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1.
Stream export of nitrogen (N) as nitrate (NO3−; the most mobile form of N) from forest ecosystems is thought to be controlled largely by plant uptake of inorganic N, such
that reduced demand for plant N during the non-growing season and following disturbances results in increased stream NO3− export. The roles of microbes and soils in ecosystem N retention are less clear, but are the dominant controls on N export
when plant uptake is low. We used a mass balance approach to investigate soil N retention during winter (December through
March) at the Hubbard Brook Experimental Forest by comparing NO3− inputs (atmospheric deposition), internal production (soil microbial nitrification), and stream output. We focused on months
when plant N uptake is nearly zero and the potential for N export is high. Although winter months accounted for only 10–15%
of annual net nitrification, soil NO3− production (0.8–1.0 g N m−2 winter−1) was much greater than stream export (0.03–0.19 N m−2 winter−1). Soil NO3− retention in two consecutive winters was high (96% of combined NO3− deposition and soil production; year 1) even following severe plant disturbance caused by an ice-storm (84%; year 2) We show
that soil NO3− retention is surprisingly high even when N demand by plants is low. Our study highlights the need to better understand mechanisms
of N retention during the non-growing season to predict how ecosystems will respond to high inputs of atmospheric N, disturbance,
and climate change. 相似文献
2.
Peter M. Homyak James O. Sickman Amy E. Miller John M. Melack Thomas Meixner Joshua P. Schimel 《Ecosystems》2014,17(7):1286-1305
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. 相似文献
3.
Anthropogenic nitrogen (N) deposition is a globally important source of N that is expected to increase with population growth.
In southern California, N input from dry deposition accumulates on vegetation and soil surfaces of chaparral and coastal sage
scrub (CSS) ecosystems during the summer and fall and becomes available as a pulse following winter rainfall. Presumably,
N input will act to stimulate the productivity and N storage of these Mediterranean-type, semi-arid shrublands because these
ecosystems are thought to be N limited. To assess whether dry-season N inputs alter ecosystem productivity and N storage,
a field experiment was conducted over a 4-year period where plots were exposed to either ambient N deposition (control) or
ambient + 50 kgN ha−1 y−1 (added N) that was added as NH4NO3 during the fall dry-season of each year. Plots exposed to added N had significantly higher accumulation of NH4 and NO3 on ion exchange resins that was due in part to direct fertilization and N mineralization, and the increase in N availability
lead to a significant increase in NO3 leaching in chaparral but not CSS. Nitrogen addition also lead to an increase in litter and tissue N concentration and a
decline in the C:N ratio, but failed to alter the ecosystem productivity and N storage of the chaparral and CSS shrublands
over the 4-year study period. The reasons for the lack of a treatment response are unknown; however, it is possible that these
semi-arid shrublands are not N limited, cannot respond rapidly enough to capture the ephemeral N pulse, are limited by other
nutrients, or the N response is dependent on the amount and/or distribution of rainfall. These results have important implications
for understanding the potential effects of anthropogenic N deposition on the C and N cycling and storage of Mediterranean-type,
semi-arid shrublands.
Author Contributions GLV conceived or designed study, performed research, analyzed data, contributed new methods or models, and wrote the article.
SCP performed research, analyzed data, and contributed to the writing of the article. RM performed research, analyzed data,
and contributed to the writing of the article. 相似文献
4.
