Assessing Nitrogen-Saturation in a Seasonally Dry Chaparral Watershed: Limitations of Traditional Indicators of N-Saturation

To evaluate nitrogen (N) saturation in xeric environments, we measured hydrologic N losses, soil N pools, and microbial processes, and developed an N-budget for a chaparral catchment (Sierra Nevada, California) exposed to atmospheric N inputs of approximately 8.5 kg N ha^?1 y^?1. Dual-isotopic techniques were used to trace the sources and processes controlling nitrate (NO₃ ^?) losses. The majority of N inputs occurred as ammonium. At the onset of the wet season (November to April), we observed elevated streamwater NO₃ ^? concentrations (up to 520 µmol l^?1), concomitant with the period of highest gaseous N-loss (up to 500 ng N m^?2 s^?1) and suggesting N-saturation. Stream NO₃ ^? δ¹⁵N and δ¹⁸O and soil N measurements indicate that nitrification controlled NO₃ ^? losses and that less than 1% of the loss was of atmospheric origin. During the late wet season, stream NO₃ ^? concentrations decreased (to <2 µmol l^?1) as did gaseous N emissions, together suggesting conditions no longer indicative of N-saturation. We propose that chaparral catchments are temporarily N-saturated at ≤8.5 kg N ha^?1 y^?1, but that N-saturation may be difficult to reach in ecosystems that inherently leak N, thereby confounding the application of N-saturation indicators and annual N-budgets. We propose that activation of N sinks during the typically rainy winter growing season should be incorporated into the assessment of ecosystem response to N deposition. Specifically, the N-saturation status of chaparral may be better assessed by how rapidly catchments transition from N-loss to N-retention.     

流域人类活动净氮输入量的估算、不确定性及影响因素

人类活动使得大量的氮素进入流域生态系统,大量氮的盈余导致了一系列生态环境问题的出现。为了评估人类活动对流域生态系统的影响,Howarth等于1996年提出了人类活动净氮输入(NANI)的概念。综述了当前人类活动净氮输入的估算方法、不确定性及影响因素,并得到以下结论:导致NANI估算结果的不确定性原因主要有:内涵分歧、数据来源、尺度转换、估算方法的分歧。影响NANI的主要因素包括:各输入项、人口密度、土地利用组成;对于各输入项而言,化肥施用是最主要的氮素输入来源,占人类活动净氮输入总量的79.0%,其次为作物固氮,占17.6%,食品/饲料氮净输入量占-14.5%,大气沉降占15.7%;对于人口密度,NANI随着人口密度的增大而增大,当人口密度高于100人/km2,人口密度对NANI的影响趋于稳定,其他因素起主导作用。对于土地利用组成:NANI与森林面积比例成负相关,而与耕地面积比例成正相关。     

High Atmospheric Nitrate Inputs and Nitrogen Turnover in Semi-arid Urban Catchments

The influx of atmospheric nitrogen to soils and surfaces in arid environments is of growing concern due to increased N emissions and N usage associated with urbanization. Atmospheric nitrogen inputs to the critical zone can occur as wet (rain or snow) or dry (dust or aerosols) deposition, and can lead to eutrophication, soil acidification, and groundwater contamination through leaching of excess nitrate. The objective of this research was to use the δ¹⁵N, δ¹⁸O, and Δ¹⁷O values of atmospheric nitrate (NO₃ ^?) (precipitation and aerosols) and NO₃ ^? in runoff to assess the importance of N deposition and turnover in semi-arid urban watersheds. Data show that the fractions of atmospheric NO₃ ^? exported from all the urban catchments, throughout the study period, were substantially higher than in nearly all other ecosystems studied with mean atmospheric contributions of 38% (min 0% and max 82%). These results suggest that catchment and stream channel imperviousness enhance atmospheric NO₃ ^? export due to inefficient N cycling and retention. In contrast, catchment and stream channel perviousness allow for enhanced N processing and therefore reduced atmospheric NO₃ ^? export. Overall high fractions of atmospheric NO₃ ^? were primarily attributed to slow N turn over in arid/semi-arid ecosystems. A relatively high fraction of nitrification NO₃ ^? (~30%) was found in runoff from a nearly completely impervious watershed (91%). This was attributed to nitrification of atmospheric NH₄ ⁺ in dry-deposited dust, suggesting that N nitrifiers have adapted to urban micro niches. Gross nitrification rates based on NO₃ ^? Δ¹⁷O values ranged from a low 3.04 ± 2 kg NO₃-N km^?2 day^?1 in highly impervious catchments to a high of 10.15 ± 1 kg NO₃-N km^?2 day^?1 in the low density urban catchment. These low gross nitrification rates were attributed to low soil C:N ratios that control gross autotrophic nitrification by regulating gross NH₄ ⁺ production rates.     

