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Abstract Anthropogenic activities add more reactive nitrogen (N) to the environment than all natural sources combined, and the fate of this N is of environmental concern. If N that is deposited on terrestrial ecosystems through atmospheric deposition is retained in plant tissues or soil organic matter, it could stimulate carbon (C) storage in plant biomass or soils. However, added N also could increase soil inorganic N concentrations and leaching, potentially polluting watersheds, particularly in areas with low-N soils and/or a high propensity for leaching, such as sandy or arid areas. Here, we assessed N allocation and retention across a 13-year experimental N addition gradient in a temperate grassland. We found that N accumulation decreased significantly at mid- to high levels of N addition compared to the Control, such that ecosystem N pools were equivalent across a 10 g m⁻² year⁻¹range of annual N addition rates (0–10 g N m⁻² year⁻¹), which spans most of the global range of N deposition. Nitrogen addition increased plant tissue percent N, but the total pool of N did not increase because of reduced plant biomass, particularly in roots. Nitrogen addition also increased soil inorganic N concentrations. Our results indicate that N addition is unlikely to increase grassland N pools, particularly in sandy or low-fertility ecosystems with a high potential for leaching because high application rates lead to N saturation, and additional inputs are lost. 

ABSTRACT Biodiversity promotes ecosystem productivity and stability, positive impacts that often strengthen over time. But ongoing global changes such as rising atmospheric carbon dioxide (CO₂) levels and anthropogenic nitrogen (N) deposition may modulate the impact of biodiversity on ecosystem productivity and stability over time. Using a quarter‐century grassland biodiversity‐global change experiment we show that diversity increasingly enhanced productivity over time irrespective of global change treatments. In contrast, the positive influence of diversity on ecosystem stability strengthened over time under ambient conditions but weakened to varying degrees under global change treatments, largely driven by a greater reduction in species asynchrony under global changes. Thus, over 25 years, CO₂and N enrichment gradually eroded some of the positive effects of biodiversity on ecosystem stability. As elevated CO₂, N eutrophication, and biodiversity loss increasingly co‐occur in grasslands globally, our results raise concerns about their potential joint detrimental effects on long‐term grassland stability. 

Abstract Ectomycorrhizal (ECM) fungi have long been thought to reduce litter decomposition in nitrogen (N)‐limited ecosystems by outcompeting saprotrophs for litter N (a phenomenon known as the ‘Gadgil effect’). However, recent research has called the generality of this effect into question, by demonstrating that ECM fungi can increase or decrease organic matter decomposition in different forests. The ecological factors driving such variation in the size and direction of ECM fungal effects on decomposition remain unclear.Here, we tested the hypothesis that ECM fungi would suppress decomposition of N‐poor, recalcitrant litter more in forests with lower N‐availability by exacerbating saprotrophic N limitation. We conducted an in situ ECM fungal and root reduction experiment (via soil trenching) in nine pine forests across three US states, which varied in soil and litter N content, climate and pine host (Pinus muricatain California,P. elliottiiin Florida and P.resinosain Minnesota). In each site, we decomposed needle litter from (1) a pine species native to that site and (2) a common litter,P. strobus, for 1 year.Contrary to our expectations, ECM fungi either stimulated (California) or had no effect on (Florida and Minnesota) pine needle decomposition. Across sites, ECM fungal stimulation of decomposition did increase with total soil N content, but was unrelated to inorganic N availability. Furthermore, despite previous work suggesting that competition for N structures ECM fungal–saprotroph interactions, trenching effects on decomposition did not differ between pine litter types, despite large differences in initial litter C:N ratios, recalcitrance and net litter N immobilization.Synthesis. Taken together, our results add to a growing body of evidence that the ‘Gadgil effect’ is not universal, even in the N‐poor litter of temperate pine forests where it was first described and is often invoked. Furthermore, the inconsistency of relationships between trenching effects with different metrics of decomposer N supply and demand calls into question the central role of N in structuring fungal interguild interactions. 