Rosa Gómez M. Isabel Arce J. Javier Sánchez M. del Mar Sánchez-Montoya 《Hydrobiologia》2012,679(1):43-59
Mediterranean climates predispose aquatic systems to both flood and drought periods, therefore, stream sediments may be exposed
to desiccation periods. Changes in oxygen concentrations and sediment water content influence the biotic processes implicated
in nitrogen dynamics. The objectives of this study were to identify (1) the changes of inorganic nitrogen in stream sediments
during the transition from wet to dry conditions, and (2) the underlying processes in N dynamics and its regulation. Extractable
sediment NO3
−-N and NH4
+-N, organic matter and extractable organic carbon content were assessed during natural desiccation in microcosms with sediments
from an intermittent Mediterranean stream. In agreement with our initial hypothesis, our results showed how the NO3
−-N content of the sediment was enhanced during the first 10 days of sediment drying, whereas NH4
+-N was lost by 14 days post-drying. During the first 10 days, sediment desiccation seemed to stimulate the net N-mineralization
and net nitrification from sediments. Afterwards, the extractable NO3
−-N concentration sharply dropped, which may be attributed to lower ammonium-oxidation rates as ammonium and organic matter
are depleted, and to an increase in NO3
−-N consumption by microbial populations. Denitrification was inhibited, with a significant decrease as % water-filled pore
space lowered. We hypothesize that the sediment inorganic N content enhanced during sediment desiccation could be released
as part of the N pulse observed after sediment rewetting. However, the stream N availability after rewetting dried sediments
would differ depending on desiccation period duration. 相似文献
5.
Amy E. Miller Joshua P. Schimel James O. Sickman Thomas Meixner Allen P. Doyle John M. Melack 《Biogeochemistry》2007,84(3):233-245
Cold-season processes are known to contribute substantially to annual carbon (C) and nitrogen (N) budgets in continental high
elevation and high-latitude soils, but their role in more temperate alpine ecosystems has seldom been characterized. We used
a 4-month lab incubation to describe temperature (−2, 0, 5°C) and moisture [50, 90% water-holding capacity (WHC)] effects
on soil C and N dynamics in two wet and one dry meadow soil from the Sierra Nevada, California. The soils varied in their
capacity to process N at and below 0°C. Only the dry meadow soil mineralized N at −2°C, but the wet meadow soils switched
from net N consumption at −2°C to net N mineralization at temperatures ≥0°C. When the latter soils were incubated at −2°C
at either moisture level (50 or 90% WHC), net NO3
− production decreased even as NH4
+ continued to accumulate. The same pattern occurred in saturated (90% WHC) soils at warmer temperatures (≥0°C), suggesting
that dissimilatory processes could control N cycling in these soils when they are frozen. 相似文献
6.
Indicators for nitrogen status and leaching in subtropical forest ecosystems,South China 总被引:7,自引:1,他引:6
The deposition of nitrogen (N) is high in subtropical forest in South China and it is expected to increase further in the
coming decades. To assess effects of increasing deposition on N cycling, we investigated the current N status of two selected
40–45-year-old masson pine-dominated Chinese subtropical forest stands at Tieshanping (TSP, near Chongqing City) and Caijiatang
(CJT in Shaoshan, Hunan province), and explored the applicability of several indicators for N status and leaching, suggested
for temperate and boreal forest ecosystems. Current atmospheric N deposition to the systems is from 25 to 49 kg ha−1 year−1. The concentration of total N in the upper 15 cm of the soil is from as low as 0.05% in the B2 horizon to as high as 0.53% in the O/A horizon. The concentration of organic carbon (C) varies from 0.74 (B2) to 9.54% (O/A). Pools of N in the upper 15 cm of the soils range from 1460 to 2290 kg N ha−1, where 25–55% of the N pool is in the O/A horizon (upper 3 cm of the soil). Due to a lack of a well-developed continuous
O horizon (forest floor), the C/N ratio of this layer cannot be used as an indicator for the N status, as is commonly done
in temperate and boreal forests. The net N mineralization rate (mg N g−1 C year−1) in individual horizons correlates significantly with the C/N ratio, which is from as high as 18.2 in the O/A horizon to
as low as 11.2 in the B2 horizon. The N2O emission flux from soil is significantly correlated with the KCl extractable NH4+–N in the O/A horizon and with the net nitrification in the upper 15 cm of the soil. However, the spatial and temporal variation
of the N2O emission rate is high and rates are small and often difficult to detect in the field. The soil flux density of mineral N,
defined as the sum of the throughfall N input rate and the rate of in situ net N mineralization in the upper 15 cm of the
soil, i.e., the combination of deposition input and the N status of the system, explains the NO3− leaching potential at 30 cm soil depth best. The seasonality of stream water N concentration at TSP and CJT is climatic and
hydrologically controlled, with highest values commonly occurring in the wet growing season and lowest in the dry dormant
season. This is different from temperate forest ecosystems, where N saturation is indicated by elevated NO3− leaching in stream water during summer. 相似文献
7.