Stream Nitrate Responds Rapidly to Decreasing Nitrate Deposition

Ecosystem acidification and eutrophication resulting from increased deposition of dissolved inorganic nitrogen (DIN) are issues of increasing global concern. Consequently, costly policy decisions are being implemented to decrease nitrogen oxide (NO _x ) emissions. Although declining DIN deposition along with rapid declines of DIN in surface waters have been reported in parts of Europe, the same observation is just emerging in North America. Here we find a significant decline in bulk deposition NO₃ ⁻ during the later part of a 28-year record in southcentral Ontario, Canada. Despite high N retention and substantial inter-annual variability in the long-term record due to periods of drought, we find significant declines in annual NO₃ ⁻ concentrations and export at six out of 11 streams that drain upland-dominated catchments. In contrast, five streams draining primarily wetland-dominated catchments with lower levels of NO₃ ⁻ show no decreasing trend in NO₃ ⁻ concentration or export. The rapid response in stream NO₃ ⁻ to declining atmospheric inputs was observed at sites with historically moderate inputs of DIN (~870 mg m⁻² y⁻¹) in bulk deposition. Topographic features such as slope, and related catchment features including wetland cover, appear to influence which catchments will respond positively to declining DIN deposition. These findings force us to revise our original conceptualization of the N saturation status of these catchments.     

Atmospheric nitrogen inputs and losses along an urbanization gradient from Boston to Harvard Forest,MA

Urbanization alters nitrogen (N) cycling, but the spatiotemporal distribution and impact of these alterations on ecosystems are not well-quantified. We measured atmospheric inorganic N inputs and soil leaching losses along an urbanization gradient from Boston, MA to Harvard Forest in Petersham, MA. Atmospheric N inputs at urban sites (12.3 ± 1.5 kg N ha^?1 year^?1) were significantly greater than non-urban (5.7 ± 0.5 kg N ha^?1 year^?1) sites with NH₄ ⁺ (median value of 77 ± 4 %) contributing thrice as much as NO₃ ^?. Proximity to urban core correlated positively with NH₄ ⁺ (R² = 0.57, p = 0.02) and total inorganic N inputs (R² = 0.61, p = 0.01); on-road CO₂ emissions correlated positively with NO ₃ ^? inputs (R² = 0.74, p = 0.003). Inorganic N leaching rates correlated positively with atmospheric N input rates (R² = 0.61, p = 0.01), but did not differ significantly between urban and non-urban sites (p > 0.05). Our empirical measurements of atmospheric N inputs are greater for urban areas and less for rural areas compared to modeled regional estimates of N deposition. Five of the nine sites had NO ₃ ^? leached that came almost entirely from nitrification, indicating that the NO₃ ^? in leachate came from biological processes rather than directly passing through the soil. A significant proportion (17–100 %) of NO ₃ ^? leached from the other four sites came directly from the atmosphere. Surprisingly, the four sites where atmospheric sources made up the largest proportion of leachate NO₃ ^? also had relatively low N leaching rates, suggesting that atmospheric N inputs added to terrestrial ecosystems can move to multiple sinks and losses simultaneously, rather than being lost via leaching only after abiotic and biotic sinks have become saturated. This study improves our understanding of atmospheric N deposition and leaching in urban ecosystems, and highlights the need to incorporate urbanization effects in N deposition models.     

