Sharon J. Hall

The Globalization of Nitrogen Deposition: Consequences for Terrestrial Ecosystems

Abstract The sources and distribution of anthropogenic nitrogen (N), including N fertilization and N fixed during fossil-fuel combustion, are rapidly becoming globally distributed. Responses of terrestrial ecosystems to anthropogenic N inputs are likely to vary geographically. In the temperate zone, long-term N inputs can lead to increases in plant growth and also can result in over-enrichment with N, eventually leading to increased losses of N via solution leaching and trace-gas emissions, and in some cases, to changes in species composition and to ecosystem decline. However, not all ecosystems respond to N deposition similarly; their response depends on factors such as successional state, ecosystem type, N demand or retention capacity, land-use history, soils, topography, climate, and the rate, timing, and type of N deposition. We point to some of the conditions under which anthropogenic impacts can be significant, some of the factors that control variations in response, and some areas where uncertainty is large due to limited information.

Effects of nitrogen deposition and empirical nitrogen critical loads for ecoregions of the United States

Human activity in the last century has led to a significant increase in nitrogen (N) emissions and atmospheric deposition. This N deposition has reached a level that has caused or is likely to cause alterations to the structure and function of many ecosystems across the United States. One approach for quantifying the deposition of pollution that would be harmful to ecosystems is the determination of critical loads. A critical load is defined as the input of a pollutant below which no detrimental ecological effects occur over the long-term according to present knowledge. The objectives of this project were to synthesize current research relating atmospheric N deposition to effects on terrestrial and freshwater ecosystems in the United States, and to estimate associated empirical N critical loads. The receptors considered included freshwater diatoms, mycorrhizal fungi, lichens, bryophytes, herbaceous plants, shrubs, and trees. Ecosystem impacts included: (1) biogeochemical responses and (2) individual species, population, and community responses. Biogeochemical responses included increased N mineralization and nitrification (and N availability for plant and microbial uptake), increased gaseous N losses (ammonia volatilization, nitric and nitrous oxide from nitrification and denitrification), and increased N leaching. Individual species, population, and community responses included increased tissue N, physiological and nutrient imbalances, increased growth, altered root : shoot ratios, increased susceptibility to secondary stresses, altered fire regime, shifts in competitive interactions and community composition, changes in species richness and other measures of biodiversity, and increases in invasive species.

Residential landscapes as social-ecological systems: a synthesis of multi-scalar interactions between people and their home environment

Residential landscapes are a common setting of human-environment interactions. These ubiquitous ecosystems provide social and ecological services, and yard maintenance leads to intended and unintended ecological outcomes. The ecological characteristics of residential landscapes and the human drivers of landscape management have been the focus of disciplinary studies, often at a single scale of analysis. However, an interdisciplinary examination of residential landscapes is needed to understand the feedbacks and tradeoffs of these complex adaptive social-ecological systems as a whole. Our aim is to synthesize the diversity of perspectives, scales of analysis, and findings from the literature in order to 1) contribute to an interdisciplinary understanding of residential landscapes and 2) identify research needs while providing a robust conceptual approach for future studies. We synthesize 256 studies from the literature and develop an interdisciplinary, multi-scalar framework on residential landscape dynamics. Complex human drivers (attitudinal, structural, and institutional factors) at multiple scales influence management practices and the feedbacks with biophysical characteristics of residential landscapes. However, gaps exist in our interdisciplinary understanding of residential landscapes within four key but understudied areas: 1) the link between social drivers and ecological outcomes of management decisions, 2) the ecosystem services provided by these landscapes to residents, 3) the interactions of social drivers and ecological characteristics across scales, and 4) generalizations of patterns and processes across cities. Our systems perspective will help to guide future interdisciplinary collaborations to integrate theories and research methods across geographic locations and spatial scales.

NUTRIENT STATUS OF TROPICAL RAIN FORESTS INFLUENCES SOIL N DYNAMICS AFTER N ADDITIONS

Soil nitrogen (N) transformations and N-oxide emissions were measured following N additions in three tropical montane rain forests in the Hawaiian Islands that differed in substrate age and nutrient status. Nitrous oxide (N2O) and nitric oxide (NO) emissions were negligible following first-time N additions in a forest where N limits primary production, and they increased significantly following 11 yr of N fertilization. Furthermore, N-oxide fluxes in the N-limited forest were relatively low in response to a range of N additions, and all doses of N except the two highest (125 and 175 kg N/ha) resulted in net negative rates of mineralization and nitrification (i.e., soils showed significant N consumption). Short-term laboratory 15N experiments supported these trends by showing that both 15NH4+ and 15NO3− were strongly consumed in soil from both the control and long-term fertilized plots in the N-limited site. Long-term N fertilization in the N-limited forest significantly increased N availability and turnov...