Fluxes of nitrogen and phosphorus in a gallery forest in the Cerrado of central Brazil 总被引:1,自引:0,他引:1
Lucilia Maria Parron Mercedes Maria Cunha Bustamante Daniel Markewitz 《Biogeochemistry》2011,105(1-3):89-104
The Gallery forests of the Cerrado biome play a critical role in controlling stream chemistry but little information about biogeochemical processes in these ecosystems is available. This work describes the fluxes of N and P in solutions along a topographic gradient in a gallery forest. Three distinct floristic communities were identified along the gradient: a wet community nearest the stream, an upland dry community adjacent to the woodland savanna and an intermediate community between the two. Transects were marked in the three communities for sampling. Fluxes of N from bulk precipitation to these forests resulted in deposition of 12.6 kg ha?1 y?1 of total N of which 8.8 kg ha?1 was as inorganic N. The throughfall flux of total N was generally <8.4 kg ha?1 year?1. Throughfall NO3?CN fluxes were higher (7?C32%) while NH4?CN and organic N fluxes were lower (54?C69% and 5?C46%) than those in bulk precipitation. The throughfall flux was slightly lower for the wet forest community compared to other communities. Litter leachate fluxes differed among floristic communities with higher NH4?CN in the wet community. The total N flux was greater in the wet forest than in the dry forest (13.5 vs. 9.4 kg ha?1 year?1, respectively). The stream water had total N flux of 0.3 kg ha?1 year?1. The flux of total P through bulk precipitation was 0.7 kg ha?1 year?1 while the mean fluxes of total P in throughfall (0.6 kg ha?1 year?1) and litter leachate (0.5 kg ha?1 year?1) declined but did not differ between communities. The low concentrations presented in soil solution and low fluxes in stream water (0.3 and 0.1 kg ha?1 year?1 for N and P, respectively) relative to other flowpaths emphasize the conservative nutrient cycling of these forests and the importance of internal recycling processes for the maintenance and conservation of riparian and stream ecosystems in the Cerrado. 相似文献
8.
Contrasting nutrient exports from a forested and an agricultural catchment in south-eastern Australia 总被引:1,自引:0,他引:1
Dissolved organic carbon (DOC) and total and inorganic nitrogen and phosphorus concentrations were determined over 3 years
in headwater streams draining two adjacent catchments. The catchments are currently under different land use; pasture/grazing
vs plantation forestry. The objectives of the work were to quantify C and nutrient export from these landuses and elucidate
the factors regulating export. In both catchments, stream water dissolved inorganic nutrient concentrations exhibited strong
seasonal variations. Concentrations were highest during runoff events in late summer and autumn and rapidly declined as discharge
increased during winter and spring. The annual variation of stream water N and P concentrations indicated that these nutrients
accumulated in the catchments during dry summer periods and were flushed to the streams during autumn storm events. By contrast,
stream water DOC concentrations did not exhibit seasonal variation.
Higher DOC and NO3
− concentrations were observed in the stream of the forest catchment, reflecting greater input and subsequent breakdown of
leaf-litter in the forest catchment. Annual export of DOC was lower from the forested catchment due to the reduced discharge
from this catchment. In contrast however, annual export of nitrate was higher from the forest catchment suggesting that there
was an additional NO3
− source or reduction of a NO3
− sink. We hypothesize that the denitrification capacity of the forested catchment has been significantly reduced as a consequence
of increased evapotranspiration and subsequent decrease in streamflow and associated reduction in the near stream saturated
area. 相似文献
9.
