Christopher Neill - Academia.edu (original) (raw)

Papers by Christopher Neill

Ecosphere, Jun 1, 2018

Stormwater ponds and retention basins are ubiquitous features throughout urban landscapes. These ... more Stormwater ponds and retention basins are ubiquitous features throughout urban landscapes. These ponds are potentially important control points for nitrogen (N) removal from surface water bodies via denitrification. However, there are possible trade-offs to this water quality benefit if high N and contaminant concentrations in stormwater pond sediments decrease the complete reduction of nitrous oxide (N 2 O), a potent greenhouse gas, to dinitrogen (N 2) during denitrification. This may occur through decreasing the abundance or efficiency of denitrifiers capable of producing the N 2 O reductase enzyme. We predicted that ponds draining increasingly urbanized landscapes would have higher N and metal concentrations in their sediments, and thereby greater N 2 O yields. We measured potential denitrification rates, N 2 O reductase (nosZ) gene frequencies, as well as sediment and porewater chemistry in 64 ponds distributed across eight U.S. cities. We found almost no correlation between the proportion of urban land cover surrounding ponds and the nutrient and contaminant concentrations in the stormwater pond sediments within or across all cities. Regression analysis revealed that the proportion of potential N 2 and N 2 O production that could be explained was under different environmental controls. Our survey raises many new questions about why N fluxes and transformations vary so widely both within and across urban environments, but also allays the concern that elevated metal concentrations in urban stormwater ponds will increase N 2 O emissions. Urban stormwater ponds are unlikely to be a problematic source of N 2 O to the atmosphere, no matter their denitrification potential.

Retirement of cultivated croplands creates potential for ecosystem and wetland restoration, but v... more Retirement of cultivated croplands creates potential for ecosystem and wetland restoration, but vegetation and soil legacies of cropping influence the development of post-agriculture vegetation. In low-lying coastal watersheds of southeastern Massachusetts, cranberries (Vaccinium macrocarpon Aiton) are cultivated in commercial "bogs" largely on diked, leveled and sanded bog beds created from historic wetlands. Current low cranberry prices and expanding cranberry production elsewhere now increase the likelihood of cranberry farmland retirement. We quantified the trajectories of plant species richness and cover, and plant characteristics (life form, native or non-native, wetland or non-wetland) in a chronosequence of cranberry bog beds that spanned from cultivated bog beds to those retired from cropping and revegetated for 90 years with no post-cropping management. Species richness increased from active bog beds to 10-20 year-old bog beds and subsequently decreased. Post-ret...

The Buzzards Bay Coalition’s Baywatchers Monitoring Program collected summertime water quality in... more The Buzzards Bay Coalition’s Baywatchers Monitoring Program collected summertime water quality information at more than 150 stations around Buzzards Bay, Massachusetts from 1992 to 2018. Baywatchers monitoring data document nutrient-related water quality and the effects of nitrogen pollution. The large majority of stations are located in sub-estuaries of the main Bay, although stations in central Buzzards Bay and Vineyard Sound were added beginning in 2007. Measurements include temperature, salinity, Secchi depth and concentrations of dissolved oxygen, ammonium, nitrate + nitrite, total dissolved nitrogen, particulate organic nitrogen, particulate organic carbon, ortho-phosphate, chlorophyll a, pheophytin a, and in lower salinity waters, total phosphorus and dissolved organic carbon. The Baywatchers dataset provides a long-term record of the water quality of Buzzards Bay and its sub-estuaries. The data have been used to identify impaired waters, evaluate discharge permits, support the development of nitrogen total maximum daily loads, develop strategies for reducing nitrogen inputs, and increase public awareness and generate support for management actions to control nutrient pollution and improve water quality. The Readme, STN_EQUIV, Stations and Methods, S_D for WFH, and Acknowledge tabs were updated and corrections were made for Time and POC on 2018 Upper Bay samples and deleted CH1 from sonde method column where no CHL data existed, moved words like "sample lost" from Chl and Pheo data columns to comments column

