Chelsea L. Crenshaw

Stream denitrification across biomes and its response to anthropogenic nitrate loading

Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing and terrestrial ecosystems are becoming increasingly nitrogen-saturated, causing more bioavailable nitrogen to enter groundwater and surface waters. Large-scale nitrogen budgets show that an average of about 20–25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins, indicating that substantial sinks for nitrogen must exist in the landscape. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks.

Stoichiometry of soil enzyme activity at global scale

Extracellular enzymes are the proximate agents of organic matter decomposition and measures of these activities can be used as indicators of microbial nutrient demand. We conducted a global-scale meta-analysis of the seven-most widely measured soil enzyme activities, using data from 40 ecosystems. The activities of beta-1,4-glucosidase, cellobiohydrolase, beta-1,4-N-acetylglucosaminidase and phosphatase g(-1) soil increased with organic matter concentration; leucine aminopeptidase, phenol oxidase and peroxidase activities showed no relationship. All activities were significantly related to soil pH. Specific activities, i.e. activity g(-1) soil organic matter, also varied in relation to soil pH for all enzymes. Relationships with mean annual temperature (MAT) and precipitation (MAP) were generally weak. For hydrolases, ratios of specific C, N and P acquisition activities converged on 1 : 1 : 1 but across ecosystems, the ratio of C : P acquisition was inversely related to MAP and MAT while the ratio of C : N acquisition increased with MAP. Oxidative activities were more variable than hydrolytic activities and increased with soil pH. Our analyses indicate that the enzymatic potential for hydrolyzing the labile components of soil organic matter is tied to substrate availability, soil pH and the stoichiometry of microbial nutrient demand. The enzymatic potential for oxidizing the recalcitrant fractions of soil organic material, which is a proximate control on soil organic matter accumulation, is most strongly related to soil pH. These trends provide insight into the biogeochemical processes that create global patterns in ecological stoichiometry and organic matter storage.

Nitrous oxide emission from denitrification in stream and river networks

Nitrous oxide (N2O) is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. Anthropogenic nitrogen (N) loading to river networks is a potentially important source of N2O via microbial denitrification that converts N to N2O and dinitrogen (N2). The fraction of denitrified N that escapes as N2O rather than N2 (i.e., the N2O yield) is an important determinant of how much N2O is produced by river networks, but little is known about the N2O yield in flowing waters. Here, we present the results of whole-stream 15N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N2O at rates that increase with stream water nitrate (NO3−) concentrations, but that <1% of denitrified N is converted to N2O. Unlike some previous studies, we found no relationship between the N2O yield and stream water NO3−. We suggest that increased stream NO3− loading stimulates denitrification and concomitant N2O production, but does not increase the N2O yield. In our study, most streams were sources of N2O to the atmosphere and the highest emission rates were observed in streams draining urban basins. Using a global river network model, we estimate that microbial N transformations (e.g., denitrification and nitrification) convert at least 0.68 Tg·y−1 of anthropogenic N inputs to N2O in river networks, equivalent to 10% of the global anthropogenic N2O emission rate. This estimate of stream and river N2O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change.

N retention and transformation in urban streams

Abstract Nutrient spiraling in theory and application provides a framework for comparing nutrient retention efficiency of urban streams to relatively unaltered streams. Previous research indicated that streams of the southwestern USA deserts are highly retentive of N because of N limitation, high productivity, and high channel complexity (in particular, extensive transient storage associated with the hyporheic zone). Most southwestern urban streams have extensively modified channels and experience N loading from urban runoff and inputs of NO3−-contaminated groundwater. Therefore, we predicted southwestern urban streams are neither N-limited nor retentive. For some urban streams, however, restoration efforts reestablish flow in long-dry channels, create nonstructural flood-management solutions, and design riparian areas as a public recreation amenity. These human modifications may, in part, restore N retention functions if channel complexity and heterogeneity are as important to N retention efficiency as believed. We conducted experimental tracer studies using 15N-NO3−, as part of the Lotic Intersite Nitrogen eXperiment (LINX) project, and several separate nutrient-addition experiments (using slight increases in NO3− concentration), to evaluate N retention in southwestern urban streams. We present preliminary results of those experiments, comparing results to similar experiments in unaltered streams to test our predictions. Our results allow an evaluation of the use of nutrient spiraling metrics as a tool for assessing the status of stream ecosystem services in urban restoration projects.