Plant and Soil N Response of Southern Californian Semi-arid Shrublands After 1 Year of Experimental N Deposition 总被引:1,自引:0,他引:1
Large inputs of atmospheric N from dry deposition accumulate on vegetation and soil surfaces of southern Californian chaparral
and coastal sage scrub (CSS) ecosystems during the late-summer and early-fall and become available as a pulse following winter
rainfall; however, the fate of this dry season atmospheric N addition is unknown. To assess the potential for dry season atmospheric
N inputs to be incorporated into soil and/or vegetation N pools, an in situ N addition experiment was initiated in a post-fire
chaparral and a mature CSS stand where 10 × 10 m plots were exposed to either ambient N deposition (control) or ambient +50 kg
N ha−1 (added N) added as NH4NO3 during a single application in October 2003. After 1 year of N addition, plots exposed to added N had significantly higher
accumulation of extractable inorganic N (NH4−N + NO3−N) on ion exchange resins deployed in the 0–10 cm mineral soil layer and higher soil extractable N in the subsurface (30–40 cm)
mineral soil than plots exposed to ambient N. Chaparral and CSS shrubs exposed to added N also exhibited a significant increase
in tissue N concentration and a decline in the tissue C:N ratio, and added N significantly altered the shrub tissue δ
15N natural abundance. Leaching of inorganic N to 1 m below the soil surface was on average 2–3 times higher in the added N
plots, but large within treatment variability cause these differences to be statistically insignificant. Although a large
fraction of the added N could not be accounted for in the shrub and soil N pools investigated, these observations suggest
that dry season N inputs can significantly and rapidly alter N availability and shrub tissue chemistry in Mediterranean-type
chaparral and CSS shrublands of southern California. 相似文献
10.
Experimental and theoretical work emphasize the role of plant nutrient uptake in regulating ecosystem nutrient losses and
predict that forest succession, ecosystem disturbance, and continued inputs of atmospheric nitrogen (N) will increase watershed
N export. In ecosystems where snowpack insulates soils, soil-frost disturbances resulting from low or absent snowpack are
thought to increase watershed N export and may become more common under climate-change scenarios. This study monitored watershed
N export from the Hubbard Brook Experimental Forest (HBEF) in response to a widespread, severe soil-frost event in the winter
of 2006. We predicted that nitrate (NO3
−) export following the disturbance would be high compared to low background streamwater NO3
− export in recent years. However, post-disturbance annual NO3
− export was the lowest on record from both reference (undisturbed) and treated experimental harvest or CaSiO3 addition watersheds. These results are consistent with other studies finding greater than expected forest NO3
− retention throughout the northeastern US and suggest that changes over the last five decades have reduced impacts of frost
events on watershed NO3
− export. While it is difficult to parse out causes from a complicated array of potential factors, based on long-term records
and watershed-scale experiments conducted at the HBEF, we propose that reduced N losses in response to frost are due to a
combination of factors including the long-term legacies of land use, process-level alterations in N pathways, climate-driven
hydrologic changes, and depletion of base cations and/or reduced soil pH due to cumulative effects of acid deposition. 相似文献
11.
Dissolved Nitrogen, Phosphorus, and Sulfur forms in the Ecosystem Fluxes of a Montane Forest in Ecuador 总被引:1,自引:0,他引:1
Rainer Goller Wolfgang Wilcke Katrin Fleischbein Carlos Valarezo Wolfgang Zech 《Biogeochemistry》2006,77(1):57-89
The N, P, and S cycles in pristine forests are assumed to differ from those of anthropogenically impacted areas, but there
are only a few studies to support this. Our objective was therefore to assess the controls of N, P, and S release, immobilization,
and transport in a remote tropical montane forest. The study forest is located on steep slopes of the northern Andes in Ecuador.
