Please have a look at these links.

https://www.ars.usda.gov/research/publications/publication/?seqNo115=239978

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3997909/

https://www.researchgate.net/publication/286745407_Levels_of_glomalin_and_their_relation_with_soil_chemical_and_biological_soil_andisol_characteristics_in_a_relic_of_native_forest_of_southern_Chile

Article Levels of glomalin and their relation with soil chemical and...

Glomalin has been found in agricultural, grassland, forest, desert, and non-cultivated soils.Although most of these findings are based on glomalin in the A horizon, both B and C horizons contain glomalin (Rillig et al. 2003) and it can be found to a depth of 140 cm in the soil profile (Harner et al. 2004). However, Paul et al. (1997), Torn et al. (1997) and Halvorson & Gonzalez (2006) have demonstrated that the stability of that protein diminishes as soil depth increases. In general it is considered that dynamics of AM fungi and glomalin have highly temporal and depth patterns, and is influenced by nutrient availability and enzymatic activity.

• Harner MJ, Ramsey PW, Rillig MC (2004) Protein accumulation and distribution in floodplain soils and river foam. Ecol Lett 7:829–836

• Halvorson, J, Gonzalez J. (2006) Bradford reactive soil protein in Appalachian soils: distribution and response to incubation, extraction reagent and tannins. Plant and Soil 286: 339-356

Nice discussion . Considering the glomalin as a  part of soil organic matter , in highly recalcitrant form , and having a  strong relation with spore density of AMF, glomalin is usually maximum in plant rhizosphere zone , then decreases down the soil depth as the root activity and organic matter continue to reduce...

Glomalin, basically an arbuscular mycorrhizal fungal soil protein or a glycoprotein produced by AM fungi  will find more under grasslands and in cereal crops field having more fibrous root system. In general you will find more upto 20 cm of soil depth and with further soil depth drustic reduction in content  may be found due to reduction of root activity as also pointed by Dr. Srivastava. In arid areas we find more glomalin content under Cenchrus cilliaris and Zea mays where crops are mainly infected by Glomus mosseae and Glomus fasciculatum.

Rhizosphere zone of a crop  is the best region where you can observe maximum concentration of glomalin , since this is the zone having maximum feeder root concentration and consequently the zone of maximum colonization by AMF  . subsurface will not observe higher concentration of glomalin because of lower concentration of feeder roots ..and other related spill over effects

Let me put up another quarry with regard to on-going discussion. How does glomalin contribute towards carbon turnover of the soil , both as a function of crop and  AM-species as well ..?