John D. Schade

THE INFLUENCE OF A RIPARIAN SHRUB ON NITROGEN CYCLING IN A SONORAN DESERT STREAM

Riparian zones often act as nutrient filters, removing NO3 from water flowing through riparian soils. The role of vegetation in NO3 retention remains unclear and may be direct (uptake) or indirect (stimulation of microbial activity). We studied the riparian shrub Baccharis salicifolia (seepwillow) in Sycamore Creek (Arizona, USA), to determine (1) if sites colonized by seepwillow were sinks for NO3, and (2) the mechanism by which seepwillow causes NO3 retention. Subsurface water was sampled along flowpaths from an uncolonized gravel bar through seepwillow sites at several depths and on several gravel bars. NO3 concentration was significantly lower in seepwillow sites than in uncolonized sites, at least to 20 cm below the water table. Predictions of three hypotheses were tested to explain NO3 losses: (Hi) by plant uptake, (H2) by stimulation of denitrification by seepwillow, and (H3) a prior condition unrelated to seepwillow. Six experiments were used to test these hypotheses. Transplant experiments, plant size relationships, and root distri- bution experiments all demonstrated the importance of seepwillow (rejection of H3). Other tests involving removal of aboveground biomass, denitrification measures, and mass balance calculations showed a predominance of denitrification over uptake (rejection of Hi). We conclude that the main effect of seepwillow is to produce organic matter creating conditions favorable to denitrification and a loss of NO3 from subsurface water. Since denitrification is a permanent loss of N to the atmosphere, and uptake only temporarily retains N, the interaction between plants and microbes has important implications for the maintenance of water quality in streams and downstream reservoirs.

Sources of nitrogen to the riparian zone of a desert stream: Implications for riparian vegetation and nitrogen retention

Riparian zones effectively remove nitrogen (N) from water flowing through riparian soils, particularly in agricultural watersheds. The mechanism of N removal is still unclear, especially the role of vegetation. Uptake and denitrification are the two most commonly studied mechanisms. Retention of groundwater N by plant uptake is often inferred from measurements of N in net incremental biomass. However, this assumes other sources of N are not contributing to the N demand of plants. The purpose of this work was to investigate the relative importance of three sources of available N to riparian trees in a desert stream—input in stream water during floods, input during baseflow, and mineralization of N from soil organic matter. Two approaches were used; a mass balance approach in which the mass of available N from each source was estimated, and a correlational approach in which indexes of each source were compared to leaf N for individual willow trees. Total N from all sources was 396 kg ha−1 y−1, with 172 kg ha−1 y−1 from mineralization, 214 kg ha−1 y−1 from the stream during baseflow, and 9.6 kg ha−1 y−1 from floods. Leaf N was significantly related to N mineralization rates and flood inputs; it was not related to baseflow inputs. We conclude that mineralization is a major source of available N for willow trees, subsidized by input of N from floods. Baseflow inputs are most likely removed by rapid denitrification at the stream–riparian edge, while higher rates of flood supply exceed the capacity of this “filter.”

Permafrost thaw and soil moisture driving CO2 and CH4 release from upland tundra

As permafrost degrades, the amount of organic soil carbon (C) that thaws during the growing season will increase, but decomposition may be limited by saturated soil conditions common in high-latitude ecosystems. However, in some areas, soil drying is expected to accompany permafrost thaw as a result of increased water drainage, which may enhance C release to the atmosphere. We examined the effects of ecosystem warming, permafrost thaw, and soil moisture changes on C balance in an upland tundra ecosystem. This study was conducted at a water table drawdown experiment, established in 2011 and located within the Carbon in Permafrost Experimental Heating Research project, an ecosystem warming and permafrost thawing experiment in Alaska. Warming and drying increased cumulative growing season ecosystem respiration by ~20% over 3 years of this experiment. Warming caused an almost twofold increase in decomposition of a common substrate in surface soil (0–10 cm) across all years, and drying caused a twofold increase in decomposition (0–20 cm) relative to control after 3 years of drying. Decomposition of older C increased in the dried and in the combined warmed + dried plots based on soil pore space 14CO2. Although upland tundra systems have been considered CH4 sinks, warming and ground thaw significantly increased CH4 emission rates. Water table depth was positively correlated with monthly respiration and negatively correlated with CH4 emission rates. These results demonstrate that warming and drying may increase loss of old permafrost C from tundra ecosystems, but the form and magnitude of C released to the atmosphere will be driven by changes in soil moisture.

