Urbanization minimizes the effects of plant traits on soil provisioned ecosystem services across climatic regions

Urbanization leads to asynchronous homogenization of soil microbial communities across biomes

Soil bacterial and fungal communities play fundamental roles in biogeochemical cycles and ecosystem stability. Urbanization alters soil properties and microbial habitats, driving shifts in community composition, yet the divergent responses of bacteria and fungi and their ecological consequences remain inadequately understood. To elucidate these differential responses, we investigated soil bacterial and fungal communities along an urbanization gradient, ranging from undisturbed reference forests to urban parks, across three distinct climatic regions. To capture different disturbance intensities, urban parks were classified by tree age into old parks (>60-year-old trees) and young parks (10-20-year-old trees). Climate had a strong influence on soil microbiota, yet urbanization still significantly altered both bacterial and fungal communities in all regions. Urban disturbances homogenized soil microbial communities: average similarity among bacterial communities increased from ∼79 % in forests to ∼85 % in young urban parks, indicating substantial homogenization, whereas fungal communities showed little homogenization. Urbanization also homogenized microbial functional traits, with a greater reduction in trait dissimilarity for bacteria than for fungi. Bacterial communities exhibited high adjustability to urban conditions, dominated by generalist taxa (∼90 %), whereas fungal communities consisted mostly of specialists (∼83 %). Despite these asynchronous responses-bacteria adjusting and homogenizing more than fungi-overlapping functional traits between bacteria and fungi help maintain functional resilience in urban ecosystems.

What drives metal resistance genes in urban park soils? Park age matters across biomes

Although resistance genes are a global concern in ecosystems, the underlying factors responsible for their worldwide dissemination, especially in urban greenspaces, are poorly known. To investigate metal and metal resistance genes (MRGs) accumulation in urban parks, we used ICP-MS to analyze metal concentrations and GeoChip functional gene arrays to analyze MRGs abundances in vegetation types with labile and recalcitrant litter across urban parks and non-urban reference sites in three distinct climatic regions: Boreal (Finland), Temperate (Baltimore, USA), and Tropical (Singapore). Our results indicate that metal concentrations and MRGs abundances in park soils increase with park age across climatic zones, especially so for the dominant metals - Fe and Al - accounting for more than 90% of the total metal content, and others, e.g., Mn, Zn, and Pb. Correspondingly, Fe and Al resistance genes were the most abundant MRGs, representing 23% of all detected MRGs. Vegetation type affected metals and MRGs only in the boreal region, not in temperate or tropical regions, suggesting that vegetation context is not generalizable across climatic zones. Our analyses also indicate that the distribution of resistance genes is only weakly affected by soil properties, but largely associated with accumulation of metals from traffic and industrial sources. Our data further indicate that MRGs and antibiotic resistance genes (ARGs) are co-selected by metal accumulation. The pattern of MRG abundance between old and young parks is similar to that of ARGs, indicating a potential risk for human health in old urban parks. Our findings emphasize the importance of park age and the corresponding cumulative effects of anthropogenic activities as a driver of metal and MRG dynamics in urban soils globally.

Does land use change decline the regional ecosystem health maintenance? Case study in subtropical coastal region, Fuzhou, China

Regional environmental management aims to maintain or improve regional ecosystem health (REH) and prevent its degradation over time. In the context of rapid urbanization and global sustainability over the last two decades, has land use change resulted in a deterioration of REH? By using the improved REH framework model as Pressure-Vigor-Organization-Resilience-Service (P-VOR-S), this paper proposed Regional Ecosystem Health Maintenance (REHM) as a dynamic quantitative indicator of REH, and detected REHM as well as its response to land use change in the coastal city of Fuzhou, China during 2003-2018. The results showed that: (1) During 2003-2018, the average REH gradually decreased in spite of the 62.50-67.55% area coverage of the REH "well" level. The REHM "maintenance" level covered 9764.69 km2 (83.74%), while the REHM "degradation" and "improvement" covered only 1485.17 km2 (12.74%) and 410.99 km2 (3.52%), respectively. (2) The REHM "degradation" in Fuzhou was predominantly caused by the conversion of forest to cultivated land and cultivated land to construction land, as well as the conversion of forest and water to construction land. (3) The REHM map highlighted the degraded areas as hotspots for environmental management concerns, and the area of REHM "degradation" or "improvement" could be served as key indicator for regional environmental management and spatial land planning.

