Human alteration of the carbon (C) and nitrogen (N) cycles is having profound detrimental impacts on natural systems. This dissertation research focused on investigating soil feedbacks to multiple anthropogenic drivers to explore unifying mechanisms applicable to global biogeochemical modeling. In Chapter 1 and 2, I investigate how changes in C, N, and temperature regulate soil CO2 production (Rsoil) through changes to the Michaelis-Menten parameters (i.e. Vmax and kM) using soils from three contrasting ecosystems in southern California in Chapter 1 and in subtropical evergreen forests of southern China in Chapter 2. Overall, the response of Rsoil to N addition was generally dependent on C:N stoichiometry, consistent with predictions from the dynamic microbial carbon-use efficiency hypothesis. Furthermore, I show the first empirical results from whole soil measurements demonstrating temperature sensitivity of both Vmax and kM, showing strong support for substrate regulation of Rsoil temperature sensitivity across diverse soils. Results from Chapters 1 and 2 demonstrate great potential for Michaelis-Menten kinetics in describing Rsoil responses to N and temperature, with implications for understanding microbial physiology and broad applicability to global biogeochemical modeling. Chapter 3 extends this research by exploring N trace gas emissions and evaluating variation in the microbial community composition along an N deposition gradient in the Colorado Desert. Results from Chapter 3 present soil N fluxes that were considerably higher than expected, demonstrating a need for greater appreciation of arid systems in global N budgets. While short term effects of experimental N addition resulted in inconsistent responses in the microbial community composition, long-term N deposition resulted in a distinct differentiation of the microbial community, with possible implications for N cycling. Microbial communities associated with nitrification, identified from 16S rRNA sequencing and qPCR of amoA, demonstrate a shift from archaeal dominance at the low deposition site to more bacterial dominance at the high deposition site. Overall, results from this dissertation contribute to mechanistic understanding of soil feedbacks to climate change and nitrogen deposition, which is necessary for predicting future changes and developing strategies for mitigation of anthropogenic global change drivers.

Urban systems, or socio-ecological systems, are complex on fine and coarse scales, and are important to humans through the effects of ecological processes. Nitrogen pollution is uniquely altered by humans and variable on fine spatial scales with understudied potential sources and sinks. We found local atmospheric nitrogen pollution concertation in Riverside correlated with traffic as a pollution source, but plant canopy did not act as a significant source. Atmospheric pollution does not remain within city limits, and effects in natural systems can be observed in the air, canopy, and soil. We observed a deposition gradient, with decreased atmospheric nitrogen further from urbanization. However the pattern was not consistent for nitrogen in the canopy and the soil, highlighting a disconnect in the gradient and the complexity of teleconnections. Urban ecology affects humans in socioecological systems and plays an important role in science policy. The Salton Sea is a hyper-resilient ecological crisis that fits within the framework of wicked problems. Through research on existing government policy and interviews with key stakeholders, we identified issues that potentially keep the sea in a degraded state and recommendation via resilience theory to address the wicked problem. The dissertation as a whole seeks to improve understanding of socio-ecological systems across scales, and quantify variation in ecosystem processes and nitrogen patterns within and beyond urban areas.

Linking variation in ecosystem functioning to landscape drivers has become an important research need for understanding ecosystem responses to global change. Due to extensive land use and land cover changes many regions have variable distributions of landscape drivers and ecosystem processes. Furthermore, changes in local- to regional-scale climate may impact ecosystem variation and sensitivity to physiological drivers. This dissertation investigates how contrasting scale-dependent drivers of soil temperature, moisture and substrate levels influence soil respiration (Rs), a key ecosystem process, using in-situ landscape surveys and experimental subsidies of water and labile carbon. Furthermore, to improve understanding of scale-dependent sources of variation of urban microclimate this dissertation investigates how land cover and vegetation influences local distributions of air temperature (Ta), land surface temperature (LST), and relative humidity (RH), using intensive and widely distributed networks of microclimate sensors. Finally, vital in estimates of urban warming, this dissertation examines the relationships between Ta and LST among common urban land covers.

I found Rs in intensively managed urban land uses has increased rates, decreased spatial variation, and decreased sensitivity to environmental conditions. Furthermore, among common urban land uses spatial variation in Rs was positively correlated with soil temperature, and negatively correlated with soil moisture and substrate. Landscape position, or land use and climate distributions, influenced Rs by altering both levels and Rs sensitivity of physiological drivers. Next, in my first microclimate study I found negative Ta and positive RH correlations with vegetation intensity. Vegetation cooling effects were greater in more arid climates and in the evening hours. Furthermore, increasing city-scale mean Ta was associated with higher spatial variation of Ta in coastal cities, and lower variation in more arid cities. In my final study I observed vertical height-dependent Ta-LST relationships associated with land cover composition. Furthermore, I observed decreased nighttime Ta-LST differences among land covers. These findings can help city planners identify potential heat risk reductions strategies associated with urban vegetation and land cover composition. Together, these systematic evaluations of landscape effects on Rs and microclimate provide a framework of understanding the effects of interactive global change drivers on urban ecosystem processes.

