Impacts of local soil and vehicle NOx emissions on ground-level NO2 concentrations on a university campus in the city of Shenyang, China

Nitrogen dioxide (NO2) is a ubiquitous atmospheric pollutant, and fossil fuel combustion is generally considered its predominant source in and around urban areas. As the total nitrogen deposition is high over here, soil NOx emissions from urban green space might also be an important local source of ground-level NO2. In this study, Willems badge samplers were employed to monitor the spatial and seasonal variations of 2-week mean atmospheric NO2 concentrations at a height of 1.7 m on an urban campus in Northeast China from November 2020 to December 2021. We found considerable small-scale spatial variations of ground-level NO2 concentrations on the campus during the growing season, with local soil NOx emissions as the main driver. According to its linear correlation with green space coverage, the increment in ground-level NO2 concentration was partitioned into two components, with one ascribed to the local soil source (referred to as NO2-Isoil) and the other the local vehicle source (NO2-Ivehicle). NO2-Isoil generally reached a maximum (as high as 25.6 μg/m3) during early spring, while its ratio to the background value generally reached a maximum (could be >1) during late spring and could reach 0.52 to 0.92 during summer. Therefore, soil NOx emissions were an important source of ground-level NO2 on the campus, with the contribution even higher than those of other anthropogenic sources during late spring. Even with light traffic on the campus, NO2-Ivehicle could reach 0.48 times the background value at a site with high frequencies of warm starts.

Anthropogenic effects on global soil nitrogen pools

The amount of nitrogen stored in terrestrial soils, its "nitrogen pool", moderates biogeochemical cycling affecting primary productivity, nitrogen pollution and even carbon budgets. The soil nitrogen pools and the transformation of nitrogen forms within them are heavily influenced by environmental factors including anthropogenic activities. However, our understanding of the global distribution of soil nitrogen with respect to anthropogenic activity and human land use remains unclear. We constructed a meta-analysis from a global sampling, in which we compare soil total nitrogen pools and the driving mechanisms affecting each pool across three major classifications of human land use: natural, agricultural, and urban. Although the size of the nitrogen pool can be similar across natural, agricultural and urban soils, the ecological and human associated drivers vary. Specifically, the drivers within agricultural and urban soils as opposed to natural soils are more complex and often decoupled from climatic and soil factors. This suggests that the nitrogen pools of those soils may be co-moderated by other factors not included in our analyses, like human activities. Our analysis supports the notion that agricultural soils act as a nitrogen source while urban soils as a nitrogen sink and informs a modern understanding of the fates and distributions of anthropogenic nitrogen in natural, agricultural, and urban soils.

Urbanization can accelerate climate change by increasing soil N2 O emission while reducing CH4 uptake

Urban land-use change has the potential to affect local to global biogeochemical carbon (C) and nitrogen (N) cycles and associated greenhouse gas (GHG) fluxes. We conducted a meta-analysis to (1) assess the effects of urbanization-induced land-use conversion on soil nitrous oxide (N₂ O) and methane (CH₄ ) fluxes, (2) quantify direct N₂ O emission factors (EF_d ) of fertilized urban soils used, for example, as lawns or forests, and (3) identify the key drivers leading to flux changes associated with urbanization. On average, urbanization increases soil N₂ O emissions by 153%, to 3.0 kg N ha^-1  year^-1 , while rates of soil CH₄ uptake are reduced by 50%, to 2.0 kg C ha^-1  year^-1 . The global mean annual N₂ O EF_d of fertilized lawns and urban forests is 1.4%, suggesting that urban soils can be regional hotspots of N₂ O emissions. On a global basis, conversion of land to urban greenspaces has increased soil N₂ O emission by 0.46 Tg N₂ O-N year^-1 and decreased soil CH₄ uptake by 0.58 Tg CH₄ -C year^-1 . Urbanization driven changes in soil N₂ O emission and CH₄ uptake are associated with changes in soil properties (bulk density, pH, total N content, and C/N ratio), increased temperature, and management practices, especially fertilizer use. Overall, our meta-analysis shows that urbanization increases soil N₂ O emissions and reduces the role of soils as a sink for atmospheric CH₄ . These effects can be mitigated by avoiding soil compaction, reducing fertilization of lawns, and by restoring native ecosystems in urban landscapes.