Labile carbon input substantially increases priming effect in urban greenspace soils

Urban greenspace soils can store equal amount of carbon, or even more, compared to agricultural and forest soils, and play an important role in carbon sequestration. Despite its importance, the patterns and drivers of the priming effect-a key and complex process in soil organic matter decomposition-in urban ecosystems remain poorly understood. Here, we sampled soils in urban lawns, suburban lawns, and forests, and conducted a 30-day microcosm incubation with 13C-labelled glucose and nitrogen additions to explore whether and how the intensity of soil organic matter priming effect differs between urbanized and forest ecosystems. We found that lawn soils in urban (7.01 mg C g-1 SOC) and suburban (5.86) areas had a significantly higher intensity of priming effect than forest soils (1.34), with further enhancement observed in urban lawn soils through simulated nitrogen deposition. Moreover, the alpha diversity of soil bacteria and fungi was found to play a crucial role in modulating the priming effect, exhibiting a positive correlation with its intensity. These findings advance our understanding of the potential mechanisms behind the soil priming effect in urban greenspaces, providing crucial insights for predicting soil carbon stocks and environmental impacts of urban development.

Protists and fungi: Reinforcing urban soil ecological functions against flash droughts

Rising instances of flash droughts are contributing to notable variability in soil moisture across terrestrial ecosystems. These phenomena challenge urban ecosystem services, yet the reaction of soil ecological functions (SEFs) to such events is poorly understood. This study investigates the responses of SEFs (about nutrient metabolism capacity and potential) and the microbiome under two specific scenarios: a flooding-drought sequence and a direct drought condition. Using quantitative microbial element cycling analysis, high-throughput sequencing, and enzyme activity measurements, we found that unlike in forests, the microbial composition in urban soils remained unchanged during flash drought conditions. However, SEFs were affected in both settings. Correlation analysis and Mantel test showed that forest soils exhibited more complex interactions among soil moisture, properties, and microbial communities. Positive linear correlation revealed that bacteria were the sole drivers of SEFs. Interestingly, while multi-threshold results suggested bacterial α diversity impeded the maximization of SEFs in urban soils, fungi and protists had a beneficial impact. Cross-domain network of urban soils had higher number of nodes and edges, but lower average degree and robustness than forest soils. Mantel test revealed that fungi and protist had significant correlations with bacterial composition in forest soils, but not in urban soils. In the urban network, the degree and eigenvector centrality of bacterial, fungal and protistan ASVs were significantly lower compared to those in the forest. These results suggest that the lower robustness of the microbial network in urban soils is attributed to limited interactions among fungi, consumer protists, and bacteria, contributing to the failure of microbial-driven ecological functions. Overall, our findings emphasize the critical role of fungi and protists in shielding urban soils from drought-induced disturbances and in enhancing the resistance of urban ecological functions amidst environmental changes.

Dynamic Responses of Soil Organic Carbon to Urbanization: A Global Perspective

Rapid global urbanization has a complex impact on soil organic carbon (SOC) stocks. Through its direct and indirect impacts on soil formation and development, urbanization greatly influences SOC stocks. However, the extent to which urbanization affects SOC stocks globally remains unclear. In this study, we utilized an urban-rural gradient approach to assess the effects of urbanization on SOC stocks at both global and national scales. First, we calculated the urbanization intensity (UI) at a 1 km scale globally, categorizing urbanization into three stages: low (0 ≤ UI ≤ 25), medium (25 < UI ≤ 75), and high (75 < UI ≤ 100). Additionally, we distinguished the contributions of natural factors and human activities and analyzed the effects of land-use changes in eight representative cities. We found the following: (1) The SOC stocks exhibit distinct trends with increasing UI, but when UI is low or high, an increase in UI is associated with decreasing SOC stocks (reductions of 6.8% and 5.4% at a depth of 30 cm; 6.4% and 3.2% at a depth of 100 cm, respectively). (2) Changes in human activities are the main drivers of SOC stock changes during urbanization. At low and medium urban intensities, the contributions of human activities reach 98% and 89%, respectively. Additionally, land-use transitions are closely correlated with SOC stock changes, particularly in areas near the urban core, across different climate zones. (3) The response of SOC to urbanization varies across climatic zones. In water-scarce arid climates, attention should be given to the negative effects of urbanization, and more targeted measures should be taken to enhance the carbon sequestration capacity of urban soils. This study provides valuable insights into the dynamic interplay between urbanization and SOC stocks, underscoring the need for tailored strategies to manage soil carbon in urban environments.