Denitrification kinetics and denitrifier abundances in sediments of lakes receiving atmospheric nitrogen deposition (Colorado, USA)

The transport and deposition of anthropogenic nitrogen (N) to downwind ecosystems is significant and can be a dominant source of new N to many watersheds. Bacterially mediated denitrification in lake sediments may ameliorate the effects of N loading by permanently removing such inputs. We measured denitrification in sediments collected from lakes in the Colorado Rocky Mountains (USA) receiving elevated (5–8?kg?N?ha^?1?y^?1) or low (<2?kg?N?ha^?1?y^?1) inputs of atmospheric N deposition. The nitrate (NO₃ ^?) concentration was significantly greater in high-deposition lakes (11.3?μmol?l^?1) compared to low-deposition lakes (3.3?μmol?l^?1). Background denitrification was positively related to NO₃ ^? concentrations and we estimate that the sampled lakes are capable of removing a significant portion of N inputs via sediment denitrification. We also conducted a dose–response experiment to determine whether chronic N loading has altered sediment denitrification capacity. Under Michaelis–Menten kinetics, the maximum denitrification rate and half-saturation NO₃ ^? concentration did not differ between deposition regions and were 765?μmol?N?m^?2?h^?1 and 293?μmol?l^?1?NO₃ ^?, respectively, for all lakes. We enumerated the abundances of nitrate- and nitrite-reducing bacteria and found no difference between high- and low-deposition lakes. The abundance of these bacteria was related to available light and bulk sediment resources. Our findings support a growing body of evidence that lakes play an important role in N removal and, furthermore, suggest that current levels of N deposition have not altered the abundance of denitrifying bacteria or saturated the capacity for sediment denitrification.     

Spatial and temporal variation in phosphorus budgets for 24 watersheds in the Lake Erie and Lake Michigan basins

We estimated net anthropogenic phosphorus inputs (NAPI) to 18 Lake Michigan (LM) and 6 Lake Erie (LE) watersheds for 1974, 1978, 1982, 1987, and 1992. NAPI quantifies all anthropogenic inputs of P (fertilizer use, atmospheric deposition, and detergents) as well as trade of P in food and feed, which can be a net input or output. Fertilizer was the dominant input overall, varying by three orders of magnitude among the 24 watersheds, but detergent was the largest input in the most urbanized watershed. NAPI increased in relation to area of disturbed land (R² = 0.90) and decreased with forested and wetland area (R² = 0.90). Export of P by rivers varied with NAPI, especially for the 18 watersheds of LM (R² = 0.93), whereas the relationship was more variable among the six LE watersheds (R² = 0.59). On average, rivers of the LE watersheds exported about 10% of NAPI, whereas LM watersheds exported 5% of estimated NAPI. A comparison of our results with others as well as nitrogen (N) budgets suggests that fractional export of P may vary regionally, as has been reported for N, and the proportion of P inputs exported by rivers appears lower than comparable findings with N.     

Nitrogen budget and riverine nitrogen output in a rice paddy dominated agricultural watershed in eastern China