We determined the concentrations of NO3-N, NH4-N, dissolved organic N (DON), PO4-P, dissolved organic P (DOP), SO4-S, dissolved organic S (DOS), and dissolved organic C (DOC) in rainfall, throughfall, stemflow, lateral flow (in the organic
layer), litter leachate, mineral soil solution, and stream water of three 8–13 ha catchments (1900–2200 m a.s.l.). The organic
forms of N, P, and S contributed, on average, 55, 66, and 63% to the total N, P, and S concentrations in all ecosystem fluxes,
respectively. The organic layer was the largest source of all N, P, and S species except for inorganic P and S. Most PO4 was released in the canopy by leaching and most SO4 in the mineral soil by weathering. The mineral soil was a sink for all studied compounds except for SO4. Consequently, concentrations of dissolved inorganic and organic N and P were as low in stream water (TDN: 0.34–0.39 mg N l−1, P not detectable) as in rainfall (TDN: 0.39–0.48 mg N l−1, P not detectable), whereas total S concentrations were elevated (stream water: 0.04–0.15, rainfall: 0.01–0.07 mg S l−1). Dissolved N, P, and S forms were positively correlated with pH at the scale of soil peda except inorganic S. Soil drying
and rewetting promoted the release of dissolved inorganic N. High discharge levels following heavy rainstorms were associated
with increased DOC, DON, NO3-N and partly also NH4-N concentrations in stream water. Nitrate-N concentrations in the stream water were positively correlated with stream discharge
during the wetter period of the year. Our results demonstrate that the sources and sinks of N, P, and S were element-specific.
More than half of the cycling N, P, and S was organic. Soil pH and moisture were important controls of N, P, and S solubility
at the scale of individual soil peda whereas the flow regime influenced the export with stream water. 相似文献
12.
Responses of terrestrial nitrogen pools and dynamics to different patterns of freeze‐thaw cycle: A meta‐analysis 下载免费PDF全文
Decai Gao Lei Zhang Jun Liu Bo Peng Zhenzhen Fan Weiwei Dai Ping Jiang Edith Bai 《Global Change Biology》2018,24(6):2377-2389
Altered freeze‐thaw cycle (FTC) patterns due to global climate change may affect nitrogen (N) cycling in terrestrial ecosystems. However, the general responses of soil N pools and fluxes to different FTC patterns are still poorly understood. Here, we compiled data of 1519 observations from 63 studies and conducted a meta‐analysis of the responses of 17 variables involved in terrestrial N pools and fluxes to FTC. Results showed that under FTC treatment, soil NH4+, NO3?, NO3? leaching, and N2O emission significantly increased by 18.5%, 18.3%, 66.9%, and 144.9%, respectively; and soil total N (TN) and microbial biomass N (MBN) significantly decreased by 26.2% and 4.7%, respectively; while net N mineralization or nitrification rates did not change. Temperate and cropland ecosystems with relatively high soil nutrient contents were more responsive to FTC than alpine and arctic tundra ecosystems with rapid microbial acclimation. Therefore, altered FTC patterns (such as increased duration of FTC, temperature of freeze, amplitude of freeze, and frequency of FTC) due to global climate warming would enhance the release of inorganic N and the losses of N via leaching and N2O emissions. Results of this meta‐analysis help better understand the responses of N cycling to FTC and the relationships between FTC patterns and N pools and N fluxes. 相似文献
13.
Muhammad Riaz Ishaq A. Mian Ambreen Bhatti Malcolm S. Cresser 《Biogeochemistry》2012,107(1-3):165-185
Surface and subsurface litter fulfil many functions in the biogeochemical cycling of C and N in terrestrial ecosystems. These were explored using a microcosm study by monitoring dissolved inorganic nitrogen (DIN) (NH4 +–N?+?NO3 ?–N), dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) concentrations and fluxes in drainage water under ambient outdoor temperatures. Subsurface litter remarkably reduced the DIN concentrations in winter, probably by microbial N uptake associated with higher C:N ratio of added litter compared with soil at 10–25?cm depth. Fluxes of DIN were generally dominated by NO3 ?–N; but NH4 +–N strongly dominated DIN fluxes during freeze–thaw events. Appreciable concentrations of NH4 +–N were observed in the drainage from the acid grassland soils throughout the experiment, indicating NH4 +–N mobility and export in drainage water especially during freeze–thaw. Litter contributed substantially to DOC and DON production and they were correlated positively (p?<?0.01) for all treatments. DOC and DON concentrations correlated with temperature for the control (p?<?0.01) and surface litter (p?<?0.001) treatments and they were higher in late summer. The subsurface litter treatment, however, moderated the effect of temperature on DOC and DON dynamics. Cumulative N species fluxes confirmed the dominance of litter as the source of DON and DOC in the drainage water. DON constituted 42, 46 and 62% of cumulative TDN flux for control, surface litter and subsurface litter treatments respectively. 相似文献
14.