Journal of Geophysical Research: Biogeosciences, 2021

In seasonally dry ecosystems, which are common in sub‐Saharan Africa, precipitation after dry per... more In seasonally dry ecosystems, which are common in sub‐Saharan Africa, precipitation after dry periods can cause large pulses of nitrous oxide (N2O), a greenhouse gas, and of nitric oxide (NO), a precursor to tropospheric ozone pollution. Agricultural practices can change soil characteristics, affecting trace N gas emissions. To evaluate the effects of land use on trace gas pulses at the start of the rainy season, we conducted laboratory measurements of N2O and NO fluxes from soils collected from four pairs of agricultural and natural savannah sites across the Sudano‐Sahelian zone. We also conducted in situ wetting experiments, measuring NO fluxes from fallow sandy soils in Tanzania and NO and N2O fluxes from clayey soils in Kenya with different histories of fertilizer use. In incubation studies, NO increased by a factor of 7 to 25 following wetting, and N2O fluxes shifted from negative to positive; cumulative NO fluxes were an order of magnitude larger than cumulative N2O fluxes. In...

Journal of Geophysical Research: Biogeosciences, 2019

Agricultural impacts on aquatic ecosystems are well studied; however, most research has focused o... more Agricultural impacts on aquatic ecosystems are well studied; however, most research has focused on temperate regions, whereas the forefront of agricultural expansion is currently in the tropics. At the vanguard of this growth is the boundary between the Amazon and Cerrado biomes in Brazil, driven primarily by expansion of soybean and corn croplands. Here we examine the impacts of cropland expansion on receiving lowland Amazon Basin headwater streams in terms of dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) composition via ultrahigh-resolution mass spectrometry. Streams draining croplands had lower DOC concentrations and DOM molecular signatures enriched in N-and S-containing formula in comparison to forested streams. Cropland streams were also enriched in aliphatic, peptide-like, and highly unsaturated and phenolic (low O/C) compound categories in comparison to forest streams (enriched in polyphenolics, condensed aromatics, and highly unsaturated and phenolic [high O/C] compound categories) indicative of the shifting of sources from organic-rich surface soils and litter layers to autochthonous and more microbial biomass. Distinct molecular assemblages were strongly correlated with cropland and forest catchments, highlighting headwater streams as sentinels for detecting change. On investigation of unique molecular formulae present in only cropland sites, four cropland markers provided the ability to track agricultural impacts in the region. Overall, these patterns indicate reduced organic matter inputs in croplands and greater microbial degradation at these sites leading to declining DOC concentrations, and DOM of more microbial character in receiving streams that is more biolabile, with clear ramifications for downstream ecology and biogeochemical cycles.

Journal of Geophysical Research: Biogeosciences, 2018

Climate and land use models predict that tropical deforestation and conversion to cropland will p... more Climate and land use models predict that tropical deforestation and conversion to cropland will produce a large flux of soil carbon (C) to the atmosphere from accelerated decomposition of soil organic matter (SOM). However, the C flux from the deep tropical soils on which most intensive crop agriculture is now expanding remains poorly constrained. To quantify the effect of intensive agriculture on tropical soil C, we compared C stocks, radiocarbon, and stable C isotopes to 2 m depth from forests and soybean cropland created from former pasture in Mato Grosso, Brazil. We hypothesized that soil disturbance, higher soil temperatures (+2°C), and lower OM inputs from soybeans would increase soil C turnover and deplete C stocks relative to nearby forest soils. However, we found reduced C concentrations and stocks only in surface soils (0-10 cm) of soybean cropland compared with forests, and these differences could be explained by soil mixing during plowing. The amount and Δ 14 C of respired CO 2 to 50 cm depth were significantly lower from soybean soils, yet CO 2 production at 2 m deep was low in both forest and soybean soils. Mean surface soil δ 13 C decreased by 0.5‰ between 2009 and 2013 in soybean cropland, suggesting low OM inputs from soybeans. Together these findings suggest the following: (1) soil C is relatively resistant to changes in land use and (2) conversion to cropland caused a small, measurable reduction in the fast-cycling C pool through reduced OM inputs, mobilization of older C from soil mixing, and/or destabilization of SOM in surface soils.

Ecosphere, 2018

Urban ecosystems are widely hypothesized to be more ecologically homogeneous than natural ecosyst... more Urban ecosystems are widely hypothesized to be more ecologically homogeneous than natural ecosystems. We argue that urban plant communities assemble from a complex mix of horticultural and regional species pools, and evaluate the homogenization hypothesis by comparing cultivated and spontaneously occurring urban vegetation to natural area vegetation across seven major U.S. cities. There was limited support for homogenization of urban diversity, as the cultivated and spontaneous yard flora had greater numbers of species than natural areas, and cultivated phylogenetic diversity was also greater. However, urban yards showed evidence of homogenization of composition and structure. Yards were compositionally more similar across regions than were natural areas, and tree density was less variable in yards than in comparable natural areas. This homogenization of biodiversity likely reflects similar horticultural source pools, homeowner preferences, and management practices across U.S. cities.