Ecological homogenization of urban USA

A visually apparent but scientifically untested outcome of land-use change is homogenization across urban areas, where neighborhoods in different parts of the country have similar patterns of roads, residential lots, commercial areas, and aquatic features. We hypothesize that this homogenization extends to ecological structure and also to ecosystem functions such as carbon dynamics and microclimate, with continental-scale implications. Further, we suggest that understanding urban homogenization will provide the basis for understanding the impacts of urban land-use change from local to continental scales. Here, we show how multi-scale, multi-disciplinary datasets from six metropolitan areas that cover the major climatic regions of the US (Phoenix, AZ; Miami, FL; Baltimore, MD; Boston, MA; Minneapolis–St Paul, MN; and Los Angeles, CA) can be used to determine how household and neighborhood characteristics correlate with land-management practices, land-cover composition, and landscape structure and ecosystem functions at local, regional, and continental scales.

Fertilization practices and soil variations control nitrogen oxide emissions from tropical sugar cane

Nitrogen (N) fertilization of agricultural systems is thought to be a major source of the increase in atmospheric N2O; NO emissions from soils have also been shown to increase due to N fertilization. While N fertilizer use is increasing rapidly in the developing world and in the tropics, nearly all of our information on gas emissions is derived from studies of temperate zone agriculture. Using chambers, we measured fluxes of N2O and NO following urea fertilization in tropical sugar cane systems growing on several soil types in the Hawaiian Islands, United States. On the island of Maui, where urea is applied in irrigation lines and soils are mollisols and inceptisols, N2O fluxes were elevated for a week or less after fertilization; maximum average fluxes were typically less than 30 ng cm−2 h−1. NO fluxes were often an order of magnitude less than N2O. Together, N2O and NO represented from 0.03 to 0.5% of the applied N. In fields on the island of Hawaii, where urea is broadcast on the surface and soils are andisols, N2O fluxes were similar in magnitude to Maui but remained elevated for much longer periods after fertilization. NO emissions were 2–5 times higher than N2O through most of the sampling periods. Together the gas losses represented approximately 1.1–2.5% of the applied N. Laboratory studies indicate that denitrification is a critical source of N2O in Maui, but that nitrification is more important in Hawaii. Experimental studies suggest that differences in the pattern of N2O/NO and the processes producing them are a result of both carbon availability and placement of fertilizer and that the more information-intensive fertilizer management practice results in lower emissions.

Assessing the homogenization of urban land management with an application to US residential lawn care

Significance This paper offers conceptual and empirical contributions to sustainability science in general and urban-ecological studies in particular. We present a new analytical framework for classifying socioecological measures along a homogenization–differentiation spectrum. This simple 2 × 2 matrix highlights the multiscale nature of the processes and outcomes of interest. Our application of the conceptual framework produces needed empirical insights into the extent to which land management appears to be homogenizing in differing biophysical settings. Results suggest that US lawn care behaviors are more differentiated in practice than in theory. Thus even if the biophysical outcomes of urbanization are homogenizing, managing the associated sustainability implications may require a multiscale, differentiated approach. Changes in land use, land cover, and land management present some of the greatest potential global environmental challenges of the 21st century. Urbanization, one of the principal drivers of these transformations, is commonly thought to be generating land changes that are increasingly similar. An implication of this multiscale homogenization hypothesis is that the ecosystem structure and function and human behaviors associated with urbanization should be more similar in certain kinds of urbanized locations across biogeophysical gradients than across urbanization gradients in places with similar biogeophysical characteristics. This paper introduces an analytical framework for testing this hypothesis, and applies the framework to the case of residential lawn care. This set of land management behaviors are often assumed—not demonstrated—to exhibit homogeneity. Multivariate analyses are conducted on telephone survey responses from a geographically stratified random sample of homeowners (n = 9,480), equally distributed across six US metropolitan areas. Two behaviors are examined: lawn fertilizing and irrigating. Limited support for strong homogenization is found at two scales (i.e., multi- and single-city; 2 of 36 cases), but significant support is found for homogenization at only one scale (22 cases) or at neither scale (12 cases). These results suggest that US lawn care behaviors are more differentiated in practice than in theory. Thus, even if the biophysical outcomes of urbanization are homogenizing, managing the associated sustainability implications may require a multiscale, differentiated approach because the underlying social practices appear relatively varied. The analytical approach introduced here should also be productive for other facets of urban-ecological homogenization.

