Manure may cause more acidity than residue addition in the rooting zones and higher acidity changes pedo-chemical environment,which may link to greater release of carbonaceous acidic exudates from crop roots that would act as source of carbon.

In Chenu et al (2019), three explanations have been proposed: long-lasting SOM are essentially derived from microbial material (Kallenbach et al., 2015, Miltner et al., 2012); easily degradable substrates are processed with a high microbial carbon use efficiency (Wieder et al., 2014 and Cotrufo et al 2013); soluble compounds migrate in soil between mineral surfaces, where they can be protected.

@ Fan, residue is having high C:N ratio so they can not help much to enhance microbial activity unless and until you add more N to reduce the C:N ratio for decomposition whereas organic carbon from manure due to low C:N ratio provides the energy source for the active microbial mass as you have already mention major source of soil organic C derived from microorganisms therefore you may get more C build up in manuring fields that residue fields.

Respected Sir,

You may go for fractional analysis of different carbon pools (I to IV) contributing organic matter fractions in soil.

Manure is already more decomposed than residues. So, naturally they contain more resistant fractions of organic carbon (in form of long chain bulky carbon molecules), that persist in soil for longer periods, leading to increase in total organic carbon content in soil.

On the other hand, residues have more vulnerable portions of carbon compounds that go utilized by different groups of soil flora & fauna, including plants. Only negligibly smaller resistant portion goes to be added to the organic carbon pool. This may be one of the causes why there's almost no increase even after prolonged application.

Hi,

The answer goes by the quality of organic C, since you keep the C input constant, so if you look at the POM fraction and silt plus clay fraction you will see which pool is accumulatlng.

In order to answer your question it is necessary you provide the rates of appplication f both manure and residues in kg C.ha-1.yr-1. Otherwise they cannot be compared. Is it farm yard manure or slurry? Provide the composition as well

If crop residues returned to the soil over 30 years have been consistent in quantity or type, then the C content of the soil would not likely change. The reason for this is that the soil C is in equilibrium with the biomass C inputs and the environmental conditions of the soil (temperature, moisture, nutrient inputs, etc.). Adding manure is essentially an importation of C which changes the equilibrium between the C sources and soil. Because the manure is of different quality, the equilibrium has to adjust to the manure source, nutrient load and other manure characteristics. If different rates of manure are applied, each manure rate would require a different set of equilibrium conditions. However, if the manure applications are consistent and of similar quantity and quality, over time you should see SOC attain a new level that will remain relatively constant until a new equilibrium is reached.

The key to changing the SOC levels in soil (both in a positive or negative direction) is to change equilibrium conditions (change residue/biomass inputs, temperature, moisture , season length, type of residue, nutrients, tillage, etc.). Changing any of these will generate equilibrium changes that will result in increase or decrease in SOC over time.

What were the rates of addition of both residues and manure, in gC/kg soil (or tC/ha). What were the C/N ratio of both?

I think SOC fractionation based on protection mechanisms will give more clear picture about this. Moreover, I agree with Claire Chenu, you should calculate C/N ratio of both manure and residues.

The rates of addition of residues and manure are 0.93 tC/ha and 3.86 tC/ha, repectively. The C/N is 20 for manure and lager than 50 for residue.(@ Claire Chenu @ Gert-Jan Noij @ Muhammad Mohsin Abrar

Both residue and manure treatments both include mineral NPK fertilizer (165 kg mineral N/ha and 50 kg mineral N/ha, repecitively), but their total amount of N (including minera and organic N) is equal.

Fan....Have you applied residues solely or in combination with manure or chemical fertilizers?

combination with chemical fertilzier both for residue and manure.

Sir,

If we simplify the terms , manure is nothing but a decomposed form of raw organic residues. Being already decomposed it contains a vital amount of resistant materials which may not decompose easily like that of fresh residue. Residues go on decomposition and lost a huge part of it through the process and ultimately comes to the same platform with manures. Thus it contributes very less than compost.

That may be a probable reason according to my mind.

In my opinion, C:N ratio of crops is low, while it is high for compost. As crop residue has low nitrogen content , it take time to decompose due to which the available nitrogen for growth of microbes and for next crop is not upto the mark. it is the reason that organic carbon of soil does not increase but reverse is the case with compost.

Article The Microbial Efficiency-Matrix Stabilization (MEMS) framewo...

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How much quantity of residues was being added every year and how much manure was added? As the Mollisol already contained 13g/kg, small change due to addition by crop residues may not be visible.

Manure enhanced the microbiology of the soil and the diversity of micro organisms. With higher levels of diversity the metabolism pathways increased too and this eventually, yielded a soil richer in stable, organic matter.

i guess manure is something in between straw and SOM.

The longer distance to destination (SOM) the more loss of cargo on the way. : )