Tropical Conservation Science, 2017

The role of tropical forests in climate is most often expressed in terms of the carbon they keep ... more The role of tropical forests in climate is most often expressed in terms of the carbon they keep out of the atmosphere if deforestation is avoided or the carbon they remove from the atmosphere as they grow. The direct role of forests, particularly in the tropics, in maintaining low surface temperatures and relatively high precipitation has been underappreciated. Recent studies in the Brazilian agricultural frontier indicate that tropical deforestation, for pasture and crop production, has led to significant regional climate change in the last 40 years of a scale much larger than that attributed to the carbon released from deforestation. Deforestation reduces net surface radiation and evapotranspiration, thus increasing sensible heat flux and land surface temperature. In Mato Grosso state, the temperature of the forested Xingu Indigenous Park is 3 C cooler than the surrounding mosaic of pasturelands, croplands, and remaining forest fragments. In the neighboring state of Rondô nia, rainfall has significantly decreased and the dry season lengthened as deforestation occurred. Numerical model studies strongly suggest that Brazil's agricultural frontier will be much warmer and dryer in coming decades as greenhouse gas concentrations increase. Thus, in Brazil, it is becoming clear that, because of their capacity to moderate regional climate, preserving tropical forests will be a key component of mitigating exogenously driven future climate change.

Urban ecosystems are widely hypothesised to be more ecologically homogeneous than natural ecosyst... more Urban ecosystems are widely hypothesised to be more ecologically homogeneous than natural ecosystems. We argue that urban plant communities assemble from a complex mix of horticultural and regional species pools and thus evaluate the homogenisation hypothesis by comparing cultivated and spontaneously occurring urban vegetation to natural area vegetation across seven major US cities. Urban yards were homogenised across cities in terms of their diversity, composition, and structure. First, cultivated and spontaneous yard flora had higher numbers of species than did natural areas but similar phylogenetic diversity, indicating that yard species were drawn from a relatively small number of lineages. Second, yards were compositionally more similar across regions than were natural areas. Finally, vegetation structure, specifically cultivated tree density, was less variable in yards than natural areas across cities. Biodiversity homogenisation likely reflects similar horticultural source po...

Ecological Applications, 2016

Fertilizer applications are poised to increase across sub-Saharan Africa (SSA), but the fate of a... more Fertilizer applications are poised to increase across sub-Saharan Africa (SSA), but the fate of added nitrogen (N) is largely unknown. We measured vertical distributions and temporal variations of soil inorganic N following fertilizer application in two maize (Zea mays L.)-growing regions of contrasting soil type. Fertilizer trials were established on a clayey soil in Yala, Kenya, and on a sandy soil in Tumbi, Tanzania, with application rates of 0-200 kg N/ha/yr. Soil profiles were collected (0-400 cm) annually (for three years in Yala and two years in Tumbi) to examine changes in inorganic N pools. Topsoils (0-15 cm) were collected every 3-6 weeks to determine how precipitation and fertilizer management influenced plant-available soil N. Fertilizer management altered soil inorganic N, and there were large differences between sites that were consistent with differences in soil texture. Initial soil N pools were larger in Yala than Tumbi (240 vs. 79 kg/ha). Inorganic N pools did not change in Yala (277 kg/ha), but increased fourfold after cultivation and fertilization in Tumbi (371 kg/ha). Intra-annual variability in NO − 3-N concentrations (3-33 μg/g) in Tumbi topsoils strongly suggested that the sandier soils were prone to high leaching losses. Information on soil inorganic N pools and movement through soil profiles can h vulnerability of SSA croplands to N losses and determine best fertilizer management practices as N application rates increase. A better understanding of the vertical and temporal patterns of soil N pools improves our ability to predict the potential environmental effects of a dramatic increase in fertilizer application rates that will accompany the intensification of African croplands.

