Kathleen A. Lohse

Linking soils and streams: sources and chemistry of dissolved organic matter in a small coastal watershed.

[1]xa0To understand the hydrologic and biogeochemical controls on the age and recalcitrance of dissolved organic matter (DOM) found in stream waters, we combined hydrometric monitoring along a topographic gradient from ridge to channel with isotopic (13C and 14C) and spectroscopic (UV and 13C nuclear magnetic resonance) analyses of soil and stream water samples in a small coastal watershed in California. With increasing discharge, dissolved organic carbon concentrations increased from 2.2 to 10.9 mg C L−1, Δ14C values increased from −125 to +120‰, δ13C values decreased from −24 to −29‰, C:N ratios increased from 6.5 to 15.4, and specific UV adsorption increased from 1.4 to 3.8 L mg C−1 m−1. These changes in DOM composition are consistent with a shift in source from old and recalcitrant soil organic matter (OM) sources found in deep soil horizons to young and relatively fresh OM sources found in the surface horizons. Results from this study suggest upland soils of the watershed become DOM production limited as indicated by a seasonal depletion and chemical shift in soil DOM, whereas highly productive soils in the hollow act as a near-infinite DOM source. Hydrologic connectivity of this DOM-rich riparian source region to the stream ultimately constrains DOM export, and the stream DOM composition reflects the combined influence of soil biogeochemical cycling of OM and hydrologic routing of water through the landscape.

Atmospheric deposition of carbon and nutrients across an arid metropolitan area.

Urbanization is increasing rapidly in semi-arid environments and is predicted to alter atmospheric deposition of nutrients and pollutants to cities as well as to ecosystems downwind. We examined patterns of wet and coarse dry deposition chemistry over a five-year period at 7 sites across the Central Arizona-Phoenix (CAP) study area, one of two urban sites within the National Science Foundations Long-Term Ecological Research (LTER) program. Wet and dry deposition of organic carbon (oC) were significantly elevated in the urban core; in contrast, mean annual wet and dry fluxes of nitrogen (N) were low (<6 kg ha(-1) yr(-1)) compared to previous estimates and did not differ significantly among sites. Wet deposition of sulfate (SO(4)2-) was high across CAP (mean 1.39 kg ha(-1) yr(-1) as S) and represented the dominant anion in rainfall. Dry deposition rates did not show strong seasonal trends with the exception of oC, which was 3-fold higher in winter than in summer; ammonium (NH4+) deposition was high but more variable. Dry deposition of NO3- and oC was strongly correlated with particulate base cations and dust-derived soluble reactive phosphorus (SRP), suggesting that urban-derived dust is scrubbing the atmosphere of acidic gases and entrained particles and increasing local deposition. Differences between measured and predicted rates of dry N deposition to the urban core may be explained by incomplete collection of gas phase N on surrogate deposition surfaces in this hot and arid environment. The extent of urban enhancement of cations and oC inputs to desert ecosystems appears to be restricted to the urbanized metropolitan area rather than extending far downwind, although a low number of sites make it difficult to resolve this spatial pattern. Nevertheless, wet and dry inputs may be important for biogeochemical cycles in nutrient and carbon-poor desert ecosystems within and near arid cities.

Sources and Transport of Nitrogen in Arid Urban Watersheds

Urban watersheds are often sources of nitrogen (N) to downstream systems, contributing to poor water quality. However, it is unknown which components (e.g., land cover and stormwater infrastructure type) of urban watersheds contribute to N export and which may be sites of retention. In this study we investigated which watershed characteristics control N sourcing, biogeochemical processing of nitrate (NO3-) during storms, and the amount of rainfall N that is retained within urban watersheds. We used triple isotopes of NO3- (δ15N, δ18O, and Δ17O) to identify sources and transformations of NO3- during storms from 10 nested arid urban watersheds that varied in stormwater infrastructure type and drainage area. Stormwater infrastructure and land cover--retention basins, pipes, and grass cover--dictated the sourcing of NO3- in runoff. Urban watersheds were strong sinks or sources of N to stormwater depending on runoff, which in turn was inversely related to retention basin density and positively related to imperviousness and precipitation. Our results suggest that watershed characteristics control the sources and transport of inorganic N in urban stormwater but that retention of inorganic N at the time scale of individual runoff events is controlled by hydrologic, rather than biogeochemical, mechanisms.

Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways

Rapid increases in human population and land transformation in arid and semi-arid regions are altering water, carbon (C) and nitrogen (N) cycles, yet little is known about how urban ephemeral stream channels in these regions affect biogeochemistry and trace gas fluxes. To address these knowledge gaps, we measured carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) before and after soil wetting in 16 ephemeral stream channels that vary in soil texture and organic matter in Tucson, AZ. Fluxes of CO2 and N2O immediately following wetting were among the highest ever published (up to 1,588xa0mg Cxa0m−2xa0h−1 and 3,121xa0μg Nxa0m−2xa0h−1). Mean post-wetting CO2 and N2O fluxes were significantly higher in the loam and sandy loam channels (286 and 194xa0mg C m−2xa0h−1; 168 and 187xa0μgxa0N m−2xa0h−1) than in the sand channels (45xa0mg C m−2xa0h−1 and 7xa0μgxa0N m−2xa0h−1). Factor analyses show that the effect of soil moisture, soil C and soil N on trace gas fluxes varied with soil texture. In the coarser sandy sites, trace gas fluxes were primarily controlled by soil moisture via physical displacement of soil gases and by organic soil C and N limitations on biotic processes. In the finer sandy loam sites trace gas fluxes and N-processing were primarily limited by soil moisture, soil organic C and soil N resources. In the loam sites, finer soil texture and higher soil organic C and N enhance soil moisture retention allowing for more biologically favorable antecedent conditions. Variable redox states appeared to develop in the finer textured soils resulting in wide ranging trace gas flux rates following wetting. These findings indicate that urban ephemeral channels are biogeochemical hotspots that can have a profound impact on urban C and N biogeochemical cycling pathways and subsequently alter the quality of localized water resources.

Climatic and landscape influences on soil moisture are primary determinants of soil carbon fluxes in seasonally snow-covered forest ecosystems

A changing climate has the potential to mobilize soil carbon, shifting seasonally snow-covered, forested ecosystems from carbon sinks to sources. To determine the sensitivity of soil carbon fluxes to changes in temperature and moisture, we quantified seasonal and spatial variability of soil carbon dioxide (CO2) fluxes (Nxa0=xa0746) and dissolved organic carbon (DOC) in leachate (Nxa0=xa0260) in high-elevation, mixed conifer forests in Arizona and New Mexico. All sites have cold winters, warm summers, and bimodal soil moisture patterns associated with snowmelt and summer monsoon rainfall. We employed a state factor approach, quantifying how distal controls (parent material, regional climate, topography) interacted with proximal variability in soil temperature (−3 to 26xa0°C) and moisture (2–76xa0%) to influence carbon effluxes. Carbon loss was dominated by CO2 flux (250–1220xa0g Cxa0m−2xa0year−1) rather than leached DOC (7.0–9.4xa0g C m−2xa0year−1). Significant differences in mean growing season CO2 flux were associated with parent material and aspect; differences appear to be mediated by how these distal controls influence primarily moisture and secondarily temperature. Across all sites, a multiple linear regression model (MLR) relying on moisture and temperature best described growing season CO2 fluxes (r2xa0=xa00.63, pxa0<xa00.001). During winter, the MLR describing soil CO2 flux (r2xa0=xa00.98, pxa0<xa00.001) relied on distal factors including snow cover, clay content, and bulk carbon, all factors that influence liquid water content. Our findings highlight the importance of state factors in controlling soil respiration primarily through influencing spatial and temporal heterogeneity in soil moisture.