Effects of Coarse Particulate Organic Matter on Fungal Biomass and Invertebrate Density in the Subsurface of a Headwater Stream

Links between groundwater invertebrates and their potential food resources were examined using biofilm development on fine wood. Little is known about biofilm development and organic matter content of lateral subsurface (i.e., parafluvial) environments and hyporheic habitats (upwelling and downwelling zones). Eighteen experimental baskets containing river rocks were paired by treatment in which 1 basket was supplemented with wood (six 7.2 cm × 12 cm strips of oak wood veneer). Nine pairs of baskets were buried 12 to 15 cm below the surface in Gallina Creek, a 1st-order mountain stream in northern New Mexico in late summer 1997. Three pairs were buried beneath the stream bank (i.e., parafluvial zone) and 6 pairs were buried in the hyporheic zone. Baskets were distributed along upwelling and downwelling reaches to assess the potential hydrologic influence of subsurface-surface exchange. Open baskets of wood veneer were placed on the streambed surface to compare fungal biomass on the surface with the subsurface. Wood in both hyporheic and parafluvial baskets was colonized by fungi, but fungal biomass was significantly greater on wood in surface water than in hyporheic and parafluvial zones. In addition, fungal biomass on hyporheic wood was significantly greater than on parafluvial wood. A similar pattern (i.e., surface > hyporheic > parafluvial) was observed for dissolved oxygen. In contrast, concentrations of retained particulate organic matter were significantly higher in the parafluvial than the hyporheic zone. Invertebrate densities were significantly greater in baskets supplemented with wood and were greater in the hyporheic zone than in the parafluvial zone. Our data suggest that wood and associated microbial biofilms represent an important food resource for interstitial invertebrate communities.

Decomposition of leaf litter from a native tree and an actinorhizal invasive across riparian habitats

Dynamics of nutrient exchange between floodplains and rivers have been altered by changes in flow management and proliferation of nonnative plants. We tested the hypothesis that the nonnative, actinorhizal tree, Russian olive (Elaeagnus angustifolia), alters dynamics of leaf litter decomposition compared to native cottonwood (Populus deltoides ssp. wislizeni) along the Rio Grande, a river with a modified flow regime, in central New Mexico (U.S.A.). Leaf litter was placed in the river channel and the surface and subsurface horizons of forest soil at seven riparian sites that differed in their hydrologic connection to the river. All sites had a cottonwood canopy with a Russian olive-dominated understory. Mass loss rates, nutrient content, fungal biomass, extracellular enzyme activities (EEA), and macroinvertebrate colonization were followed for three months in the river and one year in forests. Initial nitrogen (N) content of Russian olive litter (2.2%) was more than four times that of cottonwood (0.5%). Mass loss rates (k; in units of d(-1)) were greatest in the river (Russian olive, k = 0.0249; cottonwood, k = 0.0226), intermediate in subsurface soil (Russian olive, k = 0.0072; cottonwood, k = 0.0031), and slowest on the soil surface (Russian olive, k = 0.0034; cottonwood, k = 0.0012) in a ratio of about 10:2:1. Rates of mass loss in the river were indistinguishable between species and proportional to macroinvertebrate colonization. In the riparian forest, Russian olive decayed significantly faster than cottonwood in both soil horizons. Terrestrial decomposition rates were related positively to EEA, fungal biomass, and litter N, whereas differences among floodplain sites were related to hydrologic connectivity with the river. Because nutrient exchanges between riparian forests and the river have been constrained by flow management, Russian olive litter represents a significant annual input of N to riparian forests, which now retain a large portion of slowly decomposing cottonwood litter with a high potential for N immobilization. As a result, retention and mineralization of litter N within these forests is controlled by hydrologic connectivity to the river, which affects litter export and in situ decomposition.

