Diverging patterns at urban-rural forest grandients:biological nitrogen fixation responses to N addition

Biological nitrogen fixation (BNF), a function performed by diazotrophic microbes, plays an essential role in nitrogen (N) bioavailability in terrestrial ecosystems. However, little is known about the effects of meadow degradation on soil BNF and diazotrophic communities in alpine meadows. We investigated changes in soil BNF and their potential drivers in alpine meadows along a degradation gradient on the Tibetan Plateau (non-degraded, lightly degraded, moderately degraded, and severely degraded meadows) using real-time quantitative PCR and amplicon sequencing. Soil BNF rates decreased significantly along the meadow degradation gradient with a range of 17.34–79.84 nmol C2H4 g−1 dry soil d−1 across all sites. The highest BNF rate in the non-degraded meadow was 1.5–4.6-fold higher than that in the degraded meadows. The abundance and diversity of diazotrophs measured by nifH abundance and Shannon diversity was also decreased in the degraded meadows, accompanied by decreases in plant biomass, soil moisture, and nutrient content (C, N). Soil BNF rate was correlated with plant biomass, soil nutrient content, and diazotrophic abundance (including Nostoc, Scytonema, Rhodopseudomonas, and unidentified genera within the Rhizobiales and Proteobacteria). The community composition of diazotrophs differed markedly among sites with different levels of degradation, with both autotrophic (Cyanobacteria) and heterotrophic (Proteobacteria) diazotrophs contributing significantly to BNF. The plant functional groups, particularly the sedge family, were the primary drivers of soil BNF rates via mediating soil moisture, nutrient content (dissolved organic C and N), nifH gene abundance, and diazotrophic community composition. Our results reveal the main drivers of decreased BNF during alpine meadow degradation and emphasize the importance of plant functional groups in shaping the diazotrophic community and regulating the BNF rate. This information can be applied to the restoration of degraded meadow ecosystems.

Urban and peri-urban forests are important habitats for maintaining biodiversity in cities. In this paper, we report a method for using hoverflies as biodiversity indicators in urban forest habitats. As a case study, forest habitats in three peri-urban and urban forests were assessed and compared to rural forests in Slovenia. Rožnik (Ljubljana) was chosen as the urban forest site, Mestni log (Ljubljana) and Brdo (Kranj) were chosen as the peri-urban sites, and eight sites were chosen in rural forests in different ecoregions in Slovenia. Forest hoverfly species richness and the species composition of different biological traits were compared between the peri-urban forests, urban forest and rural forest sites. In addition, species richness was assessed for changes in response to weather conditions between years. The number of species with the investigated traits in the urban and peri-urban forests was within the range of the number of species observed in the rural forests. The number of saproxylic species was higher in the urban forest but lower in the peri-urban forests compared to the rural forests. The proportions of species with different feeding modes and different development times were similar between the peri-urban, urban and rural forests. The proportions of species with development times of less than 2 months or more than 1 year and of predatory species were similar in the urban and peri-urban forests but higher in the rural forests. The species composition of the other biological traits differed between the peri-urban, urban and rural forests. Species richness and abundance displayed large differences in phenological patterns between 2012 and 2013; these differences are related to differences in the minimum temperature for these years. The results are discussed in relation to forest management in urban forests, the usefulness of hoverflies as a biodiversity indicator and possible extrapolation to other species groups.

Comparing the carbon sequestration capacity of temperate deciduous forests between urban and rural landscapes in central Japan

Depression of soil nitrogen fixation by drying soil in a degraded alpine peatland