Substrate, climate, and land use controls over soil N dynamics and N-oxide emissions in Borneo

Nitrogen (N) enrichment of tropical ecosystems is likely to increase with rapid industrial and agricultural development, but the ecological consequences of N additions in these systems are not well understood. We measured soil N- oxide emissions and N transformations in primary rain forest ecosystems at four elevations and across two substrate types on Mt. Kinabalu, Borneo, before and after short-term experimental N additions. We also measured N pools and fluxes across a land use gradient of primary forest, burned secondary forest, and fertilized agriculture. Background soil N2O and NO emissions in primary forest decreased with elevation, and soils derived from sedimentary substrates had larger pools of inorganic N, rates of nitrification, and N-oxide fluxes than ultrabasic soils when there were significant differences between substrate types. N-oxide emissions after N additions and background rates of nitrification were low in all soils derived from ultrabasic substrates compared to sedimentary substrates, even at lowland sites supporting, diverse Dipterocarp forests growing on morphologically similar Oxisols. Rates of potential nitrification were good predictors of N-oxide emissions after N additions. N2O and NO fluxes were largest at low elevations and on sedimentary-derived soils compared to ultrabasic-derived soils, even at the smallest addition of N, 15 kg N  ha−1. Because current methods of soil classification do not explicitly characterize a number of soil chemical properties important to nutrient cycling, the use of soil maps to extrapolate biogeochemical processes to the region or globe may be limited in its accuracy and usefulness. In agricultural systems, management practices were more important than substrate type in controlling N-oxide emissions and soil N cycling. N-oxide fluxes from agricultural fields were more than an order of magnitude greater than from primary forests on the same substrate type and at the same elevation. As primary forests are cleared for intensive agriculture, soil N2O and NO emissions are likely to far exceed those from the most N-saturated tropical forest ecosystems. This study highlights the inter-dependence of climate, substrate age, N deposition, and land-use practices determining N cycling and N-oxide emissions in humid tropical regions.

Ecosystem response to nutrient enrichment across an urban airshed in the Sonoran Desert

Rates of nitrogen (N) deposition have increased in arid and semiarid ecosystems, but few studies have examined the impacts of long-term N enrichment on ecological processes in deserts. We conducted a multiyear, nutrient-addition study within 15 Sonoran Desert sites across the rapidly growing metropolitan area of Phoenix, Arizona (USA). We hypothesized that desert plants and soils would be sensitive to N enrichment, but that these effects would vary among functional groups that differ in terms of physiological responsiveness, proximity to surface N sources, and magnitude of carbon (C) or water limitation. Inorganic N additions augmented net potential nitrification in soils, moreso than net potential N mineralization, highlighting the important role of nitrifying microorganisms in the nitrate economy of drylands. Winter annual plants were also responsive to nutrient additions, exhibiting a climate-driven cascade of resource limitation, from little to no production in seasons of low rainfall (winter 2006 and 2007), to moderate N limitation with average precipitation (winter 2009), to limitation by both N and P in a season of above-normal rainfall (winter 2008). Herbaceous production is a potentially important mechanism of N retention in arid ecosystems, capable of immobilizing an amount equal to or greater than that deposited annually to soils in this urban airshed. However, interannual variability in precipitation and abiotic processes that limit the incorporation of detrital organic matter into soil pools may limit this role over the long term. In contrast, despite large experimental additions of N and P over four years, growth of Larrea tridentata, the dominant perennial plant of the Sonoran Desert, was unresponsive to nutrient enrichment, even during wet years. Finally, there did not appear to be strong ecological interactions between nutrient addition and location relative to the city, despite the nearby activity of nearly four million people, perhaps due to loss or transfer pathways that limit long-term N enrichment of ecosystems by the urban atmosphere.

Urbanization Alters Soil Microbial Functioning in the Sonoran Desert

Cities can transform ecosystems in multiple ways, through modification of land use and land cover and through exposure to altered physical, chemical, and biological conditions characteristic of urban environments. We compared the multiple impacts of urbanization on microbial carbon (C) and nutrient cycling in ecosystems across Phoenix, Arizona, one of the fastest growing metropolitan areas in the USA. Land-use/land-cover change from desert to managed ecosystems altered soil microbial functioning, primarily through changes in organic matter supply. Although residential xeriscapes often feature native plants and patchy structure like deserts, spatial heterogeneity in soil biogeochemical cycling was not tightly linked to plant canopies. Grassy lawns exhibited higher nitrogen (N) and phosphorus demand by microorganisms than other landscape types, suggesting that high C quality may effectively sequester these nutrients during periods between fertilization events. Soils in native desert remnants exposed to the urban environment had higher organic matter content, but supported lower activities of extracellular peroxidase enzymes compared to outlying deserts. Experimental N enrichment of desert systems decreased peroxidase activities to a similar extent, suggesting that protected desert remnants within the city are receiving elevated N loads that are altering biogeochemical functioning. Although some microbial processes were spatially homogenized in urban desert remnants, resource islands associated with plants remain the dominant organizing factor for most soil properties. The extent to which native desert preserves within the city functionally resemble managed xeriscapes and lawns suggests that these remnant ecosystems are being ‘domesticated’ by exposure to the urban environment.