Ecological Applications, 2015

Deforestation and fragmentation influence the microclimate, vegetation structure, and composition... more Deforestation and fragmentation influence the microclimate, vegetation structure, and composition of remaining patches of tropical forest. In the southern Amazon, at the frontier of cropland expansion, forests are converted and fragmented in a pattern that leaves standing riparian forests whose dimensions are mandated by the Brazilian National Forest Code. These altered riparian forests share many characteristics of well‐studied upland forest fragments, but differ because they remain connected to larger areas of forest downstream, and because they may experience wetter soil conditions because reduction of forest cover in the surrounding watershed raises groundwater levels and increases stream runoff. We compared forest regeneration, structure, composition, and diversity in four areas of intact riparian forest and four areas each of narrow, medium, and wide altered riparian forests that have been surrounded by agriculture since the early 1980s. We found that seedling abundance was re...

Biogeosciences Discussions, 2015

Degradation of coastal ecosystems by eutrophication is largely defined by nitrogen loading from l... more Degradation of coastal ecosystems by eutrophication is largely defined by nitrogen loading from land via surface and groundwater flows. However, indicators of water quality are highly variable due to a myriad of other drivers, including temperature and precipitation. To evaluate these drivers, we examined spatial and temporal trends in a 22 year record of summer water quality data from 122 stations in 17 embayments within Buzzards Bay, MA (USA), collected through a citizen science monitoring program managed by Buzzards Bay Coalition. To identify spatial patterns across Buzzards Bay's embayments, we used a principle component and factor analysis and found that rotated factor loadings indicated little correlation between inorganic nutrients and organic matter and chlorophyll a (Chl a) concentration. Factor scores showed that embayment geomorphology in addition to nutrient loading was a strong driver of water quality, where embayments with surface water inputs showed larger biological impacts than embayments dominated by groundwater influx. A linear regression analysis of annual summertime water quality indicators over time revealed that from 1992 to 2013, most embayments (15 of 17) exhibited an increase in temperature (mean rate of 0.082 ± 0.025 (SD) • C yr −1) and Chl a (mean rate of 0.0171 ± 0.0088 log 10 (Chl a; mg m −3) yr −1 , equivalent to a 4.0 % increase per year). However, only 7 embayments exhibited an increase in total nitrogen (TN) concentration (mean rate 0.32 ± 0.47 (SD) µM yr −1). Average summertime log 10 (TN) and log 10 (Chl a) were correlated with an indication that yield of Chl a per unit total nitrogen increased with time suggesting the estuarine response to TN may have changed because of other stressors such as warming, altered precipitation patterns, or changing light levels. These findings affirm that nitrogen loading and physical aspects of embayments are essential in explaining observed ecosystem response. However, climate-related stressors may also need to be considered by managers because increased temperature and precipitation may worsen water quality and partially offset benefits achieved by reducing nitrogen loading.

Proceedings of the National Academy of Sciences, 2014

Significance This paper offers conceptual and empirical contributions to sustainability science i... more 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.

Philosophical Transactions of the Royal Society B: Biological Sciences, 2013

Large-scale cattle and crop production are the primary drivers of deforestation in the Amazon tod... more Large-scale cattle and crop production are the primary drivers of deforestation in the Amazon today. Such land-use changes can degrade stream ecosystems by reducing connectivity, changing light and nutrient inputs, and altering the quantity and quality of streamwater. This study integrates field data from 12 catchments with satellite-derived information for the 176 000 km 2 upper Xingu watershed (Mato Grosso, Brazil). We quantify recent land-use transitions and evaluate the influence of land management on streamwater temperature, an important determinant of habitat quality in small streams. By 2010, over 40 per cent of catchments outside protected areas were dominated (greater than 60% of area) by agriculture, with an estimated 10 000 impoundments in the upper Xingu. Streams in pasture and soya bean watersheds were significantly warmer than those in forested watersheds, with average daily maxima over 4°C higher in pasture and 3°C higher in soya bean. The upstream density of impoundm...

