Here in my opinion, one of the possible reason might due to its increasing in crop straw return, which contains high C/N ratio, and subsequently increase C:N ratio in soils. 

Increasing C:N ratio depends upon microorganism activities, fertilizer applications, type of cultivated crops, pH, quality of watering, which usually increased with intensive cultivation 

This might occur via humification processes, which should result in appearing of hardly digestible substances?  Besides, the increasing microbial biomass during the composition of plant residuals should result in lowering of C:N ratio.

Did the C go up or the N go down?

It should depend both on the rates of decomposition (mineralization) and humification. In this case, I would propose that after 30 years of the decomposition microbial N is getting lower. Carbon cannot be higher if you did not add new portion of plant residues. The turnover of microbial C and N at appropriate conditions is very intense, but we may expect accumulation of hardly digestible C of soil organics due to the humification. And in humic substances, microbial N should be more able to turnover as compared to the HUMIC C.

@Boris: no worries about your english!

However, I´m a Sherlock-Holmes-fan: don´t speculate too much before you know all facts ;-)

First, thank you for your answers all above. Actually, both C and N content increased a lot during the last 30 years. While, C increased faster than N, which lead to an increase in C/N ratio. Here is also other background information of the cropland in this my study area, 1) crop rotation: wheat and corn. 2) soil type :Fluvisols 3) straw return increased 4) N fertilizer application increased.  Here i don't know the mechanism of the increasing C/N ratio under an increasing N application.  

@Caser : Could you please explain your answer more specifically?  Why intensive cultivation could result in C/N ratio increase? Thank you very much for your time. 

C/N ratio in soil is not constant. It is changeable. It may be higher with addition of fresh organic materials bearing wider C/N ratio. But C/N in the organic materials or soil become narrower upon decomposition of organic materials. However, regular addition crop roots and residues may increase the C/N ratio in the soil.

If you don’t speculate, you will have no chance to develop hypotheses! 

@Doarul Thank you for the detailed information you provided!

My guess: there was not enough water to decompose the straw added to the soil. Obviously farmers put too much (inorganic?) fertilizer on the fields. This is not so good, as much of the Surplus N is likely to end up in the nearby rivers as nitrate. This could affect the drinking water Quality. In Europe 50mg/l nitrate is the threshold for drinking water, but ir should be lower for babies and pregnant women. 

Organic matter in soils is actually a good Thing for agriculture, as it increases the soils ability to hold back water .

Do you have any concerns about the increasing C:N-ratio?

To answer this question you need more details, history, what happened in particular periods of the field experiments, etc. According to your observation, you cannot conclude why it is, no sufficient data to conclude? And Sherlock Holmes would say the same.

C:N is lowering when humification proceeds. As we have in vermicomposting and composting, for example. If C:N is getting higher during time, this means that you periodically added new portion of fresh organics. So, to study these processes you should control every addition of carbon at equal amount following C:N. Or, which is better, do not put the organics at all. I would recommend to study the effect in micro- and mesocosms modelling different conditions: organics - no organics, mineral fertilizers - no m.f., and other controls.

Photosynthesis we can exclude, therefore to get more C the only way is the addition of carbon. Lowering N in soil itself (!) is due to accumulation N in microbial biomass. So, you should analyse C and N in soil and microorganisms (active and dormant biomass)   

in most cases this is caused by the combined effects of sampling and laboratory variability background noise

thanks for your answer @Ewald. 

I have hundreds of data in different places showed the C/N ratio increased in the past 30 years. is there any chance that other factors affect the C/N ratio? but not the bias in the experiment?  thank you.

Obviously, continued addition of straw for 30 years might have increased soil C. However, as you mentioned, increase in C was more than that recorded for N, which indicates that in your system potential of N loss increased with time. It is well established in literature that addition of straw increases N loss via denitrification (from applied fertilizer as well as from native soil N pool). To verify this, you can compare the denitrification potential of two soils.        

I recommend to measure microbial C and N. During the decomposition, C and N accumulate in microbial biomass, and the C:N out of microbial mass would increase. 

To answer this question you need more details about your experimental conditions and the soil microbial activity

Regular/frequent  addition of undecomposed  or partially decomposed organics may enhance C:N ratio to some extent in cool climatic conditions where the decompostion rate is very slow.

Addition of organic materials in regular basis may increases the soil C status. On the other hand mineralization of soil humus reduces the soil C. As a results the net increase of soil C is very very slow specially in the tropics. 

hi

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