Soil microbial nitrogen cycling and nitrous oxide emissions from urban afforestation in the New York City Afforestation Project
Introduction
Afforestation, the establishment of stands of trees in previously unforested areas, is active in cities around the world with intentions of reducing storm water runoff, sequestering atmospheric carbon dioxide (CO2), reducing urban ambient temperatures and providing green spaces to improve local quality of life (Kitha and Lyth, 2011, Pataki et al., 2011, Jansson, 2013, Schäffler and Swilling, 2013). As city governments seek to increase “green infrastructure” to attain observed environmental and health benefits for city residents, there is a need to evaluate the effects of creating urban green spaces on ecosystem processes (Guo and Gifford, 2002, Churkina, 2008, Jansson, 2013, Oldfield et al., 2014). Soil processes involved in carbon (C) and nitrogen (N) cycling are critical for tree establishment and growth and influence the impact of the urban forest ecosystem on the atmosphere and hydrology (Cogliastro et al., 2003, Pouyat et al., 2006, Nowak et al., 2013, Oldfield et al., 2014). Understanding the interactions among soil biogeochemical cycles and afforestation processes is critical to meeting the diverse goals of urban afforestation policies.
The natural N cycle is greatly altered by direct anthropogenic inputs of reactive N from the combustion of fossil fuels, application of agricultural fertilizers, and conversion of landscapes from natural to human-dominated spaces (Neff and Hooper, 2002Galloway et al., 2003, Howarth, 2004, Filoso et al., 2006) and these changes may create positive feedbacks to climate change (Hungate et al., 2003, Zaehle et al., 2010). Nitrogen is made bioavailabile in soils through the microbial processes of mineralization and nitrification through which organic N is converted to ammonium (NH4+) and then to nitrate (NO3−) (Crawford and Glass, 1998) with the greenhouse gas (GHG) nitrous oxide (N2O) emitted as a by-product. This process is linked to the removal of N from the soil system by the microbial process of denitrification through which NO3− is reduced to N2O (with some release to the environment) and finally to chemically inert dinitrogen (N2) gas.
Spatial and temporal variability in the production of N2O from soils creates great uncertainty in estimating fluxes from natural and anthropogenic ecosystems (Groffman and Tiedje, 1989, Groffman et al., 2009). The environmental characteristics known to drive N2O production include soil moisture, the presence of oxygen, availability of C substrates and temperature, though the interactions of these and other factors is a topic of current investigation (Morse et al., 2015, Powell et al., 2015). Relevant environmental parameters are often difficult to characterize in human-dominated systems where conditions can be altered in unknown ways (Kaye et al., 2006, Pickett and Cadenasso, 2008, Raciti et al., 2011; Pickett et al., 2013). In suburban and urban landscapes, changes to soil structure, nutrient inputs, species composition, irrigation and impervious surfaces have been shown to alter N cycling rates and N2O fluxes to the atmosphere (Zhu et al., 2004, Kaye et al., 2006, O’Driscoll et al., 2010, Groffman et al., 2014). As research increasingly shows that human decisions in urban and exurban planning can have net negative impacts on the environment, managers and planners have sought to design more ecologically sustainable spaces (Seyfang and Smith, 2007, Kitha and Lyth, 2011, Felson et al., 2013, Baró et al., 2014). There is a strong need to determine whether efforts to mitigate climate change through improved design of urban greenspaces have positive or negative effects on N2O emissions from urban soils.
Studies which have tested the effects of afforestation of soils with different land use histories on rates of net N mineralization and nitrification and soil inorganic N content generally suggest that afforestation decreases these variables, likely due to a changing relationship between soil organic C and N over time since afforestation (Templer et al., 2005, Luo et al., 2006, Gelfand et al., 2012, Deng et al., 2014). Singh et al. (2011) provide evidence that shifts in microbial community structure alter soil N cycling and decrease N2O fluxes as a result of afforestation. There is a body of evidence which suggests that organic C and N inputs to soil from afforestation as well as the microbial community response to afforestation may drive changes in N cycling and alter the production of N2O.
In the US, the The MillionTreesNYC (MTNYC) initiative is a large-scale effort to establish one million new trees, and “healthy, multi-story forests with native trees, shrubs and herbaceous layer” in particular, in New York City (Lu et al., 2014). The MTNYC initiative is a component of New York City's PlaNYC2030 sustainability agenda (www.milliontreesnyc.org). Because of the scale of its planting goals, the initiative is among the most ambitious municipal efforts to increase urban canopy and green space. The New York City Afforestation Project (NY-CAP) is a component of the MTNYC initiative developed as a designed-experiment approach to address the effects of urban environmental stressors and management on the physical attributes of soils and tree performance in urban sites (Felson et al., 2013). The NY-CAP plots were varied in shrub presence/absence, tree species richness and soil amendment with compost. To determine the importance of stand structure and initial inputs of organic matter to N2O emissions from urban afforestation sites, we evaluated the effects of presence/absence of a shrub and herbaceous understory species and compost addition on N2O flux in the NY-CAP. We hypothesized that shrub planting and compost application would lead to greater availability of C, higher microbial biomass and higher rates of microbial respiration that would increase rates of N uptake by microbes (immobilization) and lower rates of N transformations (nitrification and denitrification), thus decreasing N2O emissions.
Section snippets
Methods
This research was conducted in Kissena Corridor Park (KCP) (40.749824̊N, −73.823136̊W) in the Flushing neighborhood of Queens County, New York, USA. Queens, NY experiences a temperate humid climate and receives an average annual rainfall of 113 cm (NOAA, 2012). The area of the study site is approximately 6.6 ha, and land use in the surrounding area is a mixture of high-density residential and commercial buildings. Soils in Kissena Corridor Park are classified as Inwood–Laguardia–Ebbets complex
Results
N2O fluxes were highest in the plots not amended compost (Fig. 1) and were higher in March compared to November and December (ANOVA, df = 41, p = 0.0541) (Fig. 2). Fluxes were non-normally distributed (p < 0.05), and while the ANOVA of the mean N2O flux values for each treatment indicates a marginal difference in the flux response of at least one treatment (ANOVA, df = 41 p = 0.048), it was not possible to discern specific significant differences between treatments. The mean N2O flux values from KCP
Discussion
We expected to find the lowest N2O fluxes from the KCP plots with shrubs and compost. This expectation was based on the idea that inputs of C and N from compost and from leaf litter in the shrub and compost treatments would lead to greater availability of C, higher microbial biomass and higher rates of microbial respiration. These increases, due to greater availability of C, would increase rates of N uptake by microbes (immobilization) and thus lower rates of N transformations that lead to N2O
Conclusions
Of the four treatments tested, we found no significant effects on losses of N2O in this study. The results suggest that the absence of a natural vegetation structure, including canopy, shrub and herbaceous species, and compost at the time of site preparation may be one among other drivers of N2O flux to the atmosphere and the variables measured in this study were not sufficient to discern the main influences of N2O production under urban afforestation conditions. We conclude that efforts to
Acknowledgements
We acknowledge and appreciate the funding support of the New York University Dean's Undergraduate Research Fund and the Millbrook Garden Club. We appreciate permission to conduct research in parks granted by the NYC Parks Department. We thank Lisa Martel, Kate Shepherd, Anita Pierre, Dana Jackson, Amee Gil, Justin Pinderhughes and Samuel Chamberlain for their laboratory and field support. Many thanks also to Dr. Mark Bradford and Dr. Alexander Felson for their editorial input and access to the
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