You are not always what we think you eat: selective assimilation across multiple whole-stream isotopic tracer studies

Analyses of 21 15 N stable isotope tracer experiments, designed to examine food web dynamics in streams around the world, indicated that the isotopic composition of food resources assimilated by primary consumers (mostly invertebrates) poorly reflected the presumed food sources. Modeling indicated that consumers assimilated only 33-50% of the N available in sampled food sources such as decomposing leaves, epilithon, and fine particulate detritus over feeding periods of weeks or more. Thus, common methods of sampling food sources consumed by animals in streams do not sufficiently reflect the pool of N they assimilate. Isotope tracer studies, combined with modeling and food separation techniques, can improve estimation of N pools in food sources that are assimilated by consumers. Food web studies that use putative food samples composed of actively cycling (more readily assimilable) and refractory (less assimilable) N fractions may draw erroneous conclusions about diets, N turnover, and trophic linkages of consumers. By extension, food web studies using stoichiometric or natural abundance approaches that rely on an accurate description of food-source composition could result in errors when an actively cycling pool that is only a fraction of the N pool in sampled food resources is not accounted for.

Dissolved inorganic nitrogen dynamics in the hyporheic zone of reference and human-altered southwestern U. S. streams

Canalization and incision are common morphological alterations associated with human land use that reduce hydrological/hydrodynamic linkages between surtace and ground waters in stream ecosystems. To explore the impacts of these anthropogenic changes on nutrient spiraling in streams, we measured the linkage between hyporheic and surface zones of reference and human-altered streams using 15 N-nitrate ( 15 NO 3 ― ) and bromide (Br ― ) tracer injection experiments. Experiments were conducted in 7 streams (3 reference, 3 agricultural and 1 urban) in Arizona and New Mexico, USA, over a 3 yr period during the Lotic Intersite Nitrogen eXperiment (LINX II). Groundwater wells (6―9 in each stream) were inserted to 30cm depth and multilevel samplers (MLS) were installed downstream from injection sites to measure inorganic nitrogen (N) and Br ― concentrations in hyporheic water. There was measurable surface water-ground water (SW-GW) connectivity, as indicated by Br ― concentrations in shallow alluvial ground water (0―30 cm depth) at all sites. SW-GW connectivity was higher in reference than in human-altered streams. Concentrations of NO 3 ― and NH 4 + in MLS cells (0―10, 10―20, and 20―30 cm below streambed) were inversely correlated, and 15 N-enrichment of both N species was measurable in groundwater wells. Hyporheic zones with lower surface-water infiltration had higher quantities of 15 NH 4 + than of 15 NO 3 ― . Whole stream uptake (k tot ; calculated during the LINX II experiments) was correlated with δ 15 NH 4 + in wells; i.e., high δ 15 NH 4 + in wells was associated with high stream uptake, and both whole stream NO 3 ― uptake and δ 15 NH 4 + were highest in the human-altered streams. The presence of enriched 15 NH 4 + in many of the wells within 24 h of injection suggested that dissimilatory nitrate reduction to ammonium (DNRA) was occurring in the hyporheic zone.

The Lotic Intersite Nitrogen Experiments: an example of successful ecological research collaboration

Collaboration is an essential skill for modern ecologists because it brings together diverse expertise, viewpoints, and study systems. The Lotic Intersite Nitrogen eXperiments (LINX I and II), a 17-y research endeavor involving scores of early- to late-career stream ecologists, is an example of the benefits, challenges, and approaches of successful collaborative research in ecology. The scientific success of LINX reflected tangible attributes including clear scientific goals (hypothesis-driven research), coordinated research methods, a team of cooperative scientists, excellent leadership, extensive communication, and a philosophy of respect for input from all collaborators. Intangible aspects of the collaboration included camaraderie and strong team chemistry. LINX further benefited from being part of a discipline in which collaboration is a tradition, clear data-sharing and authorship guidelines, an approach that melded field experiments and modeling, and a shared collaborative goal in the form of a universal commitment to see the project and resulting data products through to completion.Abstract: Collaboration is an essential skill for modern ecologists because it brings together diverse expertise, viewpoints, and study systems. The Lotic Intersite Nitrogen eXperiments (LINX I and II), a 17-y research endeavor involving scores of early- to late-career stream ecologists, is an example of the benefits, challenges, and approaches of successful collaborative research in ecology. The scientific success of LINX reflected tangible attributes including clear scientific goals (hypothesis-driven research), coordinated research methods, a team of cooperative scientists, excellent leadership, extensive communication, and a philosophy of respect for input from all collaborators. Intangible aspects of the collaboration included camaraderie and strong team chemistry. LINX further benefited from being part of a discipline in which collaboration is a tradition, clear data-sharing and authorship guidelines, an approach that melded field experiments and modeling, and a shared collaborative goal in the form of a universal commitment to see the project and resulting data products through to completion.