Thaw depth determines reaction and transport of inorganic nitrogen in valley bottom permafrost soils
Nitrate (NO3–) export coupled with high inorganic nitrogen (N) concentrations in Alaskan streams suggests that N cycles of permafrost‐influenced ecosystems are more open than expected for N‐limited ecosystems. We tested the hypothesis that soil thaw depth governs inorganic N retention and removal in soils due to vertical patterns in the dominant N transformation pathways. Using an in situ, push–pull method, we estimated rates of inorganic N uptake and denitrification during snow melt, summer, and autumn, as depth of soil–stream flowpaths increased in the valley bottom of an arctic and a boreal catchment. Net NO3– uptake declined sharply from snow melt to summer and decreased as a nonlinear function of thaw depth. Peak denitrification rate occurred during snow melt at the arctic site, in summer at the boreal site, and declined as a nonlinear function of thaw depth across both sites. Seasonal patterns in ammonium (NH4+) uptake were not significant, but low rates during the peak growing season suggest uptake that is balanced by mineralization. Despite rapid rates of hydrologic transport during snow melt runoff, rates of uptake and removal of inorganic N tended to exceed water residence time during snow melt, indicating potential for retention of N in valley bottom soils when flowpaths are shallow. Decreased reaction rates relative to water residence time in subsequent seasons suggest greater export of inorganic N as the soil–stream flowpath deepens due to thawing soils. Using seasonal thaw as a proxy for longer term deepening of the thaw layer caused by climate warming and permafrost degradation, these results suggest increasing potential for export of inorganic N from permafrost‐influenced soils to streams. 相似文献
15.
Dual isotope analyses indicate efficient processing of atmospheric nitrate by forested watersheds in the northeastern U.S. 总被引:1,自引:1,他引:0
Nitrogen from atmospheric deposition serves as the dominant source of new nitrogen to forested ecosystems in the northeastern
U.S. By combining isotopic data obtained using the denitrifier method, with chemical and hydrologic measurements we determined
the relative importance of sources and control mechanisms on nitrate (NO3
−) export from five forested watersheds in the Connecticut River watershed. Microbially produced NO3
− was the dominant source (82–100%) of NO3
− to the sampled streams as indicated by the δ15N and δ18O of NO3
−. Seasonal variations in the δ18O–NO3
− in streamwater are controlled by shifting hydrologic and temperature affects on biotic processing, resulting in a relative
increase in unprocessed NO3
− export during winter months. Mass balance estimates find that the unprocessed atmospherically derived NO3
− stream flux represents less than 3% of the atmospherically delivered wet NO3
− flux to the region. This suggests that despite chronically elevated nitrogen deposition these forests are not nitrogen saturated
and are retaining, removing, and reprocessing the vast majority of NO3
− delivered to them throughout the year. These results confirm previous work within Northeastern U.S. forests and extend observations
to watersheds not dominated by a snow-melt driven hydrology. In contrast to previous work, unprocessed atmospherically derived
NO3
− export is associated with the period of high recharge and low biotic activity as opposed to spring snowmelt and other large
runoff events. 相似文献
16.