Forest Ecology and Management, 2007

Fire is an important component of the historic disturbance regime of oak and pine forests that oc... more Fire is an important component of the historic disturbance regime of oak and pine forests that occupy sandy soils of the coastal outwash plain of the northeastern U.S. Today prescribed fire is used for fuel reduction and for restoration and maintenance of habitat for rare plant, animal and insect species. We evaluated the effects of the frequency and seasonality of prescribed burning on the soils of a Cape Cod, Massachusetts coastal oak-pine forest. We compared soil bulk density, pH and acidity, total extractable cations and total soil C and N in unburned plots and in plots burned over a 12-year period, along a gradient of frequency (every 1to-4 years), in either spring (March/April) or summer (July/August). Summer burning decreased soil organic horizon thickness more than spring burning, but only summer burning every 1 to 2 years reduced organic horizons compared with controls. Burning increased soil bulk density of the organic horizon only in the annual summer burns and did not affect bulk density of mineral soil. Burn frequency had no effect on pH in organic soil, but burning every year in summer increased pH of organic soil from 4.01 to 4.95 and of mineral soil from 4.20 to 4.79. Burning had no significant effect on organic or mineral soil percent C, percent N, C:N, soil exchangeable Ca 2+ , Mg 2+ , K + or total soil C or N. Overall effects of burning on soil chemistry were minor. Our results suggest that annual summer burns may be required to reduce soil organic matter thickness to produce conditions that would regularly allow seed germination for oak and for grassland species that are conservation targets. Managers may have to look to other measures, such as combinations of fire with mechanical treatments (e.g., soil scarification) to further promote grasses and forbs in forests where establishment of these plants is a high priority.

BioScience, 2013

Fertilizer-intensive agriculture has been integral to increasing food production over the past ha... more Fertilizer-intensive agriculture has been integral to increasing food production over the past half century but has been accompanied by environmental costs. We use case studies of phosphorus fertilizer use in the world's most productive soybean-growing regions, Iowa (United States), Mato Grosso (Brazil), and Buenos Aires (Argentina), to examine influences of management and soil type on agriculture's most prevalent phosphorusrelated environmental consequences: eutrophication and consumption of Earth's finite phosphorus reserves. With increasing phosphorus inputs, achieving high yields on tropical soils with high phosphorus-binding capacity is becoming more common. This system has low eutrophication risks but increases demands on phosphorus supplies. In contrast, production in traditional breadbaskets, on soils with lower phosphorus-binding capacities, is being sustained with decreasing phosphorus inputs. However, in these regions, historical overuse of phosphorus may mean continued eutrophication risk even as pressures on phosphorus reserves diminish. We focus here on soybean production but illustrate how achieving sustainable agriculture involves an intricate optimization of local, regional, and global considerations.

BioScience, 2012

Economic and political realities present challenges for implementing an aggressive climate change... more Economic and political realities present challenges for implementing an aggressive climate change abatement program in the United States. A high-efficiency approach will be essential. In this synthesis, we compare carbon budgets and evaluate the carbon-mitigation potential for nine counties in the northeastern United States that represent a range of biophysical, demographic, and socioeconomic conditions. Most counties are net sources of carbon dioxide (CO 2) to the atmosphere, with the exception of rural forested counties, in which sequestration in vegetation and soils exceed emissions. Protecting forests will ensure that the region's largest CO 2 sink does not become a source of emissions. For rural counties, afforestation, sustainable fuelwood harvest for bioenergy, and utility-scale wind power could provide the largest and most cost-effective mitigation opportunities among those evaluated. For urban and suburban counties, energy-efficiency measures and energy-saving technologies would be most cost effective. Through the implementation of locally tailored management and technology options, large reductions in CO 2 emissions could be achieved at relatively low costs.

Biogeochemistry, 2009

Human activities have profoundly altered the global nitrogen (N) cycle. Increases in anthropogeni... more Human activities have profoundly altered the global nitrogen (N) cycle. Increases in anthropogenic N have had multiple effects on the atmosphere, on terrestrial, freshwater and marine ecosystems, and even on human health. Unfortunately, methodological

Outlook on Agriculture, 2005

Land-use change for human settlement and agricultural purposes, especially pasture establishment,... more Land-use change for human settlement and agricultural purposes, especially pasture establishment, has caused major impacts on the Amazon Basin's environment. Development of strategies for reformation and restoration of already degraded pastures constitutes the main goal of the authors' research work. For some of this work, a homogeneous area of land in terms of soil characteristics was selected at Nova Vida ranch in Rondônia state to conduct a multidisciplinary experiment, which included agronomic, environmental and economic analyses. Since July 2001, the authors have monitored five treatments: control, herbicide, tillage, no-till rice and no-till soybean, arranged in four blocks. Early results on carbon and nitrogen stocks, nitrogen mineralization, trace-gas fluxes, dissolved organic carbon and microbial biomass are briefly presented.

Global Change Biology, 2020

The world's growing population will require increased grain production. Increases in global grain... more The world's growing population will require increased grain production. Increases in global grain demand from 60% to 110%