Friday, August 7, 2009: 11:10 AM
Grand Pavillion II, Hyatt
Background/Question/Methods Space-for-time substitutions or chronosequences are a common approach to evaluate changes in ecosystem structure and function over time, yet less is known about the role of state factors (e.g., precipitation regime and soil texture) on recovery trajectories following long term disturbance and commonalities in trajectories at cross-continental scales. We used three chronosequences comprised of independent C4-dominated grassland sites restored for different lengths of time under contrasting soil types and varying precipitation regimes to elucidate the role of soil texture and climate in the recovery of soil carbon (C) and belowground biological components instrumental to the protection (and sequestration) of soil C. We quantified changes in root biomass, total soil C, microbial biomass, fungal biomass, fungi:bacteria ratio, and aggregation in restored grasslands, continuously cultivated systems and native grasslands in fine-loamy-sand and silty-clay-loam soil from northeast and southeast Nebraska , respectively, and fine-sandy-loam soil from the Free State Province of South Africa.
Results/Conclusions Both grassland restoration chronosequences in Nebraska were dominated by C4 grasses within 4-6 years following planting. Root biomass increased across both chronosequences at rates of 21 and 31 g/m2/y in clayey soils (r2=0.88; P<0.001) and sandy soil (r2=0.48; P<0.001), respectively. There were no changes in soil C, microbial biomass, or aggregation in the sandy soil, whereas, the clayey soil exhibited linear increases in soil C (r2=0.41; P<0.0001), microbial phospholipid fatty acid (PLFA) biomass (r2=0.29; P=0.005), fungal PLFA biomass (r2=0.65; P<0.0001), and fungi:bacteria PLFA ratio (r2=0.67; P<0.0001). Root development and recovery of the soil microbial community are critical to soil aggregation, and the mean weighted aggregate diameter conformed to a 3-parameter power model (r2 = 0.52; P = 0.0002) across the clayey soil chronosequence. The chronosequence in South Africa exhibited the highest rate of soil C accrual (60.5 g C/m2/y, r2=0.62, P<0.001) within a similar time frame as the Nebraska chronosequences, likely resulting from extended periods of limited decomposition. Thus, belowground recovery during grassland restoration varies between soil textures within a precipitation regime, and cross-continental studies expand our understanding soil C accrual by incorporating variation in climate regimes.
Results/Conclusions Both grassland restoration chronosequences in Nebraska were dominated by C4 grasses within 4-6 years following planting. Root biomass increased across both chronosequences at rates of 21 and 31 g/m2/y in clayey soils (r2=0.88; P<0.001) and sandy soil (r2=0.48; P<0.001), respectively. There were no changes in soil C, microbial biomass, or aggregation in the sandy soil, whereas, the clayey soil exhibited linear increases in soil C (r2=0.41; P<0.0001), microbial phospholipid fatty acid (PLFA) biomass (r2=0.29; P=0.005), fungal PLFA biomass (r2=0.65; P<0.0001), and fungi:bacteria PLFA ratio (r2=0.67; P<0.0001). Root development and recovery of the soil microbial community are critical to soil aggregation, and the mean weighted aggregate diameter conformed to a 3-parameter power model (r2 = 0.52; P = 0.0002) across the clayey soil chronosequence. The chronosequence in South Africa exhibited the highest rate of soil C accrual (60.5 g C/m2/y, r2=0.62, P<0.001) within a similar time frame as the Nebraska chronosequences, likely resulting from extended periods of limited decomposition. Thus, belowground recovery during grassland restoration varies between soil textures within a precipitation regime, and cross-continental studies expand our understanding soil C accrual by incorporating variation in climate regimes.