Nitrogen inputs are more important than denitrifier abundances in controlling denitrification-derived N2O emission from both urban and agricultural soils

Cities are increasingly being recognized as important contributors in global warming, for example by increasing atmospheric nitrous oxide (N₂O). However, urban ecosystems remain poorly understood due to their functional complexity. Further, few studies have documented the microbial processes governing the N₂O emissions from urban soils. Here, a field study was performed to assess in situ N₂O emissions in an urban and agricultural soil located in Xiamen, China. The mechanisms underlying the difference in N₂O emission patterns in both soils were further explored in an incubation experiment. Field investigations showed that N₂O emission (3.5-19.0 μg N₂O-N m^-2 h^-1) from the urban soil was significantly lower than that from the agricultural soil (25.4-18,502.3 μg N₂O-N m^-2 h^-1). Incubation experiments showed that the urban soil initially emitted lower denitrification-derived N₂O because of the lower nirS (encoding nitrite reductases) abundances, whereas overall N₂O accumulation during the incubation was mainly controlled by the initial nitrate content in soil. Nitrate addition in a short period (5 days) did not change the total bacterial and denitrifier abundances or the soil bacterial community composition, but significantly altered the relative distribution of some key genera capable of denitrification. Although the urban soil exhibited lower N₂O emission than its agricultural counterpart in this study, the expanding urban green areas should be taken into account when building N₂O emission reduction targets.

Annual N2O emissions from conventionally grazed typical alpine grass meadows in the eastern Qinghai-Tibetan Plateau

Annual nitrous oxide (N₂O) emissions from high-altitude alpine meadow grasslands have not been effectively characterized because of the scarcity of whole-year measurements. The authors performed a year-round measurement of N₂O fluxes from three conventionally grazed alpine meadows that represent the typical meadow landscape in the eastern Qinghai-Tibetan Plateau (QTP). The results showed that annual N₂O emissions averaged 0.123±0.053 (2SD, i.e., the double standard deviation indicating the 95% confidence interval) kgNha^-1yr^-1 across the three meadow sites. N₂O flux pulses during the spring freezing-thawing period (FTP) were observed at only one site, indicating a large spatial variability in association with soil moisture differences. Approximately 34-57% (mean: 46%) of the annual N₂O emissions occurred in the non-growing season, highlighting the substantial importance of accurate flux observations during this period. The simultaneous observations showed conservative, marginal nitric oxide (NO) fluxes of 0.058±0.032 (2SD) kgNha^-1yr^-1. The N₂O fluxes across the three field sites correlated negatively with the soil nitrate concentrations during the entire year-round period (P<0.05). Furthermore, a significant joint regulatory effect of topsoil temperature and moisture on the N₂O and NO fluxes was observed during the relatively warm periods. Based on the results of the present and previous studies, a simple extrapolation roughly estimated the annual total N₂O emission from Chinese grasslands to be 73±15 (2SD) GgNyr^-1 (1Gg=10⁹g). A linear dependence of the annual N₂O fluxes on the aboveground net primary productivity (ANPP) was also found. This result may provide a simple approach for estimating the N₂O emission inventories of frigid alpine or temperate grasslands that are ungrazed either in the summer or year round. However, further confirmation of this relationship with a wider ANPP range is still needed in the future studies.

Climate Change and the Impact of Greenhouse Gasses: CO2 and NO, Friends and Foes of Plant Oxidative Stress