Abstract Understanding the determinants of urban forest diversity and structure is important for preserving biodiversity and sustaining ecosystem services in cities. However, comprehensive field assessments are resource‐intensive, and landscape‐level approaches may overlook heterogeneity within urban regions. To address this challenge, we combined remote sensing with field inventories to comprehensively map and analyze urban forest attributes in forest patches across the Minneapolis‐St. Paul Metropolitan Area (MSPMA) in a multistep process. First, we developed predictive machine learning models of forest attributes by integrating data from forest inventories (from 40 12.5‐m‐radius plots) with Global Ecosystem Dynamics Investigation (GEDI) observations and Sentinel‐2‐derived land surface phenology (LSP). These models enabled accurate predictions of forest attributes, specifically nine metrics of plant diversity (tree species richness, tree abundance, and understory plant abundance), structure (average canopy height, dbh, and canopy density), and structural complexity (variability in canopy height, dbh, and canopy density) with relative errors ranging between 11% and 21%. Second, we applied these machine learning models to predict diversity metrics for 804 additional plots from GEDI and Sentinel‐2. Finally, we applied Bayesian multilevel models to the predicted diversity metrics to assess the influence of multiple factors—patch dimensions, landscape attributes, plot position, and jurisdictional agency—on these forest attributes across the 804 predicted plots. The models showed all predictors have some degree of effect on forest attributes, presenting varying explanatory power withR²values ranging from 0.071 to 0.405. Overall, plot characteristics (e.g., distance to nearest trail, proximity to forest edge) and jurisdictional agency explained a large portion of the variability across patches, whereas patch and landscape characteristics did not. The relative effect of plot versus management sets of predictors on the marginal ΔR²was heterogeneous across metrics and ecological subsections (an ecological classification designation). The multiplicity of determinants influencing urban forests emphasizes the intricate nature of urban ecosystems and highlights nuanced, heterogeneous relationships between urban ecological and anthropogenic factors that determine forest properties. Effectively enhancing biodiversity in urban forests requires assessments, management, and conservation strategies tailored for context‐specific characteristics. 

Abstract Urban tree canopy cover is often unequally distributed across cities such that more socially vulnerable neighborhoods often have lower tree canopy cover than less socially vulnerable neighborhoods. However, how the diversity and composition of the urban canopy affect the nature of social‐ecological benefits (and burdens), including the urban forest's vulnerability to climate change, remains underexamined. Here, we synthesize tree inventories developed by multiple organizations and present a species‐specific, geolocated database of more than 600,000 urban trees across the 7‐county Minneapolis‐St. Paul (MSP) metropolitan area in the Upper Midwest of the United States. We find that tree diversity across the MSP is variable yet dominated by a few species (e.g.,Fraxinus pennsylvanica,Acer platanoides, andGleditsia triacanthos), contributing to the vulnerability of the MSP urban forest to future climate change and disturbances. In contrast to tree canopy cover, tree diversity was not well predicted by socioeconomic or demographic factors. However, our analysis identified areas where both climate and social vulnerability are high. Our results add to a growing body of literature emphasizing the importance of considering how complex and interacting social and ecological factors drive urban forest diversity and composition when pursuing management objectives. 

