Please check the below given links.

http://www.sciencedirect.com/science/article/pii/S0016706115000452

https://www.researchgate.net/publication/222824317_The_Depth_Distribution_of_Soil_Organic_Carbon_in_Relation_to_Land_Use_and_Management_and_the_Potential_of_Carbon_Sequestration_in_Subsoil_Horizons

Article The Depth Distribution of Soil Organic Carbon in Relation to...

Please have a look at this link also.

http://www.nature.com/nature/journal/v450/n7167/full/nature06275.html

Aside from the location of direct inputs one has to consider the often considerable effects of transport in soil water and of bioturbation

 Thank Arvind and Paul for their valuable information. 

If we assume a system at the steady state, the question is very similar to that: 

What is the contribution of soil respiration from different depths to the total soil respiration?

Dear Zhonkui Luo, The vast majority of the respiration and soil Carbon is situated in the top soil layers. As the depth increases the amount of both soil Carbon, microbial activity  and the critical oxygen for respiration all become limiting. For accruing soil carbon the input of Carbon must be greater that the respiration of the plant matter input over time. In the case of increasing the adding of oxygen in the soil will increase respiration and can cause the soil Carbon or organic matter will decrease. Organic rich peat soils when drained and plowed rapidlly loss their soil organic matter. This has been addressed by flooding the soil creating an aerobic environment which decreases organic matter decomposition conserves the plant residues and changing the degradative pattern of soil organic matter loss. In cool temperate regions the accrual of soil organic matter is related on the decomposition lowering which allows greater net soil organic matter accumulation. The soil organic matter flux cannot be understood unless both primary photosynthesis is viewed in tandem with respiratory activity and primary productivity minus the respiration can give a positive accrual or soil regeneration a zero sum which is sustainable constancy at the present level and a negative level which is a soil organic matter degradative pattern. Soil regeneration with a positive net primary photosynthesis and respiration subtraction the result is not only soil improvement but also the reversal of atmospheric greenhouse gas from taking of Carbon dioxide into a form soil organic matter not in the air. There it provides for improvement of water conservation and retention of crop or soil resources and results a host of environmental issues. 

Dear Paul Reed Hepperly,

Thank you very much for your detailed reply and those insightful comments. I agree with you. The majority of soil carbon is situated in the top soil layers. And soil carbon dynamics in different soil depths depend on a series of conditions such as the ecosystem type and climate. Quantifying this dynamic is a big challenge and requires more efforts as most of present researches focus on the top soil layers and total soil respiration. In addition, few studies have addressed those similar questions like that being discussed here. 

I am interested in these questions because I am very interested in the turnover times of soil organic carbon in different soil depths. The answer for this question is important because soil organic carbon in different soil depths may respond differently to global environmental and management changes at the time scale of less than several hundreds of years.   At this time scale, we can assume the steady state, for example, a natural grassland or forest, and can assume that C input = C outputs. If we know the distribution of C inputs in soil layers, we can roughly estimate C outputs from the specific soil depth. Then, if we know the soil organic carbon stock (this is relatively easy to obtain) in different soil depth, the turnover time of soil organic carbon can be estimated. 

Nice discussion . This will depend upon the relative distribution of root activity across different soil depths, in addition to water transmission properties of the soil ( hydraulic conductivity ) , root c inputs will be the major contribution than any soil  pedogenic process(s)  ...

I appreciate the time of the 11 followers (including myself:)) on this question. Thank you all very much. 

Can you respond whether the following assumption satisfies you? If it does, why? If it does not, do you have any solutions if we have to answer the question? 

1. To estimate the turnover time of soil organic carbon (SOC) in the top 0.3 m soil (question), we use the total C input (assuming we know e.g., crop residue + roots or NPP - net primary production) to represent the C inputs in the top 0.3 m soil (assumption). Please note that the SOC stock in the top 0.3 m soil can be directly measured. 

2. How about increase the SOC stock measurements to 1 m soil depth (the turnover time of SOC in the top 1 m soil) while using the same C inputs data? Do you have any other concerns because of the increase of soil depth?  For example, the SOC in the top soil and sub-soil must have different turnover times, but we can only obtain one estimate for the 1 m soil. 

Cheers,

Zhongkui 

As I understand, depth upto 30-40 cm may be o.k for agricultural field crops but upto 1 m depth may be desired for horticultural tree and plantation crops where roots may go/penetrate much deeper .Apar from root carbon input, the the dissolved  CO2 (from the decomposition of organic matter),HCO3 and H2CO3 formed and moved down the profile are also important in carbon sequestration and turnover.

Dear Luo, As Dr. Rao mentioned, interpretation of soil organic carbon  (SOC) content or any other nutrients based on soil sampling depth is important from agricultural soil point of view. For instance, we sampled soils from two distinct depths in tropical agroecosystems: One from 0-30 cm and the second from 30-100 cm. The analytical results both for SOC and other nutrients were interesting. Generally, SOC significantly decreased as the sampling depth increased. I think the article attached here may be helpful to see the trend of SOC with respect to depth.

