a valid question. I guess what s/he is getting at is whether this depth is representative and what do you know about the soil temperature and moisture profile.

An argument could be that most sensors capture the soil moisture in remote sensing up to 10 cm depth ?

See a review paper: http://dx.doi.org/10.1155/2013/424178

I would have following follow-up questions: How deep did your soil respiration chambers insert into the soil and what type of soil temperature and soil moisture sensor did you use? In case you used hand held sensors and pushed them in the soil from the top your soil moisture sensor most likely represents the soil moisture across the insertion depth and not at a specific depth, the same might be true for your soil temperature sensor depending on the sensor type.

If I was a reviewer I would be interested if both measurements really align in case you are trying to establish relationships between soil temperature/moisture and soil respiration.

I would further be interested as outlined above why you measure at this particular depth and what information you have about the soil moisture/temperature profile and soil profile. If you by chance measured soil moisture in the mineral layer of a very shallow soil your soil moisture might not be related to the respiration data...

I believe the reviewer would like to know if you had a justification to assume that the biological processes governing soil respiration at your study sites happen at the depth where you measured soil moisture and temperature. For example, did you determine in a pilot experiment that the top 10 cm layer is the most active layer for soil respiration? Very often reported relationships between soil temperature/moisture and soil respiration in field studies are not very strong because the different measurements are representing different soil layers.

On the other hand, if you used a TDR probe you might not be able to measure at 5 cm due to the length of the steel rods and probe design.

Dear Huitao Shen,

there are not so many others who can tell why you actually did the measurements at 10 cm depth, if you did not think about it aforehand yourself.

Liming Ye is perfectly right, the reviewer wants to know - and should come to know - the truth. That's what scientific publications are for, and that's what your reviewers and readers may expect from you. Even if the truth is as tedious as - maybe in your case - the 10 cm sensor being the only one at hand in the laboratory from earlier experiments. Maybe someone else in your lab thought about it before.

In reality depth (I mean 5, 10, 15cm everithing is ok!) of temperature measurement together with automatic soil respiration it is not very important. You can plot histerezis and check were is the respiratory layer - dependent on which direction goes hysterezis. Therfore you can use your measurement of soil temperature at 10cm as reference.

Since roots are the main cause of soil respiration I guess that soil temperature should be taken where roots are mainly distributed. In grasslands root distribution depends on soil depth, species and roots system typology which may reach 5-10 cm and more. However, I think that soil temperature doesn't change between 5 and 10 cm depth if the sensor measure in whole numbers.