Leaf Litter in a Sonoran Desert Stream Ecosystem

Leaf litter is an important functional component of mesic stream ecosystems; however, the importance of leaf litter has not been established for streams of the southwestern North American deserts. These streams exhibit many functional and structural characteristics that are the result of the pattern and amount of precipitation and which differ from their mesic counterparts. Our objective was to determine the influence of leaf litter on ecosystem processes of Sycamore Creek, Arizona, a typical Sonoran Desert stream, and to compare these results with knowledge gained from other regions. Patterns of nitrogen dynamics and respiration during leaf decomposition were similar for leaf packs in Sycamore Creek and previous studies of mesic streams. Macroinvertebrates in Sycamore Creek colonized natural and artificial leaf packs equally, and taxonomic composition did not differ significantly between leaf types, or between leaf pack communities and benthic communities. Shredder macroinvertebrates feeding on leaf material were absent. Leaf litter input to Sycamore Creek was low and litter residence time was reduced by flash floods. As a result, leaves played an insignificant role in terms of nutrient dynamics, energy flow, and macroinvertebrate assemblages in this ecosystem. Differences in the role of leaf litter between regions are attributed to channel and riparian form and to the frequency of severe disturbance, which are functions of meteorologic, hydrologic, and geomorphic features of the landscape.

Stoichiometric response of nitrogen-fixing and non-fixing dicots to manipulations of CO2, nitrogen, and diversity

Human activities have resulted in increased nitrogen deposition and atmospheric CO2 concentrations in the biosphere, potentially causing significant changes in many ecological processes. In addition to these ongoing perturbations of the abiotic environment, human-induced losses of biodiversity are also of major concern and may interact in important ways with biogeochemical perturbations to affect ecosystem structure and function. We have evaluated the effects of these perturbations on plant biomass stoichiometric composition (C:N:P ratios) within the framework of the BioCON experimental setup (biodiversity, CO2, N) conducted at the Cedar Creek Natural History Area, Minnesota. Here we present data for five plant species: Solidago rigida, Achillea millefolium, Amorpha canescens, Lespedeza capitata, and Lupinus perennis. We found significantly higher C:N and C:P ratios under elevated CO2 treatments, but species responded idiosyncratically to the treatment. Nitrogen addition decreased C:N ratios, but this response was greater in the ambient CO2 treatments than under elevated CO2. Higher plant species diversity generally lowered both C:N and C:P ratios. Importantly, increased diversity also led to a more modest increase in the C:N ratio with elevated CO2 levels. In addition, legumes exhibited lower C:N and higher C:P and N:P ratios than non-legumes, highlighting the effect of physiological characteristics defining plant functional types. These data suggest that atmospheric CO2 levels, N availability, and plant species diversity interact to affect both aboveground and belowground processes by altering plant elemental composition.

Light-mediated thresholds in stream-water nutrient composition in a river network

The elemental composition of solutes transported by rivers reflects combined influences of surrounding watersheds and transformations within stream networks, yet comparatively little is known about downstream changes in effects of watershed loading vs. in-channel processes. In the forested watershed of a river under a mediterranean hydrologic regime, we examined the influence of longitudinal changes in environmental conditions on water-column nutrient composition during summer base flow across a network of sites ranging from strongly heterotrophic headwater streams to larger, more autotrophic sites downstream. Small streams (0.1-10 km2 watershed area) had longitudinally similar nutrient concentration and composition with low (approximately 2) dissolved nitrogen (N) to phosphorus (P) ratios. Abrupt deviations from this pattern were observed in larger streams with watershed areas > 100 km2 where insolation and algal abundance and production rapidly increased. Downstream, phosphorus and silica concentrations decreased by > 50% compared to headwater streams, and dissolved organic carbon and nitrogen increased by approximately 3-6 times. Decreasing dissolved P and increasing dissolved N raised stream-water N:P to 46 at the most downstream sites, suggesting a transition from N limitation in headwaters to potential P limitation in larger channels. We hypothesize that these changes were mediated by increasing algal photosynthesis and N fixation by benthic algal assemblages, which, in response to increasing light availability, strongly altered stream-water nutrient concentration and stoichiometry in larger streams and rivers.

Extensive natural intraspecific variation in stoichiometric (C:N:P) composition in two terrestrial insect species

Abstract Heterotrophic organisms must obtain essential elements in sufficient quantities from their food. Because plants naturally exhibit extensive variation in their elemental content, it is important to quantify the within-species stoichiometric variation of consumers. If extensive stoichiometric variation exists, it may help explain consumer variation in life-history strategy and fitness. To date, however, research on stoichiometric variation has focused on interspecific differences and assumed minimal intraspecific differences. Here this assumption is tested. Natural variation is quantified in body stoichiometry of two terrestrial insects: the generalist field cricket, Gryllus texensis Cade and Otte (Orthoptera: Gryllidae) and a specialist curculionid weevil, Sabinia setosa (Le Conte) (Coleoptera: Curculionidae). Both species exhibited extensive intraspecific stoichiometric variation. Cricket body nitrogen content ranged from 8–12% and there was a four-fold difference in body phosphorus content, ranging from 0.32–1.27%. Body size explained half this stoichiometric variation, with larger individuals containing less nitrogen and phosphorus. Weevils exhibited an almost three-fold difference in body phosphorus content, ranging from 0.38–0.97%. Overall, the variation observed within each of these species is comparable to the variation previously observed across almost all terrestrial insect species.