Climate models are predicting that the world will become warmer and understanding how individual tree species cope with increased temperatures will be important for making predictions of species survival. City trees are valuable resources as they include trees from a wide variety of habitats under regular watering regimes with only temperatures changing. A tree’s ability to acclimate to changing temperatures may be controlled by plasticity in leaf shape. We hypothesized trees species will acclimate to increased temperatures by increasing the dissection of their leaves which would be more prominent in species with wider natural distributions. In this study we focused on 7 species growing across three sites distributed along a coastal to inland temperature gradient in southern California, USA. Selected trees had the requirement of being found in vegetated areas with regular watering in at least two of the three sites. A minimum of three specimens from each species were selected at each site. From each specimen, three recently mature, sun-exposed leaves were collected from each tree, scanned and the Dissection Index calculated. Initial measurements indicate that of the species which have been measured two, Platanus racemosa (p = 0.0265) and Schinus terebinthifolius (p < 0.001) show increasing levels of leaf dissection across the temperature gradient while one, Jacaranda mimosifolia, no difference was detected (p > 0.05) in part because of a high degree of variability across the gradient. The species with the highest increases in dissection, P. racemosa, originated in limited riparian environments while the species with lower dissection levels, S. terebinthifolius, came from a wider environmental distribution. These findings support a hypothesis of increasing dissection with increasing heat, but do not support the hypothesis that natural habitat distributions may be linked to plasticity. The high degree of variability in J. mimosifolia, which is considered invasive in tropical areas of the United States, may suggest that this species may survive by producing a wide range of individual capable of surviving in different habitats.

Urban agricultural systems, like community and home gardens, may act as oases of biodiversity in cities dominated by impervious surfaces. They have also been shown to bridge gaps in food security and provide socio-cultural benefits. Despite these benefits, little research has been conducted that evaluates factors influencing garden plant biodiversity and ecosystem services (ES). Also less intensively researched are ecosystem disservices that gardens can contribute to, like gardener exposure to heavy metals. Urban soil can act as a sink for heavy metal contamination, which is mostly deposited through anthropogenic pollution. This dissertation addresses knowledge gaps about ES with two comprehensive surveys of garden biodiversity and ES production, one on community gardens in Los Angeles (LA), CA and one on home gardens across an urbanizing gradient in Beijing, China. It also addresses disservices and bioavailability of three heavy metals (lead, arsenic, and cadmium) through a soil survey and sequential analysis of heavy metals in LA gardens.

My main results indicate an overall shift in biodiversity from provisioning (food and medicinal production) to cultural (ornamental) services with increased gardener income and access to city resources (like grocery stores or markets) in both U.S. and Chinese gardens. This result supports a hierarchy of need, where gardeners preferentially plant species that support their most pressing needs, like food security. Urbanized regions in Beijing and immigrant-run gardens in Los Angeles also formed culturally distinct assemblages of edible species based on shared agricultural experiences. As the most common use for species was food, understanding metal bioavailability is important for accurate risk assessments. Lead, particularly in reducible form, increased the most with age of neighborhood, indicating oxidized lead paint buildup. Cd and As exchangeable fractions increased with proximity to road, indicating sources from air pollution. Finally, while Cd became less bioavailable with increased organic matter, reaction with organic humic acids released reducible As into the bioavailable fraction. These quantitative results can inform land managers about valued biodiversity and provisioning service from gardens in food insecure regions, as well as valuable information on how to predict metal accumulation hotspots and reduce plant uptake of metals.

Ecosystems are undergoing unprecedented rates of change with severe consequences for biodiversity loss, yet natural resilience and restoration of ecosystems provides hope to mitigate these losses. Despite the potential for natural recovery, there are thresholds and feedback mechanisms that inhibit recovery that are often driven by invasive species. Consequently, understanding how invasive species interact with their surrounding communities and how they respond to restoration efforts is crucial information for effective management and successful restoration. My dissertation seeks to understand how the impact of invasive plants prevents effective restoration through a combination of field experiments with paired greenhouse components to capture robust processes in the field and disentangle the underlying mechanisms in a controlled greenhouse setting. My first two chapters investigate how a local and regionally obnoxious novel invasive forb, Oncosiphon pilulifer, responds to the common management technique of prescribed fire, and how Oncosiphon interacts with soil biota to inhibit native plant growth. My final chapter focuses on how the multiple factors of seed limitation, invasive litter accumulation, and soil symbiont depletion constrain restoration success in a Northern Californian annual grassland. Taken together, my dissertation projects aim to both address fundamental questions in community ecology and produce actionable science for land management practitioners.