Urban forested natural areas are valuable ecological and social resources, but long-term sustainability of these habitats is challenged by environmental and social factors associated with urban ecosystems. Advances in city-scale assessments of urban forests have increased the resolution of forest community types and conditions, allowing for improved understanding of ecological function, such as natural regeneration, in these urban habitats. By applying metrics of tree regeneration that are commonly used for the management of rural forests, urban ecologists can test the potential for traditional forest management strategies within our cities. In this study, we compare urban and rural oak-hickory forest composition and structure and the capacity for natural regeneration in the New York metropolitan area. Specifically, we use two metrics of advance regeneration that describe the abundance of seedlings and saplings at different size classes to test whether this management for natural regeneration is a viable option. We found differences in recruitment dynamics between urban and rural forests that have implications for the sustainability of these forests and new management strategies. First, after controlling for forest community type, species composition in urban and rural sites was significantly different across multiple strata and within the seed bank. Species-specific capacity for natural regeneration was different in urban and rural sites, signaling the possibility of divergent successional trajectories. Second, while differences in species composition exist, both urban and rural sites were dominated by native species across all forest strata except for urban seed banks. Third, despite finding significantly lower average annual seedling abundance in urban (1.9 seedlings/m2 ) compared to rural (7.1 seedlings/m2 ) sites, we observed greater density of saplings in urban forests, and no significant difference in stocking index between sites. These findings suggest that early-establishment barriers to recruitment are greater in urban forest sites. However, once established, seedling transition into advance regeneration stages may not be different between site types, and advance regeneration may, in some cases, be more viable in urban forested natural areas. These results highlight functional differences between urban and rural forest recruitment dynamics that may impact on the future community composition of oak-hickory forests in the two landscape settings.

Biological nitrogen fixation (BNF) by canonical molybdenum and complementary vanadium and iron-only nitrogenase isoforms is the primary natural source of newly fixed nitrogen. Understanding controls on global nitrogen cycling requires knowledge of the isoform responsible for environmental BNF. The isotopic acetylene reduction assay (ISARA), which measures carbon stable isotope (13C/12C) fractionation between ethylene and acetylene in acetylene reduction assays, is one of the few methods that can quantify isoform-specific BNF fluxes. Application of classical ISARA has been challenging because environmental BNF activity is often too low to generate sufficient ethylene for isotopic analyses. Here we describe a high sensitivity method to measure ethylene δ13C by in-line coupling of ethylene preconcentration to gas chromatography-combustion-isotope ratio mass spectrometry (EPCon-GC-C-IRMS). Ethylene requirements in samples with 10% v/v acetylene are reduced from > 500 to ~ 20 ppmv (~ 2 ppmv with prior offline acetylene removal). To increase robustness by reducing calibration error, single nitrogenase-isoform Azotobacter vinelandii mutants and environmental sample assays rely on a common acetylene source for ethylene production. Application of the Low BNF activity ISARA (LISARA) method to low nitrogen-fixing activity soils, leaf litter, decayed wood, cryptogams, and termites indicates complementary BNF in most sample types, calling for additional studies of isoform-specific BNF.

Extensive landscape modification by humans has led to the fragmentation of riparian forests across North America. We compared the vegetation of extant riparian forest along an urban-rural disturbance gradient. In 1999, twenty-five sites along Assiniboine River in Manitoba, Canada were categorized according to land use: urban, suburban, high intensity rural, low intensity rural, and relatively high quality reference forest. Differences in herbaceous, shrub, and tree species composition and diversity were related to the proportion of surrounding land use, forest patch size, connectivity, and area:perimeter ratio. Urban riparian forests were more disturbed and isolated. They were smaller and characterized by drier, more alkaline soils. Moreover, they had significantly lower native and overall understorey species diversity, and had a higher proportion of exotics including Solanum dulcamara and Hesperis matronalis. Suburban forests were less disturbed, faced greater development pressure, and had sandier soils. Although suburban understorey diversity was similar to that of rural forests, suburban sites had a higher proportion of exotic species, especially escaped horticultural and invasive species including Caragana arborescens and Rhamnus cathartica. Reference sites were relatively large and exhibited greater connectivity, but there was little difference in species composition and diversity among high intensity rural, low intensity rural, and reference sites. These site types were less disturbed than either urban or suburban forests, and reference sites were characterized by hydrophilic species including Scirpus fluviatilis and Carex aquatilis. Our results suggest that landscape measures of disturbance, and related changes in environment, may be confidently used to assess impacts of land use on vegetation along urban-rural gradients.