Hydrologic exchange and N uptake by riparian vegetation in an arid-land stream

Abstract Riparian zones can strongly influence the exchange of nutrients between streams and their watersheds. Most riparian studies have been done in mesic watersheds, which differ significantly from arid-land watersheds hydrologically. The goals of our work were to determine the strength and direction of hydrologic linkages between stream and riparian zone, and to estimate the extent of uptake of streamwater N by riparian trees in Sycamore Creek, a Sonoran Desert stream. Br− and 15NH4+ were added simultaneously to the surface stream to trace water and N from stream to riparian zone. Br− concentrations in riparian wells installed downstream of the release point increased during the addition, demonstrating a strong hydrologic linkage from stream to riparian zone. Percentage stream water in wells increased in a downstream direction, suggesting little or no input of water laterally from uplands or vertically from deep groundwater. Leaf and wood samples collected from willow trees downstream of the addition point became significantly labeled with 15N during the addition, indicating uptake of streamwater N. Other tree species did not become labeled, most likely because they were located farther from the stream channel than the willows. Results from our study provide evidence of strong hydrologic linkage between stream and riparian zone and suggest that N demand by riparian vegetation is a potentially significant sink for streamwater N.

Causes and Consequences of Herbivory on Prairie Lupine (Lupinus lepidus) in Early Primary Succession

Primary succession, the formation and change of ecological communities in locations initially lacking organisms or other biological materials, has been an important research focus for at least a century (Cowles 1899; Griggs 1933; Eggler 1941; Crocker and Major 1955; Eggler 1959; Miles and Walton 1993; Walker and del Moral 2003). At approximately 60 km2, primary successional surfaces at Mount St. Helens occupy a minor proportion of the blast zone, yet they are arguably the most compelling. The cataclysmic genesis of this landscape, its utter sterilization, and the drama of its reclamation by living organisms stimulate the imagination of scientists and nonscientists alike. These primary successional surfaces are the most intensively monitored areas at Mount St. Helens because of what they may teach us about the fundamental mechanisms governing the formation and function of biological communities. At a practical level, understanding successional processes provides a conceptual basis for the restoration of devastated landscapes (Bradshaw 1993; Franklin and MacMahon 2000; Walker and del Moral 2003). Succession is a fundamentally multitrophic process. It involves not only plants but also herbivores, predators, and decomposers. Yet, the important effects of these other trophic levels are sometimes ignored. Study of trophic interactions (i.e., interactions between consumers and resources) in primary succession can provide new insights into mechanisms of primary succession and can inform the debates surrounding what controls the level of herbivory in terrestrial ecosystems. In this chapter, we describe often-devastating attacks by insect herbivores on the prairie lupine, Lupinus lepidus var. lobbii (Dougl.), and how they have affected the (spatial) spread of this little plant, generally considered the most important colonist during the first two decades of primary succession on the Mount St. Helens Pumice Plain. We also discuss a surprising spatial pattern in the intensity of lupine herbivory on the Pumice Plain and outline two hypotheses for this pattern. One hypothesis is based on gradients in plant quality; the other is based on gradients in the density and diversity of the herbivores’ natural enemies. Both hypotheses involve processes that are inherent to primary succession and that are likely relevant to systems beyond Mount St. Helens.

Subsystems, flowpaths, and the spatial variability of nitrogen in a fluvial ecosystem

Nutrient dynamics in rivers affect biogeochemical fluxes from land to oceans and the atmosphere. Fluvial ecosystems are thus important environments for understanding spatial variability in nutrient concentrations. At the San Pedro River in semi-arid Arizona, USA, we investigated how variability in dissolved inorganic nitrogen (DIN) was regulated by subsystem type and hydrological flowpaths. The three subsystems we compared were the riparian zone, parafluvial (gravel bar) zone, and surface stream. DIN concentration was greater in the riparian zone than in the surface stream, suggesting that the riparian zone retains DIN and is a source of N for the surface stream. Parafluvial zones were too variable to generalize how they regulate DIN. Our hypothesis that subsystem type regulates DIN oxidation was too simple. The riparian and parafluvial zones host a mosaic of oxidizing and reducing conditions, as they exhibited highly variable ammonium to nitrate (NH4+:NO3−) ratios. Surface stream DIN was dominated by NO3−. Along a subsurface flowpath in the riparian zone, we did not observe spatial covariation among the N forms and transformations involved in mineralization. We also compared spatial variability in solute concentrations between flowpaths and non-flowpath reference areas. Our mixed results suggest that spatial variability is regulated in part by flowpaths, but also by solute-specific processing length along a flowpath. To understand the distribution of N in fluvial ecosystems, subsystem type and flowpaths are readily discernable guides, but they should be coupled with other mechanistic factors such as biota and soil type.