Spatial patterns of vegetation, soils, and microtopography from terrestrial laser scanning on two semiarid hillslopes of contrasting lithology

Shrublands in semiarid regions are heterogeneous landscapes consisting of infertile bare areas separated by nutrient rich vegetated areas known as resource islands. Spatial patterns in these landscapes are structured by feedbacks driven by the transport of water and nutrient resources from the intershrub space to areas below shrubs, and the retention of these resources to locally drive productivity and tight biogeochemical cycles. Most understanding of plant-soil feedbacks is based predominantly on studies of low topographic gradient landscapes, and it is unclear whether the patterns of association between soils and vegetation, and the autogenic processes that create them, also occur on more steeply sloping terrain. Here we analyze the spatial patterns of soils, vegetation, and microtopography on hillslopes of contrasting lithology (one granite at 16°, one schist at 27°) in the Sonoran desert foothills of the Catalina Mountains. We also describe a method of extracting vegetation density from terrestrial laser scanning point cloud data at 5 cm × 5 cm scales and find that it correlates well with soil organic carbon measurements. Vegetation was associated with microtopographic mounds (relative to the mean slope) extending 0.3 m downslope and 1.8 m (schist) and 0.9 m (granite) upslope on the study hillslopes. Soils below the shrub canopies exhibited 2–3 times more soil organic matter and 2–4 times higher hydraulic conductivity than the interspaces. Soils enriched with organic matter were found to extend at least two canopy radii downslope of woody shrubs, but not upslope. These plumes were clearest in the lower gradient granite site where vegetation mounds created distinct patterns of microtopographic convergence and divergence. At the steeper schist site, microtopography appeared to have a weaker control on topographic flow accumulation. Collectively, our findings suggest that the spatial structure of association between soils and microtopography and vegetation on these slopes exhibit many of the features observed in lower gradient areas. However, microtopography and soils are more asymmetric along the downslope axis of the hillslopes than lower gradient areas and vary with lithology. Alluvial and colluvial processes are likely more important in shaping vegetation and soil dynamics on hillslopes, and these factors need further consideration in scaling results to the landscape level.

High Atmospheric Nitrate Inputs and Nitrogen Turnover in Semi-arid Urban Catchments

The influx of atmospheric nitrogen to soils and surfaces in arid environments is of growing concern due to increased N emissions and N usage associated with urbanization. Atmospheric nitrogen inputs to the critical zone can occur as wet (rain or snow) or dry (dust or aerosols) deposition, and can lead to eutrophication, soil acidification, and groundwater contamination through leaching of excess nitrate. The objective of this research was to use the δ15N, δ18O, and Δ17O values of atmospheric nitrate (NO3−) (precipitation and aerosols) and NO3− in runoff to assess the importance of N deposition and turnover in semi-arid urban watersheds. Data show that the fractions of atmospheric NO3− exported from all the urban catchments, throughout the study period, were substantially higher than in nearly all other ecosystems studied with mean atmospheric contributions of 38% (min 0% and max 82%). These results suggest that catchment and stream channel imperviousness enhance atmospheric NO3− export due to inefficient N cycling and retention. In contrast, catchment and stream channel perviousness allow for enhanced N processing and therefore reduced atmospheric NO3− export. Overall high fractions of atmospheric NO3− were primarily attributed to slow N turn over in arid/semi-arid ecosystems. A relatively high fraction of nitrification NO3− (~30%) was found in runoff from a nearly completely impervious watershed (91%). This was attributed to nitrification of atmospheric NH4+ in dry-deposited dust, suggesting that N nitrifiers have adapted to urban micro niches. Gross nitrification rates based on NO3− Δ17O values ranged from a low 3.04xa0±xa02xa0kgxa0NO3-Nxa0km−2xa0day−1 in highly impervious catchments to a high of 10.15xa0±xa01xa0kgxa0NO3-Nxa0km−2xa0day−1 in the low density urban catchment. These low gross nitrification rates were attributed to low soil C:N ratios that control gross autotrophic nitrification by regulating gross NH4+ production rates.