The photochemical release of inorganic nitrogen from dissolved organic matter is an important source of bio-available nitrogen (N) in N-limited aquatic ecosystems. We conducted photochemical experiments and used mathematical models based on pseudo-first-order reaction kinetics to quantify the photochemical transformations of individual N species and their seasonal effects on N cycling in a mountain forest stream and lake (Plešné Lake, Czech Republic). Results from laboratory experiments on photochemical changes in N speciation were compared to measured lake N budgets. Concentrations of organic nitrogen (Norg; 40–58 µmol L−1) decreased from 3 to 26% during 48-hour laboratory irradiation (an equivalent of 4–5 days of natural solar insolation) due to photochemical mineralization to ammonium (NH4
+) and other N forms (Nx; possibly N oxides and N2). In addition to Norg mineralization, Nx also originated from photochemical nitrate (NO3
−) reduction. Laboratory exposure of a first-order forest stream water samples showed a high amount of seasonality, with the maximum rates of Norg mineralization and NH4
+ production in winter and spring, and the maximum NO3
− reduction occurring in summer. These photochemical changes could have an ecologically significant effect on NH4
+ concentrations in streams (doubling their terrestrial fluxes from soils) and on concentrations of dissolved Norg in the lake. In contrast, photochemical reactions reduced NO3
− fluxes by a negligible (<1%) amount and had a negligible effect on the aquatic cycle of this N form. 相似文献
17.
Winter and summer nitrous oxide and nitrogen oxides fluxes from a seasonally snow-covered subalpine meadow at Niwot Ridge,Colorado 总被引:4,自引:3,他引:1
Gianluca Filippa Michele Freppaz Mark W. Williams Detlev Helmig Daniel Liptzin Brian Seok Brad Hall Kurt Chowanski 《Biogeochemistry》2009,95(1):131-149
The soil emission rates (fluxes) of nitrous oxide (N2O) and nitrogen oxides (NO + NO2 = NO
x
) through a seasonal snowpack were determined by a flux gradient method from near-continuous 2-year measurements using an
automated system for sampling interstitial air at various heights within the snowpack from a subalpine site at Niwot Ridge,
Colorado. The winter seasonal-averaged N2O fluxes of 0.047–0.069 nmol m−2 s−1 were ~15 times higher than observed NO
x
fluxes of 0.0030–0.0067 nmol m−2 s−1. During spring N2O emissions first peaked and then dropped sharply as the soil water content increased from the release of snowpack meltwater,
while other gases, including NO
x
and CO2 did not show this behavior. To compare and contrast the winter fluxes with snow-free conditions, N2O fluxes were also measured at the same site in the summers of 2006 and 2007 using a closed soil chamber method. Summer N2O fluxes followed a decreasing trend during the dry-out period after snowmelt, interrupted by higher values related to precipitation
events. These peaks were up to 2–3 times higher than the background summer levels. The integrated N2O-N loss over the summer period was calculated to be 1.1–2.4 kg N ha−1, compared to ~0.24–0.34 kg N ha−1 for the winter season. These wintertime N2O fluxes from subniveal soil are generally higher than the few previously published data. These results are of the same order
of magnitude as data from more productive ecosystems such as fertilized grasslands and high-N-cycling forests, most likely
because of a combination of the relatively well-developed soils and the fact that subnivean biogeochemical processes are promoted
by the deep, insulating snowpack. Hence, microbially mediated oxidized nitrogen emissions occurring during the winter can
be a significant part of the N-cycle in seasonally snow-covered subalpine ecosystems. 相似文献
18.
Knowledge of the fate of deposited N in the possibly N-limited, highly biodiverse north Andean forests is important because of the possible effects of N inputs on plant performance and species composition. We analyzed concentrations and fluxes of NO3 ??CN, NH4 +?CN and dissolved organic N (DON) in rainfall, throughfall, litter leachate, mineral soil solutions (0.15?C0.30 m depths) and stream water in a montane forest in Ecuador during four consecutive quarters and used the natural 15N abundance in NO3 ? during the passage of rain water through the ecosystem and bulk ??15N values in soil to detect N transformations. Depletion of 15N in NO3 ? and increased NO3 ??CN fluxes during the passage through the canopy and the organic layer indicated nitrification in these compartments. During leaching from the organic layer to mineral soil and stream, NO3 ? concentrations progressively decreased and were enriched in 15N but did not reach the ??15N values of solid phase organic matter (??15N = 5.6?C6.7??). This suggested a combination of nitrification and denitrification in mineral soil. In the wettest quarter, the ??15N value of NO3 ? in litter leachate was smaller (??15N = ?1.58??) than in the other quarters (??15N = ?9.38 ± SE 0.46??) probably because of reduced mineralization and associated fractionation against 15N. Nitrogen isotope fractionation of NO3 ? between litter leachate and stream water was smaller in the wettest period than in the other periods probably because of a higher rate of denitrification and continuous dilution by isotopically lighter NO3 ??CN from throughfall and nitrification in the organic layer during the wettest period. The stable N isotope composition of NO3 ? gave valuable indications of N transformations during the passage of water through the forest ecosystem from rainfall to the stream. 相似文献
19.