The nitrogen (N) budget calculation approach is a useful means of evaluating the impact of human activity on the N cycle. Field scale N budget calculations may ignore the interactions between landscapes, and regional scale calculations rely on statistical data and indirect parameters. Watershed scale budget calculations allow for a more direct quantification of N inputs and outputs. We conducted N budget calculations for a rice paddy-dominated agricultural watershed in eastern China for 2007?C2009, based on intensive monitoring of stream N dynamics, atmospheric deposition, ammonia (NH₃) volatilization and household interviews about N-related agricultural activities. The results showed that although total N input to the watershed was up to 280 kg N ha^?1 year^?1, riverine discharge was only 4.2 kg N ha^?1 year^?1, accounting for 1.5% of the total N input, and was further reduced to 2.0 kg N ha^?1 year^?1 after reservoir storage and/or denitrification removal. The low riverine N output was because of the characteristics of the rice paddy-dominated landscape, which intercepts run-off and enhances soil denitrification. The watershed actually purified the N in rainwater, as N concentrations in river discharge were much lower than those in rain water. Major N outputs included food/feed export, NH₃ volatilization from chemical fertilizer and manure, and emissions from crop residue burning. Net reactive gaseous emissions (emissions minus deposition) accounted for 5.5% of the total N input, much higher than riverine discharge. Therefore, the agricultural N cycle in such paddy-dominated watersheds impacts the environment mainly through gas exchange rather than water discharge.     

Nitrogen input together with ecosystem nitrogen enrichment predict nitrate leaching from European forests

The IFEF database (Indicators of Forest Ecosystem Functioning), consisting of nitrogen deposition, nitrate leaching fluxes, and soil and ecosystem characteristics, is analysed to evaluate the C/N ratio in the organic horizon as an indicator of nitrate leaching. One hundred and eighty one forests are examined, from countries across Europe ranging from boreal to Mediterranean regions, encompassing broadleaf and coniferous sites and plot and catchment studies. N input in throughfall ranges from less than 1 kg N ha^?1 y^?1 in northern Norway and Finland to greater than 60 kg N ha^?1 y^?1 in the Netherlands and Czech Republic. The amount of NO₃^– leached covers a smaller range, between 1 and 40 kg N ha^?1 y^?1. Nitrate leaching is strongly dependent on the amount of nitrogen deposited in throughfall (N input) and simply adding the C/N ratio in the organic horizon to a regression equation does not improve this relationship. However, when the data are stratified based on C/N ratios less than or equal to 25 and greater than 25, highly significant relationships (P < 0.05) are observed between N input and NO₃^– leached. The slope of the relationship for those sites where C/N ratio is ≤ 25 (′nitrogen enriched′ sites) is twice that for those sites where C/N ratio is > 25. These empirical relationships may be used to identify which forested ecosystems are likely to show elevated rates of nitrate leaching under predicted future nitrogen deposition scenarios. Elevated NO₃^– leaching also shows a relationship with soil pH, with high rates of NO₃^– leaching only observed at sites with a pH < 4.5 and N inputs > 30 kg N ha^?1 y^?1. Tree age and species have no significant impact on the ecosystem response to N input at a regional scale.     

Sources of nitrate export during rain-on-snow events at forested catchments

Rain-on-snow (ROS) events are major drivers of nitrate (NO₃-N) export from seasonally snow-covered forested catchments and may cause episodic declines in stream pH. High intensity monitoring of throughfall, snow pack and stream water draining two proximal catchments (Harp 3A and Harp 6A) with very different NO₃-N export revealed that a very small percentage of ROS-induced stream discharge originates from throughfall and melting snow (new water; average = 6.4 %). However, this new water has a very high concentration of NO₃-N (throughfall/snowmelt average = 498 μg/L) compared with baseflow (average = 7.3 μg/L in Harp 6A; average = 41 μg/L in Harp 3A) and as a result, throughfall and snowmelt contribute the majority of NO₃-N export (average = 62 %) during ROS events. In contrast, concentrations of sulphate, dissolved organic carbon and calcium in rain and snowpack are similar to baseflow and therefore ROS-induced declines in pH (often to below pH 6.0) are attributed entirely to increases in NO₃-N concentration. Differences in absolute magnitude of ROS NO₃-N export between catchments are explained through differences in baseflow NO₃-N concentrations. The frequency and magnitude of ROS events in this region are affected by both NO₃-N deposition and winter temperature, and thus the impact of these events in the future depends on changes in both atmospheric deposition and winter climate.     

