Mechanisms of carbon and nutrient release from acid impacted soils: Investigating competitive sorption and aggregate dispersion

Dissolved organic carbon (DOC) and associated nutrients are of critical importance to natural biogeochemical cycling. In recent decades, increased amounts of DOC have been observed in Northern Hemisphere surface waters recovering from acid deposition. Such increases in DOC can have significant implications for the productivity of surface waters, yet the mechanisms controlling DOC release are yet to be understood. As soils are one of the primary sources of DOC in surface waters, this study attempts to identify mechanisms controlling DOC release from soils in the context of changing deposition chemistry. Two experiments were designed to investigate two soil-related processes that can lead to the liberation of DOC and nutrients from riparian zone (RZ) and hillslope (HS) soils. First RZ soils collected from the Sleepers River USGS research station were used to conduct a flow through experiment using simulated sulfate impacted and non-impacted soils. In this experiment DOC solution was infiltrated to test the effect of competitive sorption between DOC and sulfate, however this effect could not be confirmed. In a second experiment, a batch approach was used to test the effect of pH and ionic strength (IS) on aggregate dispersion in both RZ and HS soils. Results reveal that IS, not pH, strongly controlled DOC release in all soils presumably by impacting soil aggregation. Release of DOC and P was similar for RZ vs. HS soils, however N release was significantly higher from RZ soils, indicating soil type and landscape position matter for nutrient release. Together these results indicate that changes in deposition IS more than pH or sulfate additions play a major role in the release of DOC and nutrients from soils at Sleepers River, likely due to the connection between IS and soil aggregate dispersion

Numerical Simulations of Turbulent Molecular Clouds Regulated by Reprocessed Radiation Feedback from Nascent Super Star Clusters

Radiation feedback from young star clusters embedded in giant molecular clouds (GMCs) is believed to be important to the control of star formation. For the most massive and dense clouds, including those in which super star clusters (SSCs) are born, pressure from reprocessed radiation exerted on dust grains may disperse a significant portion of the cloud mass back into the interstellar medium (ISM). Using our radiaton hydrodynamics (RHD) code, Hyperion, we conduct a series of numerical simulations to test this idea. Our models follow the evolution of self-gravitating, strongly turbulent clouds in which collapsing regions are replaced by radiating sink particles representing stellar clusters. We evaluate the dependence of the star formation efficiency (SFE) on the size and mass of the cloud and $\kappa$, the opacity of the gas to infrared (IR) radiation. We find that the single most important parameter determining the evolutionary outcome is $\kappa$, with $\kappa \gtrsim 15 \text{ cm}^2 \text{ g}^{-1}$ needed to disrupt clouds. For $\kappa = 20-40 \text{ cm}^2 \text{ g}^{-1}$, the resulting SFE=50-70% is similar to empirical estimates for some SSC-forming clouds. The opacities required for GMC disruption likely apply only in dust-enriched environments. We find that the subgrid model approach of boosting the direct radiation force $L/c$ by a "trapping factor" equal to a cloud's mean IR optical depth can overestimate the true radiation force by factors of $\sim 4-5$. We conclude that feedback from reprocessed IR radiation alone is unlikely to significantly reduce star formation within GMCs unless their dust abundances or cluster light-to-mass ratios are enhanced.Comment: 19 pages, 18 figures, accepted for publication in Ap

Aristelliger georgeensis

Number of Pages: 2Integrative BiologyGeological Science

Aristelliger

Number of Pages: 4Integrative BiologyGeological Science