Physical pathways of nutrient supply in a small, ultraoligotrophic arctic lake during summer stratification

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/110013/1/lno20065121107.pd

Pools, transformations, and sources of P in high-elevation soils: Implications for nutrient transfer to Sierra Nevada lakes

In high-elevation lakes of the Sierra Nevada (California), increases in P supply have been inferred from shifts in P to N limitation. To examine factors possibly leading to changes in P supply, we measured pools and transformations in soil P, and developed a long-term mass balance to estimate the contribution of parent material weathering to soil P stocks. Common Sierra Nevada soils were found to not be P-deficient and to be retentive of P due to the influence of Fe- and Al-oxides. Total P averaged 867μgPg-1 in the top 10cm of soil (O and A horizons) and 597μgPg-1 in the 10-60cm depth (B horizons), of which 70% in A horizons and 60% in B horizons was freely exchangeable or associated with Fe and Al. Weathering of parent material explained 69% of the P found in soils and lost from the catchment since deglaciation, implying that long-term atmospheric P deposition (0.02kgha-1yr-1) represented the balance of P inputs (31%) during the past 10,000years of soil development. During spring snowmelt ~27% of the total soil P was transferred between organic and inorganic pools; average inorganic P pools decreased by 232μgPg-1, while organic P pools increased by 242μgPg-1. Microbial biomass P was highest during winter and decreased six-fold to a minimum in the fall. Interactions between hydrology and biological processes strongly influence the rate of P transfer from catchment soils to lakes. © 2013 Elsevier B.V

Phosphorus in sediments of high-elevation lakes in the Sierra Nevada (California): implications for internal phosphorus loading

In high-elevation lakes of the Sierra Nevada (California), increases in phosphorus (P) supply have been inferred from changes in phytoplankton growth during summer. To quantify rates of sediment P release to high-elevation Sierran lakes, we performed incubations of sediment cores under ambient and reducing conditions at Emerald Lake and analyzed long-term records of lake chemistry for Emerald and Pear lakes. We also measured concentrations of individual P forms in sediments from 50 Sierra Nevada lakes using a sequential fractionation procedure to examine landscape controls on P forms in sediments. On average, the sediments contained 1,445 µg P g−1, of which 5 % was freely exchangeable, 13 % associated with reducible metal hydroxides, 68 % associated with Al hydroxides, and the remaining 14 % stabilized in recalcitrant pools. Multiple linear regression analysis indicated that sediment P fractions were not well correlated with soluble P concentrations. In general, sediments behaved as net sinks for P even under reducing conditions. Our findings suggest that internal P loading does not explain the increase in P availability observed in high-elevation Sierran lakes. Rather, increased atmospheric P inputs and increased P supply via dissolved organic C leaching from soils may be driving the observed changes in P biogeochemistry

20th Century Atmospheric Deposition and Acidification Trends in Lakes of the Sierra Nevada, California, USA

We investigated multiple lines of evidence to determine if observed and paleo-reconstructed changes in acid neutralizing capacity (ANC) in Sierra Nevada lakes were the result of changes in 20th century atmospheric deposition. Spheroidal carbonaceous particles (SCPs) (indicator of anthropogenic atmospheric deposition) and biogenic silica and δ(13)C (productivity proxies) in lake sediments, nitrogen and sulfur emission inventories, climate variables, and long-term hydrochemistry records were compared to reconstructed ANC trends in Moat Lake. The initial decline in ANC at Moat Lake occurred between 1920 and 1930, when hydrogen ion deposition was approximately 74 eq ha(-1) yr(-1), and ANC recovered between 1970 and 2005. Reconstructed ANC in Moat Lake was negatively correlated with SCPs and sulfur dioxide emissions (p = 0.031 and p = 0.009). Reconstructed ANC patterns were not correlated with climate, productivity, or nitrogen oxide emissions. Late 20th century recovery of ANC at Moat Lake is supported by increasing ANC and decreasing sulfate in Emerald Lake between 1983 and 2011 (p < 0.0001). We conclude that ANC depletion at Moat and Emerald lakes was principally caused by acid deposition, and recovery in ANC after 1970 can be attributed to the United States Clean Air Act

Influence of soil moisture on the seasonality of nitric oxide emissions from chaparral soils, Sierra Nevada, California, USA

Soil nitric oxide (NO) emissions are variable in both space and time, and are important pathways for N loss in seasonally dry ecosystems that undergo abrupt transitions from dry-to-wet soil conditions. We measured soil NO emissions from a chaparral catchment to characterize seasonal variability of, and triggers for enhanced NO losses. Pulses in NO emissions were observed in the summer and autumn when dry soils (soil water content (θ) < 6%) were wetted naturally and artificially (range: 97–513 ng NO–N m−2 s−1). The rapidity and magnitude of these pulses suggest that abiotic processes may influence NO emissions. Outside of the observed pulses, NO emissions were highest during the dry season (θ < 6%; dry season mean = 3.4 ng NO–N m−2 s−1) and lowest during the winter wet season (θ > 20%; wet season mean = 0.14 ng NO–N m−2 s−1). These observed seasonal patterns contrast with previous DAYCENT simulations of NO emissions in our catchment, which predicted higher NO emissions during the wet season. Our field observations are consistent with sustained rates of nitrification, reduced plant N uptake, and high soil gas diffusivity observed during the dry season in arid environments