Glacier ecosystem response to episodic nitrogen enrichment in Svalbard, European High Arctic

We describe the climatology, hydrology and biogeochemistry of an extreme nitrogen deposition event that occurred in the highly glacierised environment of the European High Arctic during June 1999. Meteorological analysis, three-dimensional air mass trajectories and a 3D transport model show that blocking high pressures over Scandinavia and the rapid advection of western European pollution toward Svalbard were sufficient to cause the most concentrated (1.15 ppm NO₃–N and 1.20 ppm NH₄–N), high magnitude (total 26 mm and up to 2.4 mm h^?1 at 30 m above sea level) nitrogen deposition event on record in this sensitive, high Arctic environment (78.91° N, 11.93° E). Since the event occurred when much of the catchment remained frozen or under snow cover, microbial utilisation of nitrogen within snowpacks and perennially unfrozen subglacial sediments, rather than soils, were mostly responsible for reducing N export. The rainfall event occurred long before the annual subglacial outburst flood and so prolonged (ca. 10 day) water storage at the glacier bed further enhanced the microbial assimilation. When the subglacial outburst eventually occurred, high runoff and concentrations of NO₃ ^? (but not NH₄ ⁺) returned in the downstream rivers. Assimilation accounted for between 53 and 72% of the total inorganic nitrogen deposited during the event, but the annual NO₃ ^? and NH₄ ⁺ runoff yields were still enhanced by up to 5 and 40 times respectively. Episodic atmospheric inputs of reactive nitrogen can therefore directly influence the biogeochemical functioning of High Arctic catchments, even when microbial activity takes place beneath a glacier at a time when terrestrial soil ecosystems remain frozen and unresponsive.     

Landscape-level nitrogen import and export in an ecosystem with complex terrain, Colorado Front Range

Knowledge of import, export, and transport of nitrogen (N) in headwater catchments is essential for understanding ecosystem function and water quality in mountain ecosystems, especially as these ecosystems experience increased anthropogenic N deposition. In this study, we link spatially explicit soil and stream data at the landscape scale to investigate import, export and transport of N in a 0.89?km² site at the alpine-subalpine ecotone in the Front Range of the Rocky Mountains, Colorado, U.S.A. For two of the major N inputs to our site, N deposition in the snowpack and N fixation, a complementary relationship was found across the study site, with greater abundance of N-fixing plants in areas with less snow and substantial snow inputs in areas with low N fixer abundance. During the initial phases of snowmelt, mixing model end members for oxygen isotopes in nitrate (NO₃ ^?) indicated that a substantial quantity of NO₃ ^? is transported downhill into the forested subalpine without being assimilated by soil microbes. After this initial pulse, much less NO₃ ^? entered the stream and most but not all of it was microbial in origin. Rising δ¹⁵N in stream NO₃ ^? indicated greater influence of fractionating processes such as denitrification later in the season. NO₃ ^? from both atmospheric and microbial sources was not exported from our site because it was consumed within the first several hundred meters of the stream; ultimately, N exports were in the form of dissolved organic nitrogen (DON) and particulate N (PN). The results of this study suggest that the highest elevation dry alpine meadows rely more heavily on N fixation as an N source and experience less of the effects of anthropogenic N deposition than mid and lower elevation areas that have more snow. Our data also suggest that mid-elevation krummholz, moist meadows, and talus slopes are exporting N as NO₃ ^? shortly after the onset of snowmelt, but that this NO₃ ^? is rapidly consumed as the stream flows through the subalpine forest. This consumption by assimilation and/or denitrification currently provides a buffer against increased inorganic N availability downstream.     

