Glomalin-Related Soil Proteins as Indicator of Soil Quality in Pig-Fertigated and Rainfed Systems

Pig slurry fertigation can modify soil biochemical properties by promoting glomalin production and shifting microbial communities; however, its impacts under varying water regimes remain insufficiently quantified. We assessed irrigated and rainfed systems by integrating the soil quality index (SQI) with total and easily extractable glomalin (T-GRSP, EE-GRSP), determining microbial diversity via eDNA amplicon sequencing, and evaluating enzyme activities across three soil depths (0–10, 10–20, and 20–30 cm). Robust regression revealed that T-GRSP and EE-GRSP accounted for 75% of the SQI variability in irrigated soils and 46% in rainfed soils (p < 0.001), with the strongest correlations in the 0–10 cm layer. Irrigation increased T-GRSP concentrations by 66% (1.78 vs. 1.07 mg g−1) and raised its contribution to total soil carbon from 2.0% to 3.2%. The EE-GRSP levels were slightly lower in the irrigated soils (0.73 vs. 0.76 mg g−1) yet remained a sensitive early-warning indicator of moisture stress in rainfed plots. Microbial profiling showed a 19% increase in Shannon bacterial diversity (3.44 vs. 2.89), even more bacterial communities under irrigation, intermediate fungal diversity, higher fungal abundance, and no detectable arbuscular mycorrhizal fungi in either system. Combining GRSP fractions with microbial and enzymatic markers provides a responsive framework for assessing soil health and guiding organic amendment strategies in fertigation-based agriculture under fluctuating water availability.