Influence of soil moisture on the seasonality of nitric oxide emissions from chaparral soils, Sierra Nevada, California, USA

Soil nitric oxide (NO) emissions are variable in both space and time, and are important pathways for N loss in seasonally dry ecosystems that undergo abrupt transitions from dry-to-wet soil conditions. We measured soil NO emissions from a chaparral catchment to characterize seasonal variability of, and triggers for enhanced NO losses. Pulses in NO emissions were observed in the summer and autumn when dry soils (soil water content (θ) < 6%) were wetted naturally and artificially (range: 97–513 ng NO–N m−2 s−1). The rapidity and magnitude of these pulses suggest that abiotic processes may influence NO emissions. Outside of the observed pulses, NO emissions were highest during the dry season (θ < 6%; dry season mean = 3.4 ng NO–N m−2 s−1) and lowest during the winter wet season (θ > 20%; wet season mean = 0.14 ng NO–N m−2 s−1). These observed seasonal patterns contrast with previous DAYCENT simulations of NO emissions in our catchment, which predicted higher NO emissions during the wet season. Our field observations are consistent with sustained rates of nitrification, reduced plant N uptake, and high soil gas diffusivity observed during the dry season in arid environments

Correcting for background nitrate contamination in KCl-extracted samples during isotopic analysis of oxygen and nitrogen by the denitrifier method.

RationalePrevious research has shown that the denitrifying bacteria Pseudomonas chlororaphis ssp. aureofaciens (P. aureofaciens) can be used to measure the δ(15)N and δ(18)O values of extracted soil nitrate (NO3(-)) by isotope ratio mass spectrometry. We discovered that N2O production from reference blanks made in 1 M KCl increased relative to blanks made of deionized water (DIW). Further investigation showed that isotopic standards made in KCl yielded δ(15)N and δ(18)O values different from the standards prepared in DIW.MethodsThree grades of crystalline KCl were dissolved in DIW to create solutions of increasing molarity (0.1 M to 2 M), which were added to P. aureofaciens broth and measured as blanks. Reference standards USGS-32, USGS-34, and USGS-35 were then dissolved in a range of KCl concentrations to measure isotopic responses to changing KCl molarity. Reference blanks and standards created in DIW were analyzed as controls to measure the impact of KCl on the δ(15)N and δ(18)O values.ResultsThe amount of N2O in the KCl blanks increased linearly with increasing molarity, but at different rates for each KCl grade. The isotopic values of the reference standards measured in KCl were systematically different from those measured in DIW, suggesting contamination by background NO3(-) in the KCl reagents. However, we also noted reduced conversion of NO3(-) into N2O as the KCl molarity increased, suggesting there is a physiological response of P. aureofaciens to KCl.ConclusionsThere is a small amount of NO3(-) present in crystalline KCl, which can bias isotopic measurement of NO3(-) at low sample concentrations. This can be minimized by making standards and blanks in the same KCl as is used in samples, diluting all samples and standards to the appropriate NO3(-) concentration using matched KCl solutions, and adding samples and standards to the broth at a constant volume to standardize the KCl molarity in the reaction vial

Influence of soil moisture on the seasonality of nitric oxide emissions from chaparral soils, Sierra Nevada, California, USA

Soil nitric oxide (NO) emissions are variable in both space and time, and are important pathways for N loss in seasonally dry ecosystems that undergo abrupt transitions from dry-to-wet soil conditions. We measured soil NO emissions from a chaparral catchment to characterize seasonal variability of, and triggers for enhanced NO losses. Pulses in NO emissions were observed in the summer and autumn when dry soils (soil water content (θ) < 6%) were wetted naturally and artificially (range: 97–513 ng NO–N m−2 s−1). The rapidity and magnitude of these pulses suggest that abiotic processes may influence NO emissions. Outside of the observed pulses, NO emissions were highest during the dry season (θ < 6%; dry season mean = 3.4 ng NO–N m−2 s−1) and lowest during the winter wet season (θ > 20%; wet season mean = 0.14 ng NO–N m−2 s−1). These observed seasonal patterns contrast with previous DAYCENT simulations of NO emissions in our catchment, which predicted higher NO emissions during the wet season. Our field observations are consistent with sustained rates of nitrification, reduced plant N uptake, and high soil gas diffusivity observed during the dry season in arid environments