The Role of Critical Zone Observatories in Critical Zone Science

Chapter 2 The Role of Critical Zone Observatories in Critical Zone Science Timothy White*, Susan Brantley**, Steve Banwart † , Jon Chorover ‡ , William Dietrich § , Lou Derry ¶ , Kathleen Lohse †† , Suzanne Anderson ‡‡ , Anthony Aufdendkampe §§ , Roger Bales***, Praveen Kumar ††† , Dan Richter ‡‡‡ , and Bill McDowell §§§ Earth and Mineral Sciences, Penn State University, Pennsylvania State University, State College, Pennsylvania, USA; ** Earth and Environmental Systems Institute, Pennsylvania State University, University Park, Pennsylvania, USA; † Kroto Research Institute, North Campus, University of Sheffield, Broad Lane, Sheffield, UK; ‡ Department of Soil Water and Environmental Science, University of Arizona, Tucson, Arizona, USA; § Earth and Planetary Science, University of California, Berkeley, California, USA; ¶ Earth and Atmospheric Sciences, Cornell University, Cornell, New York, USA; †† Soil and Watershed Biogeochemistry, Biological Sciences, Idaho State University, Pocatello, Idaho, USA; ‡‡ Department of Geography, University of Colorado at Boulder, Boulder, Colorado, USA; §§ Stroud Water Research Center, West Grove, Pennsylvania, USA; *** Sierra Nevada Research Institute, University of California, Merced, California, USA; Colonel Harry F. and Frankie M. Lovell Endowed Professor of Civil and Environmental Engineering, Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA; ‡‡‡ Soils and Forest Ecology, Duke University, Durham, North Carolina, USA; §§§ New Hampshire Water Resources Research Center, Department of Natural Resources & the Environment, University of New Hampshire, Durham, New Hampshire, USA 2.1 THE CRITICAL ZONE The Critical Zone (CZ), a term first coined by the US National Research Council (2001), encompasses the thin outer veneer of Earth’s surface extending from the top of the vegetation canopy down to the subsurface depths of fresh groundxad water. Complex biogeochemical-physical processes combine in the CZ to transxad form rock and biomass into soil that in turn supports much of the terrestrial biosphere. Processes in the Critical Zone are represented by coupled physical, biological, and chemical processes (Fig. 2.1), and scientific expertise from an array of disciplines is needed to understand the zone and its processes: geology, soil science, biology, ecology, geochemistry, hydrology, geomorphology, atmosxad pheric science, and many more. The zone sustains most aboveground terrestrial life including humanity. Yet, the science of coupled human-natural systems is far Developments in Earth Surface Processes, Vol. 19. http://dx.doi.org/10.1016/B978-0-444-63369-9.00002-1 Copyright

Combined impact of catchment size, land cover, and precipitation on streamflow and total dissolved nitrogen: A global comparative analysis

Nitrogen (N) loading is a global stressor to fresh and salt water systems with cascading effects on ecosystem processes. However, it is unclear if generalized global response patterns exist between discharge and N sourcing and retention with respect to land cover and precipitation. Using data compiled from 78 catchments from across the world, we identified how discharge and total dissolved nitrogen (TDN) vary with precipitation and land cover and how TDN yields deviate from a generalized global response pattern. Area-weighted discharge regressions indicate that catchment size and the absence of vegetation largely control hydrologic responses. TDN concentrations and yields varied significantly (Pu2009 u20090.05) of discharge, suggesting that these sites are less sensitive to shifts in discharge associated with global climate change, but are more sensitive to shifts in hydrologic partitioning in response to land cover change. Clustering based on precipitation and stepwise multiple linear regression analyses show a shift in TDN responses from physical transport controls on TDN sourcing at the most arid and water limited sites to climate and biologically mediated controls on TDN retention at the wetter sites. Combined, these results indicate that terrestrial systems may have differential response to changes in precipitation based on existing land use and that the impact of land use change on N fate and transport occurs within the context of climate conditions.

Influence of Road Reclamation Techniques on Forest Ecosystem Recovery

Road reclamation has emerged as an integral part of ecological restoration strategies, particularly on public lands. However, there are no consistent techniques for how road reclamation should be implemented to restore ecosystem structure and function. Resource managers are hindered by critical research gaps regarding the linkages between, as well as the effects of different restoration actions on, above- and belowground ecological and hydrological properties. In the western US, we examined how two road reclamation methods (recontouring and abandonment) affect ecosystem properties relative to “never-roaded” areas. Recontoured and abandoned sites displayed similar aboveground properties but exhibited notable differences in below-ground properties, including soil hydraulic conductivity, organic matter, total carbon, and total nitrogen, among others. Our findings suggest that recontouring can dramatically accelerate recovery of above- and belowground properties so they resemble never-roaded reference conditi...