Bryophytes attenuate anthropogenic nitrogen inputs in boreal forests

Productivity in boreal ecosystems is primarily limited by available soil nitrogen (N), and there is substantial interest in understanding whether deposition of anthropogenically derived reactive nitrogen (N_r) results in greater N availability to woody vegetation, which could result in greater carbon (C) sequestration. One factor that may limit the acquisition of N_r by woody plants is the presence of bryophytes, which are a significant C and N pool, and a location where associative cyanobacterial N‐fixation occurs. Using a replicated stand‐scale N‐addition experiment (five levels: 0, 3, 6, 12, and 50 kg N ha^?1 yr^?1; n=6) in the boreal zone of northern Sweden, we tested the hypothesis that sequestration of N_r into bryophyte tissues, and downregulation of N‐fixation would attenuate N_r inputs, and thereby limit anthropogenic N_r acquisition by woody plants. Our data showed that N‐fixation per unit moss mass and per unit area sharply decreased with increasing N addition. Additionally, the tissue N concentrations of Pleuorzium schreberi increased and its biomass decreased with increasing N addition. This response to increasing N addition caused the P. schreberi N pool to be stable at all but the highest N addition rate, where it significantly decreased. The combined effects of changed N‐fixation and P. schreberi biomass N accounted for 56.7% of cumulative N_r additions at the lowest N_r addition rate, but only a minor fraction for all other treatments. This ‘bryophyte effect’ can in part explain why soil inorganic N availability and acquisition by woody plants (indicated by their δ¹⁵N signatures) remained unchanged up to N addition rates of 12 kg ha^?1 yr^?1 or greater. Finally, we demonstrate that approximately 71.8% of the boreal forest experiences N_r deposition rates at or below 3 kg ha^?1 yr^?1, suggesting that bryophytes likely limit woody plant acquisition of ambient anthropogenic N_r inputs throughout a majority of the boreal forest.     

N2-fixation by methanotrophs sustains carbon and nitrogen accumulation in pristine peatlands

Symbiotic relationships between N₂-fixing prokaryotes and their autotrophic hosts are essential in nitrogen (N)-limited ecosystems, yet the importance of this association in pristine boreal peatlands, which store 25 % of the world’s soil (C), has been overlooked. External inputs of N to bogs are predominantly atmospheric, and given that regions of boreal Canada anchor some of the lowest rates found globally (~1 kg N ha^?1 year^?1), biomass production is thought to be limited primarily by N. Despite historically low N deposition, we show that boreal bogs have accumulated approximately 12–25 times more N than can be explained by atmospheric inputs. Here we demonstrate high rates of biological N₂-fixation in prokaryotes associated with Sphagnum mosses that can fully account for the missing input of N needed to sustain high rates of C sequestration. Additionally, N amendment experiments in the field did not increase Sphagnum production, indicating that mosses are not limited by N. Lastly, by examining the composition and abundance of N₂-fixing prokaryotes by quantifying gene expression of 16S rRNA and nitrogenase-encoding nifH, we show that rates of N₂-fixation are driven by the substantial contribution from methanotrophs, and not from cyanobacteria. We conclude biological N₂-fixation drives high sequestration of C in pristine peatlands, and may play an important role in moderating fluxes of methane, one of the most important greenhouse gases produced in peatlands. Understanding the mechanistic controls on biological N₂-fixation is crucial for assessing the fate of peatland carbon stocks under scenarios of climate change and enhanced anthropogenic N deposition.     