Dr Getachew , how do you feel , the necessary  depth of  soil sampling  for normal SOC inventory is different  than the dynamics of SOC ..? Do we need to keep these two aspects independent of each other..?

Dear colleagues,

For your kind information, I  present hereunder data on depth distribution of Organic C in a soil profile (Typic Paleustalf) located in humid tropical belt of eastern India:

0-15 cm: 0.40%, 15-30 cm: 0.33%, 30-45 cm: 0.25%, 45-60 cm: 0.20%, 60-90 cm: 0.13%, 90-120 cm: 0.07%, 120-150 cm: 0.04%.

Hope you find this input interesting and useful in the ongoing discussion.

Reference: Singh R, Kundu DK and Das M (2000) Characterization of soils at Deras Farm, Mendhasal in Khurda District. Technical Report. Water Technology Centre for Eastern Region, Bhubaneswar 751023, India. 32 p. 

Dr.Kundu,what is the vegetation or crop on the farm before sampling?.Gradual decrease in organic carbon content is observed upto 60cm depth.What is the native vegetation in the farm area before introduction of agricutural crops?

Dear Dr. Rao,

Hereunder I give the basic information on the land, land use, climate and other salient properties of the soil for which we determined depth distribution pattern of Organic Carbon:  

This is a mid upland, lateritic parent material, sandy loam soil with good surface drainage, acidic (pH 5.6) surface layer and near-neutral (pH 6.6) subsurface layer, cultivated to rice in kharif/ wet season and potato in rabi/ dry season, clay content increases from 12% in 0-15 cm layer to 22% in 15-30 cm layer to 27% in 30-45 cm layer to 34% in 120-150 cm layer, bulk density varies from 1.41 to 1.50 g/cc. The land has 1 to 3% slope and is situated about 95 m above MSL. The site receives a mean annual rainfall of 1520 mm, and mean annual air temperature of 27 degree Celcius.

Hope the information is useful for interpretation of the organic C distribution in the soil profile. 

Dear Dr.Kundu thank you for the information provided.I think it is a lateritic soil profile with increasing clay content with depth.The crops mentioned by you may not much influence the organic matter content beyond 60cm depth.The temperature and moisture regimes also may favour soil organic matter build up (if input is there).What could be the reason for the presence of organic matter beyond 60cm and upto 1.5m?

Hi Annangi, C transport in soil water and/or bioturbation mentioned by Paul would be one of the reason for the presence of organic matter beyond 60 cm. 

Another question is that why is the soil C in the soil 0-15 cm soil so low (0.40%) in the soil mentioned by Dilip? Any temporal measurements? The rainfall and temperature should support high plant productivity. No stubble was retained at all? 

Let's go back to our initial questions. I found an old paper that would help define the C inputs to different soil depths: A global analysis of root distributions for terrestrial biomes (see the link below). As below-ground C inputs are dominated by root C inputs, the information of root distribution will be very useful. 

http://link.springer.com/article/10.1007/BF00333714

Dear Zhongkui, belowground C input mainly related to roots. Roots distributed with soil depth might quite different for different crops. We establish root distribution for different crops, which might be helpful to you. This can be used to estimate C input for different soil depth. And also C turnover rates should be different for different depth. It is necessary to expand to whole profile (or at least 1 m) for C stock estimation. But if just estimate SOC change, that will be different.

Article Root distribution by depth for temperate agricultural crops

Drs Luo and Fan,your comments are interesting and I agree with your comments.Both the papers attached by you are interesting and enlightening.I could see in most of the agricultural crops of temperate region 50%of roots are  in 20 cm depth,though the root sysems are extending upto 100cm or beyond.It is interesting from  study attached by Luo that desert plants,shrubs and certain grasses go much deeper in profile and have greater carbon sequestration potential(Am I right ?).Even root systems of perennial trees are predominantly in less than 40 cm depth.Also as the root systems are extending upto 100cm or beyond, carbon can accumulate in lower depths even though in small amounts.It wll be interesting to know how the temperature and moisture regimes fluctuate in different soil depths and upto which depth the microbes will be active( as aeration,oxygen supply is also imporant for them).

Ms.Tapasya,interesting questions.Soil layers 0-15or 0-20 and 15-30 or 20-40 cms are normally distubed due to cultivation,irrigation and application of inputs of manures and fertilizers (especially the top layer).As mentioned in my earlier reply/comments the dominant activity of roots also confined to the top two layers.What is the oxygen and microbial gradient in soil?If the soil is not disturbed and also the amount of carbon is less or small,it is possible the carbon may be old and there may be more mineral- organic matter interaction compared microbial- organic matter interaction.But fresh roots may also enter into deeper layers depending on crop species and moisture regimes especially in dry/tropical regions.