Here, we review information on how plants face redox imbalance caused by climate change, and focus on the role of nitric oxide (NO) in this response. Life on Earth is possible thanks to greenhouse effect. Without it, temperature on Earth's surface would be around -19°C, instead of the current average of 14°C. Greenhouse effect is produced by greenhouse gasses (GHG) like water vapor, carbon dioxide (CO₂), methane (CH₄), nitrous oxides (N_xO) and ozone (O₃). GHG have natural and anthropogenic origin. However, increasing GHG provokes extreme climate changes such as floods, droughts and heat, which induce reactive oxygen species (ROS) and oxidative stress in plants. The main sources of ROS in stress conditions are: augmented photorespiration, NADPH oxidase (NOX) activity, β-oxidation of fatty acids and disorders in the electron transport chains of mitochondria and chloroplasts. Plants have developed an antioxidant machinery that includes the activity of ROS detoxifying enzymes [e.g., superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), glutathione peroxidase (GPX), and peroxiredoxin (PRX)], as well as antioxidant molecules such as ascorbic acid (ASC) and glutathione (GSH) that are present in almost all subcellular compartments. CO₂ and NO help to maintain the redox equilibrium. Higher CO₂ concentrations increase the photosynthesis through the CO₂-unsaturated Rubisco activity. But Rubisco photorespiration and NOX activities could also augment ROS production. NO regulate the ROS concentration preserving balance among ROS, GSH, GSNO, and ASC. When ROS are in huge concentration, NO induces transcription and activity of SOD, APX, and CAT. However, when ROS are necessary (e.g., for pathogen resistance), NO may inhibit APX, CAT, and NOX activity by the S-nitrosylation of cysteine residues, favoring cell death. NO also regulates GSH concentration in several ways. NO may react with GSH to form GSNO, the NO cell reservoir and main source of S-nitrosylation. GSNO could be decomposed by the GSNO reductase (GSNOR) to GSSG which, in turn, is reduced to GSH by glutathione reductase (GR). GSNOR may be also inhibited by S-nitrosylation and GR activated by NO. In conclusion, NO plays a central role in the tolerance of plants to climate change.

Nonlinear response of nitric oxide fluxes to fertilizer inputs and the impacts of agricultural intensification on tropospheric ozone pollution in Kenya

Crop yields in sub-Saharan Africa remain stagnant at 1 ton ha^-1 , and 260 million people lack access to adequate food resources. Order-of-magnitude increases in fertilizer use are seen as a critical step in attaining food security. This increase represents an unprecedented input of nitrogen (N) to African ecosystems and will likely be accompanied by increased soil emissions of nitric oxide (NO). NO is a precursor to tropospheric ozone, an air pollutant and greenhouse gas. Emissions of NO from soils occur primarily during denitrification and nitrification, and N input rates are a key determinant of emission rates. We established experimental maize plots in western Kenya to allow us to quantify the response function relating NO flux to N input rate during the main 2011 and 2012 growing seasons. NO emissions followed a sigmoid response to fertilizer inputs and have emission factors under 1% for the roughly two-month measurement period in each year, although linear and step relationships could not be excluded in 2011. At fertilization rates above 100 kg N ha^-1 , NO emissions increased without a concomitant increase in yields. We used the geos-chem chemical transport model to evaluate local impacts of increased NO emissions on tropospheric ozone concentrations. Mean 4-hour afternoon tropospheric ozone concentrations in Western Kenya increased by up to roughly 2.63 ppbv under fertilization rates of 150 kg N ha^-1 or higher. Using AOT40, a metric for assessing crop damage from ozone, we find that the increased ozone concentrations result in an increase in AOT40 exposure of approximately 110 ppbh for inputs of 150 kg N ha^-1 during the March-April-May crop growing season, compared with unfertilized simulations, with negligible impacts on crop productivity. Our results suggest that it may be possible to manage Kenyan agricultural systems for high yields while avoiding substantial impacts on air quality.

Acidity and organic matter promote abiotic nitric oxide production in drying soils

