Date of Award
Master of Science
Tallgrass prairies have some of the deepest and most fertile topsoil on earth. Widespread conversion of these grasslands to agriculture has decreased soil Carbon (C) storage by exacerbating erosion and disrupting aggregates that protect C from decomposition, coupled with lower plant C inputs. Thus, a primary goal of some grassland restorations is to improve soil structure and functioning. Conversion of cultivated systems to perennial grasslands often increases soil C, microbial biomass, and soil aggregate size and stability. A few studies have documented changes in soil microbial community structure after restoration. The objective of this work was two fold: 1) to explore the importance of soil texture and 2) determine plant diversity effects on recovery of soil biotic and abiotic properties. In the first study changes in soil microbial phospholipid fatty acid (PLFA) profiles and soil aggregates were examined in two 0-19 year chronosequences of restored grasslands in Nebraska on soils differing in texture, silty clay loam (SCL) and loamy fine sand (LFS), and compared them to native prairie. Soil was collected from the 0-10 cm soil depth at each site in May of 2007 and 2008. The SCL chronosequence exhibited increases in total PLFA biomass P<0.05, r2=0.29), PLFA richness (P<0.0001, r2=0.25), fungi (P<0.0001, r2=0.65), fungal:bacterial ratio (P<0.0001, r2=0.67), Gram (+) bacteria (P=0.02, r2=0.22), Gram (-) bacteria (P=0.05, r2=0.16), and actinomycetes (P=0.02, r2=0.23). Average soil aggregate diameter also increased (p=0.0002, r2=0.52). However, LFS sites showed no change across the chronosequence for any parameter. Total PLFA biomass (ANOVA, P<0.0001), richness (P<0.0001), and fungi (P=0.005) were greater on SCL restorations than LFS, but LFS had greater fungal:bacterial ratios (P=0.02). Soil microbial groups and soil aggregates were highly correlated, especially in the LFS choronosequence indicating that structural recovery is key to microbial community recovery. The second study investigated high diversity restorations with low diversity restorations on silty clay loam. In this study, high diversity and low diversity restorations in southeast Nebraska, aged 4 and 8 years were compared. The quantity of forbs seeded was too low and high diversity communities were a mixture of dominant C4 grasses (Andropogon gerardii Vitman, Schizachyrium scoparium (Michx.) Nash, Panicum virgatum L., Bouteloua curtipendula (Michx.) Torr. and Sorghastrum nutans (L.) Nash) and subdominant C3 grasses (Elymus canadensis L., Pascopyrum smithii (Rybd.) A. Löve, and Elymus virginicus L.). Eight year old plantings had greater root biomass, root C storage, root C:N ratio (P<0.05 for all), microbial biomass (low diversity only, PC<0.1, PN<0.05), PLFA richness (low diversity only, P<0.05), mycorrhizal fungi (P<0.05), and C mineralization (low diversity only, P<0.05) than 4 year old plantings. Low diversity plantings, which contained almost exclusively dominant C4 prairie grasses, had greater root C storage (P<0.1), mycorrhizal fungi (8 years only, P<0.1), and C mineralization (8 years only, P<0.05). Thus, C4 grasses and their associated arbuscular mycorrhizal fungi seem to drive recovery of soil C, soil respiration, and soil microbial communities over time. Overall, this work indicates that rates and success of belowground recovery are dependent on both abiotic and biotic factors in restoration. Restored plant communities affected soil recovery as dominant C4 grasses appeared to drive belowground recovery, but recovery depended on soil texture.
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