Distinct latitudinal patterns and drivers of topsoil nitrogen and phosphorus across urban forests in eastern China

Nitrogen (N) and phosphorus (P) are the two most important macronutrients supporting forest growth. Unprecedented urbanization has created growing areas of urban forests that provide key ecosystem services for city dwellers. However, the large-scale patterns of soil N and P content remain poorly understood in urban forests. Based on a systematic soil survey in urban forests from nine large cities across eastern China, we examined the spatial patterns and key drivers of topsoil (0-20 cm) total N content, total P content, and N:P ratio. Topsoil total N content was found to change significantly with latitude in the form of an inverted parabolic curve, while total P content showed an opposite latitudinal pattern. Variance partition analysis indicated that regional-scale patterns of topsoil total N and P contents were dominated by climatic drivers and partially regulated by time and pedogenic drivers. Conditional regression analyses showed a significant increase in topsoil total N content with lower mean annual temperature (MAT) and higher mean annual precipitation (MAP), while topsoil total P content decreased significantly with higher MAP. Topsoil total N content also increased significantly with the age of urban park and varied with pre-urban soil type, while no such effects were found for topsoil total P content. Moreover, topsoil N:P ratio showed a latitudinal pattern similar to that of topsoil total N content and also increased significantly with lower MAT and higher MAP. Our findings demonstrate distinct latitudinal trends of topsoil N and P contents and highlight a dominant role of climatic drivers in shaping the large-scale patterns of topsoil nutrients in urban forests.

Global distribution of surface soil organic carbon in urban greenspaces

Urban greenspaces continue to grow with global urbanization. The global distribution and stock of soil organic carbon (SOC) in urban greenspaces remain largely undescribed and missing in global carbon (C) budgets. Here, we synthesize data of 420 observations from 257 cities in 52 countries to evaluate the global pattern of surface SOC density (0-20 cm depth) in urban greenspaces. Surface SOC density in urban greenspaces increases significantly at higher latitudes and decreases significantly with higher mean annual temperature, stronger temperature and precipitation seasonality, as well as lower urban greenness index. By mapping surface SOC density using a random forest model, we estimate an average SOC density of 55.2 (51.9-58.6) Mg C ha-1 and a SOC stock of 1.46 (1.37-1.54) Pg C in global urban greenspaces. Our findings present a comprehensive assessment of SOC in global urban greenspaces and provide a baseline for future urban soil C assessment under continuing urbanization.

Precipitation, Not Land Use, Primarily Determines the Composition of Both Plant and Phyllosphere Fungal Communities

Plant communities and fungi inhabiting their phyllospheres change along precipitation gradients and often respond to changes in land use. Many studies have focused on the changes in foliar fungal communities on specific plant species, however, few have addressed the association between whole plant communities and their phyllosphere fungi. We sampled plant communities and associated phyllosphere fungal communities in native prairie remnants and post-agricultural sites across the steep precipitation gradient in the central plains in Kansas, USA. Plant community cover data and MiSeq ITS2 metabarcode data of the phyllosphere fungal communities indicated that both plant and fungal community composition respond strongly to mean annual precipitation (MAP), but less so to land use (native prairie remnants vs. post-agricultural sites). However, plant and fungal diversity were greater in the native remnant prairies than in post-agricultural sites. Overall, both plant and fungal diversity increased with MAP and the communities in the arid and mesic parts of the gradient were distinct. Analyses of the linkages between plant and fungal communities (Mantel and Procrustes tests) identified strong correlations between the composition of the two. However, despite the strong correlations, regression models with plant richness, diversity, or composition (ordination axis scores) and land use as explanatory variables for fungal diversity and evenness did not improve the models compared to those with precipitation and land use (ΔAIC < 2), even though the explanatory power of some plant variables was greater than that of MAP as measured by R2. Indicator taxon analyses suggest that grass species are the primary taxa that differ in the plant communities. Similar analyses of the phyllosphere fungi indicated that many plant pathogens are disproportionately abundant either in the arid or mesic environments. Although decoupling the drivers of fungal communities and their composition - whether abiotic or host-dependent - remains a challenge, our study highlights the distinct community responses to precipitation and the tight tracking of the plant communities by their associated fungal symbionts.