Dominance of biologically produced nitrate in upland waters of Great Britain indicated by stable isotopes

Atmospheric deposition of nitrogen (N) compounds is the major source of anthropogenic N to most upland ecosystems, where leaching of nitrate (NO ₃ ^? ) into surface waters contributes to eutrophication and acidification as well as indicating an excess of N in the terrestrial catchment ecosystems. Natural abundance stable isotopes ratios, ¹⁵N/¹⁴N and ¹⁸O/¹⁶O (the “dual isotope” technique) have previously been used in biogeochemical studies of alpine and forested ecosystems to demonstrate that most of the NO ₃ ^? in upland surface waters has been microbially produced. Here we present an application of the technique to four moorland catchments in the British uplands including a comparison of lakes and their stream inflows at two sites. The NO ₃ ^? concentrations of bulk deposition and surface waters at three sites are very similar. While noting the constraints imposed by uncertainty in the precise δ¹⁸O value for microbial NO ₃ ^? , however, we estimate that 79–98% of the annual mean NO ₃ ^? has been microbially produced. Direct leaching of atmospheric NO ₃ ^? is a minor component of catchment NO ₃ ^? export, although greater than in many similar studies in forested watersheds. A greater proportion of atmospheric NO ₃ ^? is seen in the two lake sites relative to their inflow streams, demonstrating the importance of direct NO ₃ ^? deposition to lake surfaces in catchments where terrestrial ecosystems intercept a large proportion of deposited N. The dominance of microbial sources of NO ₃ ^? in upland waters suggests that reduced and oxidised N deposition may have similar implications in terms of contributing to NO ₃ ^? leaching.     

Contemporary and pre-industrial global reactive nitrogen budgets

Increases and expansion of anthropogenic emissions of both oxidized nitrogen compounds, NO_x, and a reduced nitrogen compound, NH₃, have driven an increase in nitrogen deposition. We estimate global NO_x and NH₃ emissions and use a model of the global troposphere, MOGUNTIA, to examine the pre-industrial and contemporary quantities and spatial patterns of wet and dry NO_y and NH_x deposition. Pre-industrial wet plus dry NO_x and NH_x deposition was greatest for tropical ecosystems, related to soil emissions, biomass burning and lightning emissions. Contemporary NO_y+NH_x wet and dry deposition onto Northern Hemisphere (NH) temperate ecosystems averages more than four times that of preindustrial N deposition and far exceeds contemporary tropical N deposition. All temperate and tropical biomes receive more N via deposition today than pre-industrially. Comparison of contemporary wet deposition model estimates to measurements of wet deposition reveal that modeled and measured wet deposition for both NO ₃ ^– and NH ₄ ⁺ were quite similar over the U.S. Over Western Europe, the model tended to underestimate wet deposition of NO ₃ ^– and NH ₄ ⁺ but bulk deposition measurements were comparable to modeled total deposition. For the U.S. and Western Europe, we also estimated N emission and deposition budgets. In the U.S., estimated emissions exceed interpolated total deposition by 3-6 Tg N, suggesting that substantial N is transported offshore and/or the remote and rural location of the sites may fail to capture the deposition of urban emissions. In Europe, by contrast, interpolated total N deposition balances estimated emissions within the uncertainty of each.Abbreviations EMEP European Monitoring and Evaluation Program - GEIA Global Emissions Inventory Activity - NADP/NTN National Atmospheric Deposition Program/National Trends Network in the US - NH Northern Hemisphere - NH_x=NH₃+NH ₊ ⁴ NO_x=NO+NO₂ NO_y total odd nitrogen=NO_x+HNO₃+HONO+HO₂NO₂+NO₃+radical (NO₃ ^.)+Peroxyacetyl nitrates+N₂O₅+organic nitrates - SH Southern Hemisphere - Gg 10⁹ g - Tg 10¹² g     

Covariance in water- and nutrient budgets of Dutch peat polders: what governs nutrient retention?

