Hi Cara,

The consensus would be that yes SOM will stabilize given consistent soil management, inputs/outputs of crop/manure, and climate.

Have you seen the work of Johan Six? Maybe start here:

Six, J., E. T. Elliott, and K. Paustian. "Aggregate and soil organic matter dynamics under conventional and no-tillage systems." Soil Science Society of America Journal 63.5 (1999): 1350-1358.

Hi Cara

I think my  review paper could help you. please see the below link:

https://www.academia.edu/23118280/Effect_of_conservation_tillage_on_soil_fertility_factors_A_review

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwjiiPTMgcPPAhWF6xoKHTTdCZ0QFggiMAE&url=https%3A%2F%2Fwww.researchgate.net%2Fpublication%2F284284298_Effect_of_conservation_tillage_on_soil_fertility_factors_A_review&usg=AFQjCNEukxBSBNydnWGFS4GAs8aF1_ztkw&sig2=MKdoahwBa85DWC2S1Y628g&bvm=bv.134495766,d.bGs

https://www.academia.edu/23118280/Effect_of_conservation_tillage_on_soil_fertility_factors_A_review

In Aridisol under Arid environment , we had an experiment for last 12 years between the tillage versus no-tillage. We found that marginal (only 2%)  overall decline in organic matter level  under tillage fields with the crop pearl millet while legumes like clusterbean, mung bean and moth bean fields make hardly any differences in organic matter level. But under no tillage field there was an appreciable increase in organic matter level (4-5%) with the cultivation of all four crops mentioned.

I have done experiment on clusterbean-wheat sequence under different tillage practices resulted in stabilized organic matter levels under zero tillage with residue plots while under conventional tillage plots, the conventional tillage plots, the organic matter declined slightly and it is negligible may be because of legume-cereal sequence. Stabilization mainly depends on the management factors and the input application.   

Dear Cara,

The long-term (11 and 15 yr) effect of tillage system and crop rotation on soil quality was studied in two field experiments at Ridgetown, Canada by Van Eerd et al. (2014). They recorded higher soil organic carbon (SOC) with no-till (NT) than fall moldboard plough with spring cultivation (conventional tillage, CT). In both long-term trials, NT had ca. 21 Mg ha−1 more or 14% higher SOC than CT in the 0- to 100-cm soil profile.

Reference: Van Eerd LL, Congreves, KA, Hayes A, Verhallen A and Hooker DC (2014) Long-term tillage and crop rotation effects on soil quality, organic carbon, and total nitrogen. Canadian Journal of Soil Science, 2014, 94(3): 303-315.

Hope you find this input interesting and useful.

under Mediterranean environmental conditions,  OM content in soils trends to be low. Traditional tillage system enhances this OM depletion reaching contents below 1%. In my profile you can find some references of OM content in soils under tillage system in Castilla-La Mancha (Spain).

Hi Cara,

Is it tillage the unique variable that you are considering? And, is it tillage the only / main factor changing in the system you are analyzing? 

I’m asking because, as I am sure you know, changes in soil are multifactorial, combinatorial and accumulative. Generally speaking and based on the ample soil research, tillage decreases OM content as a result of the breakage of aggregates. Consequently, tillage disturbs the physical, chemical and biological properties that sustain OM accumulation. But tillage is not the only factor affecting soil OM content. If you combine tillage with leguminous cover crops, for instance, the OM content will be different than in a scenario of tilling (or no-tilling) followed by consecutive monocrops, or than another where organic soil amendments are applied. The dynamic of the changes will vary in time.

My point is that the explanation to the variations you are seeing may not the only due to a single factor. It may be useful to consider others from the management and ecological context, as well as possible combinatorial effects in time (unless your are modeling observations from fully controlled conditions).

Good luck!

Very good discussion is going on. I understand the tillage over all tends to promote soil aeration and decomposition of soil OM. Consequently OM will decrease with tillage. During 1989, I have seen some place in USA where depth or surface of Organic Soils have lowered by several feet. I have forgot the exact location of site.

A down to earth and conspicuous illustration of no tillage effect, is evident from forest ecosystem on organic matter buildup. As very rightly stated by Georgina it is next to impossible to delineate the effect of tillage on organic matter decline or improvement due to operation of multiple and complex variables. Moreover, monoculture has become obsolete considering its soil quality deterioration with time. It has been replaced by crop rotations to protect and maintain soil productivity. Further, rice cultivation which involves extensive tillage (puddling) leads to deterioration of soil structure as the object of tillage is to retain water at the solum to maintain aquatic environment for the rice which is semi-aquatic in habit.   

You are writing from California and I do not know if that is where your modeling work is being done. Equally, I do not know if you are working on swamp rice or on upland rice because I think this would be important. In swampy conditions it is expected that organic matter mineralization would be much less than in aerated environments. Equally, the altitude of the area in which you are working would influence temperature and hence the rate of mineralization of organic matter. The lower the temperatures the less the amount of organic matter mineralized and the more its accumulation in soil. Also, the soil type in which you are working would have a significant impact on the behavior of the organic matter under continuous tillage. Soils with 2;1 minerals would have more resistance to mineralization due to the formation of clay-organic complexes. However, in the humid tropics where Oxisols are predominant, the dominant mineralogy is sesquioxidic with low surface area and the ability to form less stable organic-clay complexes which would reduce the rate of mineralization. In the oxisol systems, the textures are pseudo-sand and pseudo-silt implying that these systems tend to behave as those with high sand contents which are very well aerated and subject to high mineralization rates. That is why these oxisols are red indicating the presence of Fe in the ferric state due to good aeration. This environment fosters high organic matter mineralizatison. It has also been observed that in these soils, following continuous tillage organic matter mineralization is very fast due to high microbial activity associated with the high humidity, which condition is not obtainable in the temperate regions; the mineralized products are quickly leached due to the high rainfall.

Classic soil organic matter loss curves are the so called bath tub shape. The degradative practices causes loss of soil organic matter but the loss becomes diminishing over years or cycles and will eventually reach a depleted equilibrium. The use of soil organic matter accruing practices show a reverse bath tub curve when rapid substantial increases over time become diminishing until a new equilibrium level is reached. No till raw manure cover crop  and compost have increasing ability to have more dramatic and a higher new soil organic matter equilibrium. These practices are not exclusive and there is no silver bullet practice either.