Dear Dr. Magboul - An interesting observation. I guess OC rich  layers may appear to be  an alluvial deposit  of the Nile river in the past when the climate was more wet than today. These soils are now polygenetic and  in equilibrium with the present dry climate. I request you to consult a geomorphologist  who has working experience in this area to confirm this episode.

Very Eagar to  know though I  am not in the field.

Dear Dr. Magboul - An interesting observation. I guess OC rich  layers may appear to be  an alluvial deposit  of the Nile river in the past when the climate was more wet than today. These soils are now polygenetic and  in equilibrium with the present dry climate. I request you to consult a geomorphologist  who has working experience in this area to confirm this episode.

Dear Prof. Dilip

Thank you so much for your comment, and the suggestion will be taken into account.

Interesting question, we are also getting same types of analysis data in Aridisol under arid environment of India. The probable reasons may be:

1. The fine organic matter accumulating in the surface soil is consistently eroding due to high wind velocity under arid conditions.

2. Sub-soil is weekly oxidized because lack of moisture.

3. Due to deep and parallel rooting patterns of the arid trees and shrubs , the accumulation of root decaying organic matter staying in the sub-soil.

4. The soils are mostly sandy, which helps in fast leaching of rain water and irrigation water along with organic matter in the surface horizon.

Very interesting observation

Interesting discussion on  higher amount of  organic carbon in subsoil in arid environment.I appreciate the points mentioned by Dr.Tarafdar especially the third point.

Following could be some reasons for observed accumulation of organic carbon in the subsoil:

(1) Deeper soil horizons may have a high capacity to sequester significant amount of organic C as the turnover time and chemical recalcitrance of soil organic matter increases with depth.

(2) The subsoil C sequestration may be achieved by higher inputs of fairly stable organic matter to deeper soil horizons. This can be contributed directly by plants with deeper and thicker root systems that are high in chemical recalcitrant compounds like suberin.

(3) The surface input of organic matter favors production of dissolved organic carbon that can be transported to deeper soil horizons and thus contribute to the subsoil C storage.

(4)  Activity of the soil fauna like earthworm may promote transfer of organic matter to deeper soil layers and stabilization there.

Accumulation of more OC  in the subsoils than in the surface in sandy Aridisols is a paradoxical situation because the  pedogenic processes are not likely to be operated in this direction as prominently evident in the depth distribution of  OC ,which shows a sharp decline of OC with depth in most Aridisols of the country. Accumulation of OC in soils of hot arid climate is a huge problem because soil temperature remains at much higher temperature than the atmospheric one in the long summer  and the short rainy  months. Development of such soils appears to be related to their depositional  history  of the alluvium brought down by the adjacent river in the present and also in the past. Geomorphologists  working in the such areas including some deserts including India often unfold such interesting facts on soil developments in the present hot arid climates.

Hi,

providing soil descriptions , pictures and how the increase of OC occurs (sharp or progessive)  will greatly help to identify if a we have "buried A horizon".

Dear all.

Thanks for interesting discussion. I agree with all justifications related to the Aridisols, but the situation seem to be differ in Vertisols.

I propose that the vertical pedoturbation may be the main reason for unexpected accumulation of high OC in subsoil of Vertisols rather than the topsoil. Kovda et al. (2010) studied the vertic processes (shrinking/swelling, lateral shearing and vertical turbation) and specificity of OM properties and distribution in Vertisols, they conclude that vertic processes affected the vertical and lateral distribution of SOM, its chemical and isotopic characteristics.

More discussion related to the Vertisols is requested.

Indian soil scientists suggest that the pedoturbation  is a partially functional  process in Vertisols and thus is not strong enough to overshadow the more significant long-term clay illuviation process. Vertisols of humid and arid bio-climates of India have a depth distribution of OC that decreases with depth. In support of this a pdf is  attached for reference.

Thank you Prof. Pal for the attached pdf.