The response of glomalin-related soil proteins to fire or tillage

Glomalin-related soil proteins (GRSP) are glycoproteins abundantly produced by arbuscular mycorrhizal fungi in roots and soil. They can be a significant component of soil organic matter, which effectively acts to bind mineral particles together, so improving soil structure. Although it is known that high temperatures and tillage both have a negative impact on GRSP, there is no study aimed at comparing and contrasting the effects of fire and enduring cultivation on GRSP content in contiguous areas undergoing the two forms of soil disturbance. We studied such a situation in Kurdistan, Iran, where the top 5 cm of soil was sampled in i) an unmanaged and unburned oak sparse forest, both inside and outside sprout clumps, ii) a vineyard, both under and outside the canopy, and iii) a burned portion of the forest in between. In the latter, the samples were collected from areas burned with high and moderate severity inside sprout clumps, and areas burned with low severity outside sprout clumps. All soil samples were air-dried and sieved to obtain four aggregate size fractions (8-2, 2-1, 1-0.25, and < 0.25 mm in diameter). Total organic carbon (TOC), easily extractable glomalin (EEG) and residual glomalin (RG) were determined in all fractions. Both TOC and GRSP were significantly lower in the cultivated soil than in the unburned forest. Fire did not change the content of TOC, although GRSP increased with increasing fire severity. The relative amount of small aggregates was higher in the burned and cultivated soils, likely because of the breakdown of macroaggregates by heating and tillage, respectively. TOC, EEG, and RG generally increased in concentration with decreasing aggregate size. The linear regression analysis revealed significant relations between GRSP and TOC in all studied treatments; however, the GRSP/TOC ratio was significantly higher in the soil burned with the highest severity and in the vineyard compared to the unburned soil: Overall, this study demonstrated that GRSP in soil is significantly affected by both severe fire and cultivation, and suggests that total amount of GRSP and GRSP distribution between the aggregate size fractions might be two reliable indicators of these types of perturbations of the soil environment.