Soils are an important source of NO, particularly in dry lands because of trade-offs that develop between biotic and abiotic NO-producing processes when soils dry out. Understanding how drier climates may offset the balance of these trade-offs as soils transition toward more arid states is, therefore, critical to estimating global NO budgets, especially because drylands are expected to increase in size. We measured NO emission pulses after wetting soils from similar lithologies along an altitudinal gradient in the Sierra Nevada, CA, where mean annual precipitation varied from 670 to 1500 mm. Along the gradient, we measured field NO emissions, and used chloroform in the laboratory to reduce microbial activity and partition between biotic and abiotic NO-producing processes (i.e., chemodenitrification). Field NO emission pulses were lowest in the acidic and SOM-rich soils (4-72 ng NO-N m^-2 s^-1 ), but were highest in the high-elevation barren site (~560 ng NO-N m^-2 s^-1 ). In the laboratory, NO emission pulses were up to 19× greater in chloroform-treated soils than in the controls, and these abiotic pulses increased with elevation as pH decreased (6.2-4.4) and soil organic matter (SOM) increased (18-157 mg C g^-1 ). Drought can shift the balance between the biotic and abiotic processes that produce NO, favoring chemodenitrification during periods when biological processes become stressed. Acidic and SOM-rich soils, which typically develop under mesic conditions, are most vulnerable to N loss via NO as interactions between pH, SOM, and drought stimulate chemodenitrification.

Aridity and plant uptake interact to make dryland soils hotspots for nitric oxide (NO) emissions

Nitric oxide (NO) is an important trace gas and regulator of atmospheric photochemistry. Theory suggests moist soils optimize NO emissions, whereas wet or dry soils constrain them. In drylands, however, NO emissions can be greatest in dry soils and when dry soils are rewet. To understand how aridity and vegetation interact to generate this pattern, we measured NO fluxes in a California grassland, where we manipulated vegetation cover and the length of the dry season and measured [δ(15)-N]NO and [δ(18)-O]NO following rewetting with (15)N-labeled substrates. Plant N uptake reduced NO emissions by limiting N availability. In the absence of plants, soil N pools increased and NO emissions more than doubled. In dry soils, NO-producing substrates concentrated in hydrologically disconnected microsites. Upon rewetting, these concentrated N pools underwent rapid abiotic reaction, producing large NO pulses. Biological processes did not substantially contribute to the initial NO pulse but governed NO emissions within 24 h postwetting. Plants acted as an N sink, limiting NO emissions under optimal soil moisture. When soils were dry, however, the shutdown in plant N uptake, along with the activation of chemical mechanisms and the resuscitation of soil microbial processes upon rewetting, governed N loss. Aridity and vegetation interact to maintain a leaky N cycle during periods when plant N uptake is low, and hydrologically disconnected soils favor both microbial and abiotic NO-producing mechanisms. Under increasing rates of atmospheric N deposition and intensifying droughts, NO gas evasion may become an increasingly important pathway for ecosystem N loss in drylands.

Multivariate regulation of soil CO2 and N2 O pulse emissions from agricultural soils

Climate and land-use models project increasing occurrence of high temperature and water deficit in both agricultural production systems and terrestrial ecosystems. Episodic soil wetting and subsequent drying may increase the occurrence and magnitude of pulsed biogeochemical activity, affecting carbon (C) and nitrogen (N) cycles and influencing greenhouse gas (GHG) emissions. In this study, we provide the first data to explore the responses of carbon dioxide (CO2 ) and nitrous oxide (N2 O) fluxes to (i) temperature, (ii) soil water content as percent water holding capacity (%WHC), (iii) substrate availability throughout, and (iv) multiple soil drying and rewetting (DW) events. Each of these factors and their interactions exerted effects on GHG emissions over a range of four (CO2 ) and six (N2 O) orders of magnitude. Maximal CO2 and N2 O fluxes were observed in environments combining intermediate %WHC, elevated temperature, and sufficient substrate availability. Amendments of C and N and their interactions significantly affected CO2 and N2 O fluxes and altered their temperature sensitivities (Q10 ) over successive DW cycles. C amendments significantly enhanced CO2 flux, reduced N2 O flux, and decreased the Q10 of both. N amendments had no effect on CO2 flux and increased N2 O flux, while significantly depressing the Q10 for CO2 , and having no effect on the Q10 for N2 O. The dynamics across DW cycles could be attributed to changes in soil microbial communities as the different responses to wetting events in specific group of microorganisms, to the altered substrate availabilities, or to both. The complex interactions among parameters influencing trace gas fluxes should be incorporated into next generation earth system models to improve estimation of GHG emissions.