Jirko Holst Chunyan Liu Nicolas Brüggemann Klaus Butterbach-Bahl Xunhua Zheng Yuesi Wang Shenghui Han Zhisheng Yao Jin Yue Xingguo Han 《Ecosystems》2007,10(4):623-634
Gross rates of N mineralization and nitrification, and soil–atmosphere fluxes of N2O, NO and NO2 were measured at differently grazed and ungrazed steppe grassland sites in the Xilin river catchment, Inner Mongolia, P. R.
China, during the 2004 and 2005 growing season. The experimental sites were a plot ungrazed since 1979 (UG79), a plot ungrazed
since 1999 (UG99), a plot moderately grazed in winter (WG), and an overgrazed plot (OG), all in close vicinity to each other.
Gross rates of N mineralization and nitrification determined at in situ soil moisture and soil temperature conditions were
in a range of 0.5–4.1 mg N kg−1 soil dry weight day−1. In 2005, gross N turnover rates were significantly higher at the UG79 plot than at the UG99 plot, which in turn had significantly
higher gross N turnover rates than the WG and OG plots. The WG and the OG plot were not significantly different in gross ammonification
and in gross nitrification rates. Site differences in SOC content, bulk density and texture could explain only less than 15%
of the observed site differences in gross N turnover rates. N2O and NO
x
flux rates were very low during both growing seasons. No significant differences in N trace gas fluxes were found between
plots. Mean values of N2O fluxes varied between 0.39 and 1.60 μg N2O-N m−2 h−1, equivalent to 0.03–0.14 kg N2O-N ha−1 y−1, and were considerably lower than previously reported for the same region. NO
x
flux rates ranged between 0.16 and 0.48 μg NO
x
-N m−2 h−1, equivalent to 0.01–0.04 kg NO
x
-N ha−1 y−1, respectively. N2O fluxes were significantly correlated with soil temperature and soil moisture. The correlations, however, explained only
less than 20% of the flux variance. 相似文献
20.
Microbial transformations of C and N in a boreal forest floor as affected by temperature 总被引:9,自引:0,他引:9
The effects of temperature on N mineralization were studied in two organic surface horizons (LF and H) of soil from a boreal
forest. The soil was incubated at 5 °C and 15 °C after adding 15 N and gross N fluxes were calculated using a numerical simulation model. The model was calibrated on microbial C and N, basal
respiration, and KCl-extractable NH4
+, NO3
−, 15NH4
+ and 15 NO3
−. In the LF layer, increased temperature resulted in a faster turnover of all N pools. In both layers net N mineralization
did not increase at elevated temperature because both gross NH4
+ mineralization and NH4
+ immobilization increased. In the H layer, however, both gross NH4
+ mineralization and NH4
+ immobilization were lower at 15 °C than at 5 °C and the model predicted a decrease in microbial turnover rate at higher temperature
although measured microbial activity was higher. The decrease in gross N fluxes in spite of increased microbial activity in
the H layer at elevated temperature may have been caused by uptake of organic N. The model predicted a decrease in pool size
of labile organic matter and microbial biomass at elevated temperature whereas the amount of refractory organic matter increased.
Temperature averaged microbial C/N ratio was 14.7 in the LF layer suggesting a fungi-dominated decomposer community whereas
it was 7.3 in the H layer, probably due to predominance of bacteria. Respiration and microbial C were difficult to fit using
the model if the microbial C/N ratio was kept constant with time. A separate 15N-enrichment study with the addition of glucose showed that glucose was metabolized faster in the LF than in the H layer.
In both layers, decomposition of organic matter appeared to be limited by C availability.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献