Dear Dr. Pal Sir,

Thank you sir  first of all introducing another important topic for discussion. In hot humid tropical climate with heavy rainfall there is good chances of SOC accumulation. High vegetative cover with well spread out root growth promote good amount soil fauna to physically disintegrate organic waste products and allow soil microflora to act on it under high temperature and dry spell of around 4-5 months . These decomposed organic matter infiltrates to lower layers owing to good permeability of soils of humid tropics and reduce the losses as carbon dioxide. High acidic state of soil also prevent over-mineralization and respiratory losses of organic carbon as carbon dioxide and instead get fixed as biomass through photosynthesis with the help of autotrophic plants and microlflora and chemo autotrophs. this sort of accumulation of SOC is common in high hills, highlands, foot hills and midland hills, which buffers the soils against high acidity and aluminium, iron or manganese toxicity. These solis are grouped under Humults, Ustults, Ustolls, Ustepts and rarely Orthents

Acid suphate, potential acid sulphate or acid saline soils and marine sediments rich in organic carbon and S are the other big source of OC and can be sequestered towards scientific agriculture paddy cum prawn culture etc. these solis are grouped under Hemists, Aquepts, Aquents and some times Psamments and Orthents

anil

I wonder, if there were any studies concerning the relationship between organic carbon content and soil moisture status (or, more simply - drainage) in these soils:

It is widely known, that not only bad (poor) but even imperfect drainage favours accumulation of organic matter, because it prevents OM decomposition. According to my small observations if the soil contains about 3-5% or more percent of organic C it is combined certain drainage problems. My Poland is not a tropical country, however many soil processes occur in many different climatic zones.

My opinion is concordant with previous and first answer of Dr. Anil Kumar about soils such as Hemists (Histosols), Aquepts and Aquents (soils with aguic moisture regime). Moreover, there are some sandy soils with high ground water level, which also tend to accumulate organic matter. I would not classify them as psamments, but according to WRB they could be referred as, e.g. Umbric Gleysols (Arenic), Gleyic Phaeozems (Arenic) or Gleyic Umbrisols (Arenic).

Michał

Dear Drs. Michal  and Anil- Thanks for your intervention. Factors that favour OC accumulation in soils of HT climate are already known well.The fact is that none of the  acidic Alfisols, Mollisols and Ultisols mentioned by me has any water logging problem. The most important point is  to get any further  pedological/ mineralogical explanation  for the OC enrichment in these three   fairly to highly weathered HT soils even when their clay CEC is low, caused by the dominance of  K-HIV clay mineral.  It is well known that OC accumulation is soils is favoured by the larger surface area provided by the clay minerals. It is a paradoxical situation and needs to be resolved to gain a new knowledge on the other unknown factors of OC accumulation in these kinds of soils.

dear pal sir and stephen sir,

now the topic gaining the momentum we can interact more on these two specific situations under hot humid tropics one under uplands with with good vegetative cover and other one submerged marine sediment based systems both the cases accumulation pretty high

anil

I meant not only water saturated soils, but, above all, soils with drainage slightly slower than normal. So I thought not about typical logging problem, although yest it may occur from time to time after intensive rains. I think it is possible especially in soils with subsoil more rich in clay, such as Alfisols and Ultisols.

But, if these soils have no even temporal drainage problems it would be interesting the composition of organic matter, especially the percentage of fresh and only partially decomposed organic matter and particular humus fractions? And the balance of organic matter - is the mineralization and humification rate comparable with other soils?

Intensive wetland rice cropping (rice-rice or rice-rice-rice) helps to accumulate OC in soil. Because OC mineralization rate is slow under wetland rice condition.

Dear Dr. Islam - True. This has been the experience in the rice cropping areas in the IGP, India.

Thank you Dilip Kumar Pal sir. In our farm of Bangladesh Rice Research Institute, we observed this under long-term experiment. Another interesting findings of the long-term experiment is that the substantial increase of available S status in soil. This might be due to the industrialization around the farm which may increased the SO2 concentration in the atmosphere and finally it is added to soil with rain water.

Dear Dr. Islam - Thanks for your response.It is very interesting to note the increase in S content of soils. Since when the industrialization is introduced? Do you have any data on pH  values of rice soils in the 0-30  before and after the industrialization? pH values are expected to drop. Hope in rice fields  urea as N fertilizer is used. Use of NH4-sulphate  would also cause  decrease in pH. In case you have such data from long-term experiments, please share with your colleagues all over the world.  

Dear Dr. Islam and Dr. Pal Sir,

Marine sediments containing sulfidic materials can be another good source of sulphur as this region forms part of coastal ecosystem

anil