Diurnal and spatial variations of soil NOx fluxes in the northern steppe of China

NOx emissions from biogenic sources in soils play a significant role in the gaseous loss of soil nitrogen and consequent changes in tropospheric chemistry. In order to investigate the characteristics of NOx fluxes and factors influencing these fluxes in degraded sandy grasslands in northern China, diurnal and spatial variations of NOx fluxes were measured in situ. A dynamic flux chamber method was used at eight sites with various vegetation coverages and soil types in the northern steppe of China in the summer season of 2010. Fluxes of NOx from soils with plant covers were generally higher than those in the corresponding bare vegetation-free soils, indicating that the canopy plays an important role in the exchange of NOx between soil and air. The fluxes of NOx increased in the daytime, and decreased during the nighttime, with peak emissions occurring between 12:00 and 14:00. The results of multiple linear regression analysis indicated that the diurnal variation of NOx fluxes was positively correlated with soil temperature (P<0.05) and negatively with soil moisture content (P<0.05). Based on measurement over a season, the overall variation in NOx flux was lower than that of soil nitrogen contents, suggesting that the gaseous loss of N from the grasslands of northern China was not a significant contributor to the high C/N in the northern steppe of China. The concentration of NOx emitted from soils in the region did not exceed the 1-hr National Ambient Air Quality Standard (0.25 mg/m3).

Annual nitric and nitrous oxide fluxes from Chinese subtropical plastic greenhouse and conventional vegetable cultivations

As intensive vegetable cultivation is rapidly expanding in China and elsewhere worldwide, its environmental consequences on nitrous oxide (N(2)O) and nitric oxide (NO) emissions deserve attention. We measured N(2)O and NO fluxes simultaneously for a full year from Chinese subtropical vegetable fields. Clearly, both N(2)O and NO emissions varied greatly in different vegetable crop seasons within a year, highlighting the importance of whole-year measurement for achieving temporally accurate annual direct emission factors. A revised "hole-in-the-pipe" model well described quantitative relationships between N(2)O plus NO fluxes and soil-specific conditions. Annual background N(2)O and NO emissions were 0.73-5.0 and 0.26-0.56 kg N ha(-1) yr(-1), respectively, for the vegetable cultivations. The farmers' fertilization practice increased N(2)O and NO emissions. Annual direct emission factors for greenhouse and conventional vegetable fields, respectively, were 1.1% and 1.9% for N(2)O, and 0.36% and 0.32% for NO, indicating there is a need to consider a differentiation of emission factors for managed vegetable cultivations.

Sources of variation in home lawn soil nitrogen dynamics

Urban, suburban, and exurban lawns are an increasingly important ecosystem type in the United States. There is great concern about the environmental performance of lawns, especially nitrate (NO) leaching and nitrous oxide (NO) flux associated with nitrogen (N) fertilizer use. Previous studies of lawn N dynamics have produced conflicting results, with some studies showing high NO leaching and NO flux and others showing lower losses and high retention and cycling of N inputs. We hypothesized that this variation is caused by differences in lawn management and soil properties that control root and soil organic matter (SOM) dynamics that influence N cycling processes. We tested these hypotheses by making measurements of soil NO, root biomass, rates of potential net N mineralization and nitrification, NO flux, and SOM levels in samples from the front and backyards of residential homes in suburban and exurban neighborhoods with contrasting soil types in the Baltimore metropolitan area. There were no differences between front and backyards, between suburban and exurban neighborhoods, or between different soil types. Further, there were no significant relationships between root biomass, SOM, soil NO levels, and NO fluxes. These results suggest that lawns have uniformly high rates of plant productivity that underlies high levels of SOM and N retention in these ecosystems across the Baltimore metropolitan area.