Abstract Planting diverse forests has been proposed as a means to increase long‐term carbon (C) sequestration while providing many co‐benefits. Positive tree diversity–productivity relationships are well established, suggesting more diverse forests will lead to greater aboveground C sequestration. However, the effects of tree diversity on belowground C storage have the potential to either complement or offset aboveground gains, especially during early stages of afforestation when potential exists for large losses in soil C due to soil decomposition. Thus, experimental tests of the effects of planted tree biodiversity on changes in whole‐ecosystem C balance are needed. Here, we present changes in above‐ and belowground C pools 6 years after the initiation of the Forests and Biodiversity experiment (FAB1), consisting of high‐density plots of one, two, five, or 12 tree species planted in a common garden. The trees included a diverse range of native species, including both needle‐leaf conifer and broadleaf angiosperm species, and both ectomycorrhizal and arbuscular mycorrhizal species. We quantified the effects of species richness, phylogenetic diversity, and functional diversity on aboveground woody C, as well as on mineral soil C accumulation, fine root C, and soil aggregation. Surprisingly, changes in aboveground woody C pools were uncorrelated to changes in mineral soil C pools, suggesting that variation in soil C accumulation was not driven by the quantity of plant litter inputs. Aboveground woody C accumulation was strongly driven by species and functional identity; however, plots with higher species richness and functional diversity accumulated more C in aboveground wood than expected based on monocultures. We also found weak but significant effects of tree species richness, identity, and mycorrhizal type on soil C accumulation. To assess the role of the microbial community in mediating these effects, we further compared changes in soil C pools to phospholipid fatty acid (PLFA) profiles. Soil C pools and accumulation were more strongly correlated with specific microbial clades than with total microbial biomass or plant diversity. Our results highlight rapidly emerging and microbially mediated effects of tree biodiversity on soil C storage in the early years of afforestation that are independent of gains in aboveground woody biomass. 

ABSTRACT Anthropogenic nitrogen (N) deposition is unequally distributed across space and time, with inputs to terrestrial ecosystems impacted by industry regulations and variations in human activity. Soil carbon (C) content normally controls the fraction of mineralized N that is nitrified (ƒ_nitrified), affecting N bioavailability for plants and microbes. However, it is unknown whether N deposition has modified the relationships among soil C, net N mineralization, and net nitrification. To test whether N deposition alters the relationship between soil C and net N transformations, we collected soils from coniferous and deciduous forests, grasslands, and residential yards in 14 regions across the contiguous United States that vary in N deposition rates. We quantified rates of net nitrification and N mineralization, soil chemistry (soil C, N, and pH), and microbial biomass and function (as beta‐glucosidase (BG) andN‐acetylglucosaminidase (NAG) activity) across these regions. Following expectations, soil C was a driver ofƒ_nitrifiedacross regions, whereby increasing soil C resulted in a decline in net nitrification andƒ_nitrified. Theƒ_nitrifiedvalue increased with lower microbial enzymatic investment in N acquisition (increasing BG:NAG ratio) and lower active microbial biomass, providing some evidence that heterotrophic microbial N demand controls the ammonium pool for nitrifiers. However, higher total N deposition increasedƒ_nitrified, including for high soil C sites predicted to have lowƒ_nitrified, which decreased the role of soil C as a predictor ofƒ_nitrified. Notably, the drop in contemporary atmospheric N deposition rates during the 2020 COVID‐19 pandemic did not weaken the effect of N deposition on relationships between soil C andƒ_nitrified. Our results suggest that N deposition can disrupt the relationship between soil C and net N transformations, with this change potentially explained by weaker microbial competition for N. Therefore, past N inputs and soil C should be used together to predict N dynamics across terrestrial ecosystems. 

Abstract Despite interest in the contribution of evapotranspiration (ET) of residential turfgrass lawns to household and municipal water budgets across the United States, the spatial and temporal variability of residential lawn ET across large scales is highly uncertain. We measured instantaneous ET (ET_inst) of lawns in 79 residential yards in six metropolitan areas: Baltimore, Boston, Miami, Minneapolis‐St. Paul (mesic climates), Los Angeles and Phoenix (arid climates). Each yard had one of four landscape types and management practices: traditional lawn‐dominated yards with high or low fertilizer input, yards with water‐conserving features, and yards with wildlife‐friendly features. We measured ET_instin situ during the growing season using portable chambers and identified environmental and anthropogenic factors controlling ET in residential lawns. For each household, we used ET_instto estimate daily ET of the lawn (ET_daily) and multiplied ET_dailyby the lawn area to estimate the total volume of water lost through ET of the lawn (ET_vol). ET_dailyvaried from 0.9 ± 0.4 mm d¹in mesic cities to 2.9 ± 0.7 mm d⁻¹in arid cities. Neither ET_instnor ET_dailywas significantly influenced by yard landscape types and ET_instpatterns indicated that lawns may be largely decoupled from regional rain‐driven climate patterns. ET_volranged from ∼0 L d⁻¹to over 2,000 L d⁻¹, proportionally increasing with lawn area. Current irrigation and lawn management practices did not necessarily result in different ET_instor ET_dailyamong traditional, water‐conserving, or wildlife‐friendly yards, but smaller lawn areas in water‐conserving and wildlife‐friendly yards resulted in lower ET_vol. 