Landscape fragmentation has created large areas of forest edge. Understanding how soil organic carbon (SOC) stocks within forest edges respond to fragmentation is essential to assess carbon budgets; however, the causes and magnitude of edge effects on SOC stocks have been poorly characterized. Here, we quantify the edge effects on SOC stocks within and across urban to rural forest patches from three fragmented urban forests to a large patch of rural forest. The SOC stocks within 20 m of the rural forest edge (1.86kgCm−2) is on average 80% lower than the interiors of rural forest (10.47kgCm−2). We found that biotic factors, including annual litterfall mass (R2 > 0.94), peak leaf area index (R2 > 0.92), and fine-root mass density (R2 > 0.77), explained the spatial variation in SOC stocks within the rural forest. In urban forests, human activities at forest edges led to contrasting edge effects on SOC stocks, for instance, the SOC stocks at the east edges (4.74 kgC m−2) were 56% greater than at the west edges (3.03 kgC m−2) explained by the adjacent land uses (e.g., paved roads vs. non-paved soils) and in-situ litterfall management. We also found significant differences in summer soil temperature (ΔTS > 2.8 °C) and soil moisture (ΔVWC > 0.05 m3 m−3) between the east and west forest edges. Our results reveal that the factors responsible for the edge effects on SOC stocks in rural forests are biotic factors, while heterogeneous human activities at the local scale lead to complex edge effects on urban forest SOC stocks.

The largest natural biological sink for the radiatively active trace gas methane (CH4) is bacteria in soils that consume CH4 as an energy and carbon source. This sink has been shown to be sensitive to nitrogen (N) inputs and alterations of soil physical conditions. Given this sensitivity, conversion of native ecosystems to urban, suburban, and exurban managed lawns thus has potential to affect regional CH4 budgets. We measured CH4 fluxes monthly from four urban forest, four rural forest and four urban lawn plots in the Baltimore, MD, metropolitan area from 2001 to 2005. Our objectives were to evaluate the effects of urban atmospheric and land use change on CH4 uptake and the importance of these changes relative to other greenhouse forcings in the urban landscape. Rural forests had a high capacity for CH4 uptake (1.68 mg m(-2) day(-1)). This capacity was reduced in urban forests (0.23 mg m(-2) day(-1)) and almost completely eliminated in lawns. Possible mechanisms for these reductions include increases in atmospheric N deposition and CO2 levels, fertilization of lawns, and alteration of soil physical conditions that influence diffusion. Although conversion of native forests to lawns had dramatic effects on CH4 uptake, these effects do not appear to be significant to statewide greenhouse gas forcing.

The landscape patterns of urban forests not only reflect the influence of urbanization on urban forests, but also determines its function in urban ecosystem services. In the case of mastering the overall forest landscape pattern of a city, a study of the structure of urban forest landscapes at different scales and in urbanized regions is beneficial to a comprehensive understanding of the forest characteristics of a city. In the present study, an attempt was made to map and monitor the spatio-temporal dynamics of an urban forest in Shanghai from 2004 to 2014 using remote sensing techniques. Methods of landscape ecology analysis are followed to quantify the spatiotemporal patterns of an urban forest landscape by urban and rural gradient regionalization. The results show that the spatial structure of an urban forest landscape is essentially consistent with an urban landscape pattern. Due to strong interference from human activities, the ecological quality of forest landscapes is low. At the landscape level, the urban forest coverage rate increased from 11.43% in 2004 to 16.02% in 2014, however, the number of large patches decreased, there was a high degree of urban forest landscape fragmentation, landscape connectivity was poor, landscape patch boundaries were uniform, and weak links were present between ecological processes. Different urban and rural gradient division methods exhibit obvious gradient characteristics along the urban–rural gradient in Shanghai. The regional differences in the urban forest landscape ecological characteristics have further increased as a result of urban planning and zoning. The total amount of urban forest is located closer to the urban center, which has the smallest total amount of forest; however, in terms of urban forest coverage, the suburbs have more coverage than do the outer suburbs and the central urban areas. The urban forest landscape’s spatial distribution area is evidently different. Urbanization affects the areas closest to urban residential areas, which are markedly disturbed by humans, and the urban forest landscape has a high degree of fragmentation. The forest patches have become divided and unconnected, and the degree of natural connectivity has gradually decreased over the past 10 years. At the landscape class level, broadleaf forests are dominant in Shanghai, and their area exhibits an increasing trend; shrublands and needleleaf forests, however, show a decreasing trend. Compared with other forest types, the spatial distribution of broadleaf forest is concentrated in the suburbs, and the aggregation effect is relatively apparent. From the perspective of urban forest landscape pattern aggregation characteristics in Shanghai, the spatial distribution of urban forest landscape point patterns in the study area exhibit extremely uneven characteristics. The point density of urban forest patches larger than 1 ha in Shanghai increased from 2004 to 2014. However, the total number of patches with areas larger than 5 ha decreased, and this decrease plays an important role in the ecological environment. In the past 10 years, the concentration characteristics of urban forests with large patches has gradually decreased. In 2014, the urban forest landscapes decreased by 5 km compared to the intensity of aggregates in 2004, which also indicates that urban forests in Shanghai tend to be fragmented. The results of this study can be useful to help improve urban residents’ living environments and the sustainable development of the urban ecosystem, and they will also be vital to future management.

