussion , though , i am bit alte into the discussion..I am enclosing the abstract of very good work entitled Soil Organic Carbon Thresholds and Nitrogen Management in Tropical Agroecosystems: Concepts and Prospects

Soil organic carbon (SOC) is a potential soil fertility indicator for regulating nitrogen application in tropical farming systems. However, there are limited studies that have discussed SOC thresholds above or below which crop production could be diminished, or at which no or high response to nitrogen (N) application can be realized. This review explores the drivers of SOC concentration relevant for the establishment of thresholds. We further evaluate existing SOC thresholds for provoking no yield response or significant response to added N fertilizer. Key drivers for SOC concentration relevant in establishing thresholds are mainly climate, topography, texture, and land use management. Soil organic carbon threshold for sustaining soil quality is widely suggested to be about 2% below which deterioration may occur. For added N fertilizer management, specific SOC thresholds seem quite complex and are only valid after assuming other factors are non-limiting. In some soils, SOC levels as low as 0.5% result in fertilizer responses and soils as high as 2% SOC also respond to small N doses. Minimum SOC thresholds can be identified for a given soil type, but maximum thresholds depend on crop N requirements, crop N use efficiency and amount of N applied. However, there seem to exist critical total SOC ranges that could be targeted for optimal indigenous N supply and integrative soil functional benefits. These can be targeted as minimum levels in soil fertility restoration. In all, it is still difficult to establish a single minimum or maximum SOC threshold value that can be universally or regionally accepted. Source ; Journal of Sustainable Development; Vol. 6, No. 12; 2013 

PDF enclosed fro further reading ...hope you like the attachment..