Water and nutrient budgets were constructed for 13 low-lying peat polders in the Netherlands that varied in elevation relative to sea level (?0.2 to ?2.4 m below sea level), land use (7–70% of the total polder area covered by agriculture; largely dairy farming), and surface water prevalence (6–43%). Water balances were verified with chloride budgets and accepted when both met the criterion (total inflows ? total outflows)/(total inflows) <0.05. Apart from precipitation and evapotranspiration (overall means 913 vs. 600 mm), in- and outlet (171 vs. 420 mm) as well as in- and outward seepage (137 vs. 174 mm) were important items in the water budgets. Nutrient budgets, however, were dominated by terms related to agricultural land use (~60% of all inputs, 90% of N-removal and 80% of P removal) rather than water fluxes (8% and 5% of N and P inputs; 6 and 18% of outputs). After agriculture (200 kg N ha^?1 y^?1), mineralisation of the peat soil and atmospheric deposition appear to be important inputs (about 94 and 21 kg N ha^?1 y^?1). Major output terms were agricultural output (209 kg N ha^?1 y^?1) and denitrification (95 kg N ha^?1 y^?1). The average N budget was in balance (difference ~1 kg N ha^?1 y^?1), whereas P accumulated in most polders, particularly those under agriculture. The mean P surplus (15 kg P ha^?1 y^?1 in the 9 mainly agricultural polders) corresponds well with the accumulated difference observed elsewhere (700 kg P ha^?1 in the upper 50 cm in a nature reserve versus 1400 under agriculture) after over 50 years of dairy farming. Bulk retention of N and P in these polders is taking place in the peat soil, through temporary sorption to the matrix and N is lost through denitrification. In a principal components analysis combining land use, landscape pattern, water balance and nutrient budget terms, the three-first principal components explained 63% of the variability. The first component (PC) correlated strongly with the percentage of land under agriculture (r = 0.82) and negatively with the percentage covered by surface water (r = ?0.74). Most input and output terms of the nitrogen budget also correlated with this PC. The second PC covaried distinctly with the total area of a polder (r = ?0.79) and human population density at municipality level (r = 0.75). Phosphorus loads in inlet and outlet water correlated with this PC. This suggests that the variability in nutrient budgets among polders is largely governed by agricultural land use.     

Small differences in ombrotrophy control regional-scale variation in methane cycling among <Emphasis Type="Italic">Sphagnum</Emphasis>-dominated peatlands

Across northern Alberta, Canada, bogs experience periodic wildfire and, in the Fort McMurray region, are exposed to increasing atmospheric N deposition related to oil sands development. As the fire return interval shortens and/or growing season temperatures increase, the regional peatland CO₂–C sink across northern Alberta will likely decrease, but the magnitude of the decrease could be diminished if increasing atmospheric N deposition alters N cycling in a way that stimulates post-fire successional development in bogs. We quantified net ammonification, nitrification, and dissolved organic N (DON) production in surface peat along a post-fire chronosequence of five bogs where we also experimentally manipulated N deposition (no water controls plus 0, 10, and 20 kg N ha^?1 yr^?1 simulated deposition, as NH₄NO₃). Initial KCl-extractable NH₄⁺–N, NO₃^?–N and DON averaged 176?±?6, 54?±?0.2, and 3580?±?40 ng N cm^?3, respectively, with no consistent changes as a function of time since fire and no consistent effects of experimental N addition. Net ammonification, nitrification, and DON production averaged 3.8?±?0.3, 1.6?±?0.2, and 14.3?±?2.0 ng N cm^?3 d^?1, also with no consistent changes as a function of time since fire and no consistent effects of experimental N addition. Our hypothesis that N mineralization would be stimulated after fire because root death would create a pulse of labile soil organic C was not supported, most likely because ericaceous plant roots typically are not killed in boreal bog wildfires. The absence of any N mineralization response to experimental N addition is most likely a result of rapid immobilization of added NH₄⁺–N and NO₃^?–N in peat with a wide C:N ratio. In these boreal bogs, belowground N cycling is likely characterized by large DON pools that turn over relatively slowly and small DIN pools that turn over relatively rapidly. For Alberta bogs that have persisted at historically low N deposition values and begin to receive higher N deposition related to anthropogenic activities, peat N mineralization processes may be largely unaffected until the peat C:N ratio reaches a point that no longer favors immobilization of NH₄⁺–N and NO₃^?–N.