Key factors driving soil diazotrophic community assembly and nitrogenase activity in Qinghai-Tibet alpine meadows

Current and future biomass carbon uptake in Boston's urban forest

Urban soils are a crucial component of urban ecosystems, especially in public green spaces, because of the ecosystem services they provide (e.g. public recreation, urban cooling or water infiltration). In this study, we describe the chemical, physical and hydrostructural characteristics of 180 forest and lawn surface soil samples, taken along an urban–rural gradient in the Paris region. This was done in order to identify how these soils have been affected by urbanization. Forests and lawns are the main vegetation types found in this region and represent 21% and 22.2% of the territory's surface area, respectively. Many of the properties of urban forest soils differed from those of other sites (e.g. texture, organic carbon content, total nitrogen and carbonate contents), possibly because the urban forests are much older than the lawns and because of the legacy of the historical management of soils in this region (Haussmann period). Urban lawn soils were more compacted than urban forests, probably due to higher foot traffic. The effects of urbanization were, at times, confounded with other factors (e.g. sandier texture of urban forests), which suggests that surface soil characteristics were influenced by past urban planning. Finally, this study constitutes a baseline analysis for the monitoring of soil quality in the region.

Tundra ecosystems are typically carbon (C) rich but nitrogen (N) limited. Since biological N2 fixation is the major source of biologically available N, the soil N2-fixing (i.e., diazotrophic) community serves as an essential N supplier to the tundra ecosystem. Recent climate warming has induced deeper permafrost thaw and adversely affected C sequestration, which is modulated by N availability. Therefore, it is crucial to examine the responses of diazotrophic communities to warming across the depths of tundra soils. Herein, we carried out one of the deepest sequencing efforts of nitrogenase gene (nifH) to investigate how 5 years of experimental winter warming affects Alaskan soil diazotrophic community composition and abundance spanning both the organic and mineral layers. Although soil depth had a stronger influence on diazotrophic community composition than warming, warming significantly (P < 0.05) enhanced diazotrophic abundance by 86.3% and aboveground plant biomass by 25.2%. Diazotrophic composition in the middle and lower organic layers, detected by nifH sequencing and a microarray-based tool (GeoChip), was markedly altered, with an increase of α-diversity. Changes in diazotrophic abundance and composition significantly correlated with soil moisture, soil thaw duration, and plant biomass, as shown by structural equation modeling analyses. Therefore, more abundant diazotrophic communities induced by warming may potentially serve as an important mechanism for supplementing biologically available N in this tundra ecosystem.IMPORTANCE With the likelihood that changes in global climate will adversely affect the soil C reservoir in the northern circumpolar permafrost zone, an understanding of the potential role of diazotrophic communities in enhancing biological N2 fixation, which constrains both plant production and microbial decomposition in tundra soils, is important in elucidating the responses of soil microbial communities to global climate change. A recent study showed that the composition of the diazotrophic community in a tundra soil exhibited no change under a short-term (1.5-year) winter warming experiment. However, it remains crucial to examine whether the lack of diazotrophic community responses to warming is persistent over a longer time period as a possibly important mechanism in stabilizing tundra soil C. Through a detailed characterization of the effects of winter warming on diazotrophic communities, we showed that a long-term (5-year) winter warming substantially enhanced diazotrophic abundance and altered community composition, though soil depth had a stronger influence on diazotrophic community composition than warming. These changes were best explained by changes in soil moisture, soil thaw duration, and plant biomass. These results provide crucial insights into the potential factors that may impact future C and N availability in tundra regions.

Effects of applying maize stover on soil diazotroph community diminish with depth