Abstract Legumes are an important component of plant diversity that modulate nitrogen (N) cycling in many terrestrial ecosystems. Limited knowledge of legume effects on soil N cycling and its response to global change factors and plant diversity hinders a general understanding of whether and how legumes broadly regulate the response of soil N availability to those factors. In a 17‐year study of perennial grassland species grown under ambient and elevated (+180 ppm) CO₂and ambient and enriched (+4 g N m⁻² year⁻¹) N environments, we compared pure legume plots with plots dominated by or including other herbaceous functional groups (and containing one or four species) to assess the effect of legumes on N cycling (net N mineralization rate and inorganic N pools). We also examined the effects of numbers of legume species (from zero to four) in four‐species mixed plots on soil N cycling. We hypothesized that legumes would increase N mineralization rates most in those treatments with the greatest diversity and the greatest relative limitation by and competition for N. Results partially supported these hypotheses. Plots with greater dominance by legumes had greater soil nitrate concentrations and mineralization rates. Higher species richness significantly increased the impact of legumes on soil N metrics, with 349% and 505% higher mineralization rates and nitrate concentrations in four‐species plots containing legumes compared to legume‐free four‐species plots, in contrast to 185% and 129% greater values, respectively, in pure legume than nonlegume monoculture plots. N‐fertilized plots had greater legume effects on soil nitrate, but lower legume effects on net N mineralization. In contrast, neither elevated CO₂nor its interaction with legumes affected net N mineralization. These results indicate that legumes markedly influence the response of soil N cycling to some, but not all, global change drivers. 

Abstract Urbanization has a homogenizing effect on biodiversity and leads to communities with fewer native species and lower conservation value. However, few studies have explored whether or how land management by urban residents can ameliorate the deleterious effects of this homogenization on species composition. We tested the effects of local (land management) and neighborhood‐scale (impervious surface and tree canopy cover) features on breeding bird diversity in six US metropolitan areas that differ in regional species pools and climate. We used a Bayesian multiregion community model to assess differences in species richness, functional guild richness, community turnover, population vulnerability, and public interest in each bird community in six land management types: two natural area park types (separate and adjacent to residential areas), two yard types with conservation features (wildlife‐certified and water conservation) and two lawn‐dominated yard types (high‐ and low‐fertilizer application), and surrounding neighborhood‐scale features. Species richness was higher in yards compared with parks; however, parks supported communities with high conservation scores while yards supported species of high public interest. Bird communities in all land management types were composed of primarily native species. Within yard types, species richness was strongly and positively associated with neighborhood‐scale tree canopy cover and negatively associated with impervious surface. At a continental scale, community turnover between cities was lowest in yards and highest in parks. Within cities, however, turnover was lowest in high‐fertilizer yards and highest in wildlife‐certified yards and parks. Our results demonstrate that, across regions, preserving natural areas, minimizing impervious surfaces and increasing tree canopy are essential strategies to conserve regionally important species. However, yards, especially those managed for wildlife support diverse, heterogeneous bird communities with high public interest and potential to support species of conservation concern. Management approaches that include the preservation of protected parks, encourage wildlife‐friendly yards and acknowledge how public interest in local birds can advance successful conservation in American residential landscapes.