I'd suggest that the causality might be the other way around. Due to slower mineralization organic substance can accumulate. There are many possible reasons for limited mineralization, but one of the most important ones is oxygen limitation.

This may explained better with release in stress. As stress decreases due to increase in SOC/matter, microbes respire less per unit of SOC. And with increase in stress due to decrease in SOC/ matter microbes respire more to get energy from less available organic carbon.

Over what domain are you observing a negative relationship between soil organic matter and soil respiration? The reasons could be different for a pattern observed across a local landscape than for a pattern that spanned a large region with varying climate and ecosystems.

As previous responder points out soil organic matter builds up if decomposition is slow, and oxygen limitation can be an important reason for that. Consider soil moisture in particular, as saturated soil can lead to anaerobic conditions.

Also keep in mind that soil respiration is not only driven by microbial decomposition of soil organic matter, but also has a large component of respiration by roots are supported by root exudates

May be the available C supply is limited in this environment. Respiration decreases avaliable C and thus can show a negative relationship if there are only limited amount of mineralizable C available.

I agree with Roland. Depending on the dataset, aerobic conditions from excessive soil moisture may result in both high soil C and low respiration. If soils are well drained and not anoxic, then you would expect a positive relationship if temperature and root mass are equal. However, "soil respiration" includes the delicate interactions between soil chemistry, soil climate, autotrophic mass, and the activity of that mass. I poked around with this a bit in a publication from a few years back:

https://www.researchgate.net/publication/222411032_Modeling_soil_respiration_based_on_carbon_nitrogen_and_root_mass_across_diverse_Great_Lake_forests?ev=prf_pub

I'm sure there are many other better analyses now. One caveat, the more we explore and more we measure the less we think that CO2 from the soil can be explained by the simple chemical kinetics of decomposition constrained by the amount of stuff available for decomposition. Microbial priming and facilitation or other processes driven by the flow of carbon from roots are new frontiers that may overshadow diel, seasonal, or inter-annual patterns that we've thought were driven by temperature and moisture constraints on decomposition. There are some good references to this phenomenon in:

https://www.researchgate.net/publication/222111851_High-frequency_analysis_of_the_complex_linkage_between_soil_CO2_fluxes_photosynthesis_and_environmental_variables?ev=prf_pub

Best of luck and enjoy unraveling:)

Article Modeling soil respiration based on carbon, nitrogen, and roo...

Article High-frequency analysis of the complex linkage between soil ...

There is also a lot of discussion in a way that in case of very low amounts of carbon, it is less likely that the "right" microorganisms will meet their "right" substrates and the "right" conditions. This applies especially to dry soils, as diffusive transport between microhabitats is limited and microorganisms and organic matter may sit in isolated niches. Joshua Schimel has published quite some work on this.

In situ or in vitro respiration??

In the field, high organic matter content can be related to: slower heat transport, slower soil wetup. And perhaps most importantly in some ecosystems: less earthworm activity!

I've seen bare patches razed by earth worms, which of course have high respiration.

Thanks a lot everyone. It is really appreciated to have all of your comments. I can confirm that I have found this observation in a small man-made ecosystem where it had a well drained aerobic soil. In these series of in situ experiments, I have obtained a positive relationship of the soil respiration with the soil moisture content as well. However I could not measure the soil temperature due to the unavailability of the techniques and instruments. But I am planning to get that in vitro condition where until such time I have stored the respective soil samples in the refrigerator.

If you got a negative correlation in experiment with aerobic soil, then I would start from critical self-assessment of the used technique. In spite of apparent simplicity, there could be artifacts/misinterpretations of both "soil respiration" and "SOC".

What protocols do you use? When correctly done, respiration in aerobic soil either is positively correlating with SOC (total or water-soluble organic matter) or there is a 'cloud' of data points, e.g. if SOC varies in narrow range within experimental error.

Beside the C:N ratio and other hardly degradable content (legnin, pectin,etc.) look for for available soil moisture dynamics as well temperature regime in relation to SR. As optimal mix of water and temperature content is necessary for microbial activities. You may also look for aerobic or anaerobic soil conditions. Good luck.

Thanks a lot....

For Nicolai S Panikov, I have used the Walkley and Black method for the determination of soil organic carbon and Anderson method for the determination of the soil respiration.

To make sure, I have identified your techniques correctly:

- to measure soil C you use dichromate oxidation by determining the consumption of oxidant (dichromate) rather than direct assessment of C;

- respiration assay is done in presence of NaOH which absorbs CO2, by the end of incubation you titrate residual NaOH to assess the total CO2. Correct? Usually Anderson method is referred to so called 'substrate-acceleration respiration', so you might have added also glucose as stimulant.

Both techniques are old and of low precision. But the most important, they are not direct. The dichromate oxidation results are sensitive to presence of inorganic oxidizable compounds, say, Cl- or sulfide ions. If they vary in your experiments -- it could be the main reason. Soil respiration is very sensitive to amount of added organic compounds and indigenous carbonate in soil. Abiotic release of CO2 could explain the abnormal relationship with soil C. But the most probable reason still remains huge experimental error which normally is associated with non-instruemntal data. Especially when you have limited number of data points.

Thanks Nicolai S Panikov, you have identified the techniques correctly except for the substrate-acceleration respiration step as I have not added any glucose or any other substrates. With the available facilities these are the only possibilities to measure the organic carbon and the soil respiration at the moment. I have just initiated these as we have no data available at the moment on this regard and still searching ways and means of improving the experiments. Any suggestions to improve these studies without using sophisticated equipment like element analyzers or IRGA etc are highly appreciated. Thanks a lot.

Dear Thushari. Any analytical instrument for measuring soil respiration (to be correct, CO2 exchange rate between soil and atmosphere) costs at least 5K (the simplest IRGA). Soil C can be done by soil ignition, the cheapest model would costs you about 1000$ if you buy an used pyrolytic unit (device to burn soil to CO2) and then use the same IRGA. My advice is to apply for some equipment acquisition grant and then produce high quality research data. Before you need to read literature, go through web sites. With titration alone you cannot move forward. Sorry.

Good luck

To Nicolai S Panikov, I am in the process of preparing a research proposal for the equipment grants Sir. Since it takes time I just wanted to give it a try with these methods. Thanks a lot for the your valuable comments.