Winter climate change effects on soil C and N cycles in urban grasslands

Despite growing recognition of the role that cities have in global biogeochemical cycles, urban systems are among the least understood of all ecosystems. Urban grasslands are expanding rapidly along with urbanization, which is expected to increase at unprecedented rates in upcoming decades. The large and increasing area of urban grasslands and their impact on water and air quality justify the need for a better understanding of their biogeochemical cycles. There is also great uncertainty about the effect that climate change, especially changes in winter snow cover, will have on nutrient cycles in urban grasslands. We aimed to evaluate how reduced snow accumulation directly affects winter soil frost dynamics, and indirectly greenhouse gas fluxes and the processing of carbon (C) and nitrogen (N) during the subsequent growing season in northern urban grasslands. Both artificial and natural snow reduction increased winter soil frost, affecting winter microbial C and N processing, accelerating C and N cycles and increasing soil : atmosphere greenhouse gas exchange during the subsequent growing season. With lower snow accumulations that are predicted with climate change, we found decreases in N retention in these ecosystems, and increases in N2 O and CO2 flux to the atmosphere, significantly increasing the global warming potential of urban grasslands. Our results suggest that the environmental impacts of these rapidly expanding ecosystems are likely to increase as climate change brings milder winters and more extensive soil frost.

Inconsistent definitions of "urban" result in different conclusions about the size of urban carbon and nitrogen stocks

There is conflicting evidence about the importance of urban soils and vegetation in regional C budgets that is caused, in part, by inconsistent definitions of "urban" land use. We quantified urban ecosystem contributions to C stocks in the Boston (Massachusetts, USA) Metropolitan Statistical Area (MSA) using several alternative urban definitions. Development altered aboveground and belowground C and N stocks, and the sign and magnitude of these changes varied by land use and development intensity. Aboveground biomass (live trees, dbh > or = 5 cm) for the MSA was 7.2 +/- 0.4 kg C/m2 (mean +/- SE), reflecting a high proportion of forest cover. Vegetation C was highest in forest (11.6 +/- 0.5 kg C/m2), followed by residential (4.6 +/- 0.5 kg C/m2), and then other developed (2.0 +/- 0.4 kg C/m2) land uses. Soil C (0-10 cm depth) followed the same pattern of decreasing C concentration from forest, to residential, to other developed land uses (4.1 +/- 0.1, 4.0 +/- 0.2, and 3.3 +/- 0.2 kg C/m2, respectively). Within a land use type, urban areas (which we defined as > 25% impervious surface area [ISA] within a 1-km(2) moving window) generally contained less vegetation C, but slightly more soil C, than nonurban areas. Soil N concentrations were higher in urban areas than nonurban areas of the same land use type, except for residential areas, which had similarly high soil N concentrations. When we compared our definition of urban to other commonly used urban extents (U.S. Census Bureau, Global Rural-Urban Mapping Project [GRUMP], and the MSA itself), we found that urban soil (1 m depth) and vegetation C stocks spanned a wide range, from 14.4 +/- 0.8 to 54.5 +/- 3.4 Tg C and from 4.2 +/- 0.4 to 27.3 +/- 3.2 Tg C, respectively. Conclusions about the importance of urban soils and vegetation to regional C and N stocks are very sensitive to the definition of urban used by the investigators. Urban areas, regardless of definition, are rapidly expanding in their extent; a systematic understanding of how our development patterns influence ecosystems is necessary to inform future development choices.

Denitrification in suburban lawn soils

There is great uncertainty about the fate of nitrogen (N) added to urban and suburban lawns. We used direct flux and in situ chamber methods to measure N and NO fluxes from lawns instrumented with soil O sensors. We hypothesized that soil O, moisture, and available NO were the most important controls on denitrification and that N and NO fluxes would be high following fertilizer addition and precipitation events. While our results support these hypotheses, the thresholds of soil O, moisture, and NO availability required to see significant N fluxes were greater than expected. Denitrification rates were high in saturated, fertilized soils, but low under all other conditions. Annual denitrification was calculated to be 14.0 ± 3.6 kg N ha yr, with 5% of the growing season accounting for >80% of the annual activity. Denitrification is thus an important means of removing reactive N in residential landscapes, but varies markedly in space, time, and with factors that affect soil saturation (texture, structure, compaction) and NO availability (fertilization). Rates of in situ NO flux were low; however, when recently fertilized soils saturated with water were incubated in the laboratory, we saw extraordinarily high rates of NO production for the first few hours of incubation, followed by rapid NO consumption later in the experiment. These findings indicate a lag time between accelerated NO production and counterbalancing increases in NO consumption; thus, we cannot yet conclude that lawns are an insignificant source of NO in our study area.

