Glomalin and Soil Structure Following Woody Plant Invasion of Grasslands.

Many grass-dominated ecosystems around the world have experienced increased woody plant abundance during the past century. Our prior studies in Texas indicate that sites invaded by woody plants often have higher rates of above- and belowground productivity. These changes in plant productivity could influence the productivity of associated arbuscular mycorrhizal fungi and quantities of glomalin in the soil, and alter soil aggregate structure. We hypothesized that glomalin concentrations and soil physical structure would increase following woody plant invasion of grassland. To test this hypothesis, soil cores (0-10 cm) were collected from remnant grasslands and from adjacent patches invaded by woody plants at two locations: (i) a subtropical mesquite savanna in southern Texas, (ii) and a post oak savanna in east-central Texas. Glomalin-related soil protein (GRSP) was extracted from 1 g soil with 8 ml of 50 mM citrate-buffer, pH 8.0 at 121 °C, and quantified using the Bradford protein assay. GRSP concentrations ranged from 0.7-0.8 mg/g in remnant grasslands, and from 2.1-2.3 mg/g in wooded areas in both the post oak and mesquite savannas. In the mesquite savanna, GRSP increased linearly during the first 40 yrs of woody patch development, and then remained relatively constant in woody patches ranging in age from 40-90 yrs. Simultaneously, macroaggregates (>250 um) increased from 5% of whole-soil weight in remnant grasslands to 35% of whole soil weight in woody patches >80 yrs of age. Concentrations of glomalin within soils of these savanna ecosystems were somewhat low but comparable with others measured in Texas. Our results show that woody plant cover significantly affects GRSP concentration and soil aggregate structure. Thus, increased pool sizes of glomalin following woody plant encroachment into grassland may represent an important mechanism that promotes the formation and stabilization of soil aggregates.