Ky Ky Considering SOIL as an example and when we are talking about the effects of pH on the Organic Matter I can add that with a overall increase in the organic matter, the condition or the soil surely recovers the natural buffer capacity and this is what I meant that the overall increase in pH in acid soils and this can be seen and understood too.

When the pH of a soil is increased, the release of H from carboxyl groups helps to buffer the increase in pH and at the same time creates the CEC (negative charge). With an increase in organic matter, the soil recovers its natural buffer capacity; this means an increase in pH in acid soils.In the natural environment, soil pH has an enormous influence on soil biogeochemical processes. Soil pH is, therefore, described as the “master soil variable” that influences myriads of soil biological, chemical, and physical properties and processes that affect plant growth and biomass yield. For instance, soil pH is controlled by the leaching of basic cations such as Ca, Mg, K, and Na far beyond their release from weathered minerals, leaving H+ and Al3+ ions to dominant exchangeable cations; the dissolution of CO2 in soil water producing carbonic acid, which dissociates and releases H+ ions; humic residues from the humification of soil organic matter, which produces high-density carboxyl and phenolic groups that dissociate to release H+ ions; nitrification of to produces H+ ions; removal of N in plant and animal products; and inputs from acid rain and N uptake by plants .

Soil organic matter exists in different fractions ranging from simple molecules such as amino acids, monomeric sugars, etc. to polymeric molecules such as cellulose, protein, lignin, etc. These occur together with undecomposed and partly decomposed plant and microbial residues . The solubility and mobility of the fractions differ during and after decomposition and could lead to the leaching of dissolved organic carbon and nitrogen in some soils. Dissolved organic carbon is defined as the size of organic carbon that passes through a 0.45 mm diameter filter. Soil pH increases the solubility of soil organic matter by increasing the dissociation of acid functional groups and reduces the bonds between the organic constituents and clays. Thus, the content of dissolved organic matter increases with soil pH and consequently mineralizable C and N .This explains the strong effects of alkaline soil pH conditions on the leaching of dissolved organic carbon and dissolved organic nitrogen observed in many soils containing substantial amounts of organic matter. The same observation has been made on the dissolved organic carbon concentration in peatland soils. The pH-dependence of dissolved organic carbon concentration gets more pronounced beyond

Soil pH controls the solubility of organic matter in two major ways:

1. Its influence on the charge density of the humic compounds, and

2. Either the stimulation or repression of microbial activity. The former is found to be more pronounced than the latter.

Additionally, organic anions and other negatively charged chemical functional groups present in organic matter can undergo association reactions with H+ ions [

Ky Ky Yes i do agree with the comment of Prem Baboo and it makes sense as i was reading both the comments and comparing my comment with the Prem Baboo and obviously many reasons to be shared

I appreciate your help, but I'm more concerned about the acidity of organic matter in water. Just like the article I mentioned in the question, the author expressed that the form of organic matter in biogas slurry changes under acidic conditions. Is there any other theoretical support for this view?