Carbonyl sulfide and dimethyl sulfide fluxes in an urban lawn and adjacent bare soil in Guangzhou, China

Carbonyl sulfide (COS) and dimethyl sulfide (DMS) fluxes from an urban Cynodon dactylon lawn and adjacent bare soil were measured during April-July 2005 in Guangzhou, China. Both the lawn and bare soil acted as sinks for COS and sources for DMS. The mean fluxes of COS and DMS in the lawn (-19.27 and 18.16 pmol/(m2 sec), respectively) were significantly higher than those in the bare soil (-9.89 and 9.35 pmol/(m2 sec), respectively). Fluxes of COS and DMS in mowed lawn were also higher than those in bare soils. Both COS and DMS fluxes showed diurnal variation with detectable but much lower values in the nighttime than in the daytime. COS fluxes were related significantly to temperature and the optimal temperature for COS uptake was 29 degrees C. While positive linear correlations were found between DMS fluxes and temperature. COS fluxes increased linearly with ambient COS mixing ratios, and had a compensation point of 336 ppt.

Nitrate leaching and nitrous oxide flux in urban forests and grasslands

Urban landscapes contain a mix of land-use types with different patterns of nitrogen (N) cycling and export. We measured nitrate (NO(3)(-)) leaching and soil:atmosphere nitrous oxide (N(2)O) flux in four urban grassland and eight forested long-term study plots in the Baltimore, Maryland metropolitan area. We evaluated ancillary controls on these fluxes by measuring soil temperature, moisture, and soil:atmosphere fluxes of carbon dioxide on these plots and by sampling a larger group of forest, grass, and agricultural sites once to evaluate soil organic matter, microbial biomass, and potential net N mineralization and nitrification. Annual NO(3)(-) leaching ranged from 0.05 to 4.1 g N m(-2) yr(-1) and was higher in grass than forest plots, except in a very dry year and when a disturbed forest plot was included in the analysis. Nitrous oxide fluxes ranged from 0.05 to >0.3 g N m(-2) yr(-1), with few differences between grass and forest plots and markedly higher fluxes in wet years. Differences in NO(3)(-) leaching and N(2)O flux between forests and grasslands were not as high as expected given the higher frequency of disturbance and fertilization in the grasslands. Carbon dioxide flux, organic matter, and microbial biomass were as high or higher in urban grasslands than in forests, suggesting that active carbon cycling creates sinks for N in vegetation and soil in these ecosystems. Although urban grasslands export more N to the environment than native forests, they have considerable capacity for N retention that should be considered in evaluations of land-use change.

Recovery of nitrogen pools and processes in degraded riparian zones in the southern appalachians

Establishment of riparian buffers is an effective method for reducing nutrient input to streams. However, the underlying biogeochemical processes are not fully understood. The objective of this 4-yr study was to examine the effects of riparian zone restoration on soil N cycling mechanisms in a mountain pasture previously degraded by cattle. Soil inorganic N pools, fluxes, and transformation mechanisms were compared across the following experimental treatments: (i) a restored area with vegetation regrowth; (ii) a degraded riparian area with simulated effects of continued grazing by compaction, vegetation removal, and nutrient addition (+N); and (iii) a degraded riparian area with simulated compaction and vegetation removal only (-N). Soil solution NO(3)(-) concentrations and fluxes of inorganic N in overland flow were >90% lower in the restored treatment relative to the degraded (+N) treatment. Soil solution NO(3)(-) concentrations decreased more rapidly in the restored treatment relative to the degraded (-N) following cattle (Bos taurus) exclusion. Mineralization and nitrification rates in the restored treatment were similar to the degraded (-N) treatment and, on average, 75% lower than in the degraded (+N) treatment. Nitrogen trace gas fluxes indicated that restoration increased the relative importance of denitrification, relative to nitrification, as a pathway by which N is diverted from the receiving stream to the atmosphere. Changes in soil nutrient cycling mechanisms following restoration of the degraded riparian zone were primarily driven by cessation of N inputs. The recovery rate, however, was influenced by the rate of vegetation regrowth.