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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.
Snow depth, soil freezing and nitrogen cycling in a northern hardwood forest landscape 总被引:2,自引:0,他引:2
Peter M. Groffman Janet P. Hardy Samuel Fashu-Kanu Charles T. Driscoll Natalie L. Cleavitt Timothy J. Fahey Melany C. Fisk 《Biogeochemistry》2011,102(1-3):223-238
Increases in soil freezing associated with decreases in snow cover have been identified as a significant disturbance to nitrogen (N) cycling in northern hardwood forests. We created a range of soil freezing intensity through snow manipulation experiments along an elevation gradient at the Hubbard Brook Experimental Forest (HBEF) in the White Mountains, NH USA in order to improve understanding of the factors regulating freeze effects on nitrate (NO3 ?) leaching, nitrous oxide (N2O) flux, potential and in situ net N mineralization and nitrification, microbial biomass carbon (C) and N content and respiration, and denitrification. While the snow manipulation treatment produced deep and persistent soil freezing at all sites, effects on hydrologic and gaseous losses of N were less than expected and less than values observed in previous studies at the HBEF. There was no relationship between frost depth, frost heaving and NO3 ? leaching, and a weak relationship between frost depth and winter N2O flux. There was a significant positive relationship between dissolved organic carbon (DOC) and NO3 ? concentrations in treatment plots but not in reference plots, suggesting that the snow manipulation treatment mobilized available C, which may have stimulated retention of N and prevented treatment effects on N losses. While the results support the hypothesis that climate change resulting in less snow and more soil freezing will increase N losses from northern hardwood forests, they also suggest that ecosystem response to soil freezing disturbance is affected by multiple factors that must be reconciled in future research. 相似文献
3.
Whole-system Estimates of Nitrification and Nitrate Uptake in Streams of the Hubbard Brook Experimental Forest 总被引:3,自引:2,他引:1
Although they drain remarkably similar forest types, streams of the Hubbard Brook Experimental Forest (HBEF) vary widely in
their NO3
− concentrations during the growing season. This variation may be caused by differences in the terrestrial systems they drain
(for example, varying forest age or composition, hydrology, soil organic matter content, and so on) and/or by differences
between the streams themselves (for example, contrasting geomorphology, biotic nitrogen [N] demand, rates of instream nitrogen
transformations). We examined interstream variation in N processing by measuring NH4
+ and NO3
− uptake and estimating nitrification rates for 13 stream reaches in the HBEF during the summers of 1998 and 1999. We modeled
nitrification rates using a best-fit model of the downstream change in NO3
− concentrations following short-term NH4
+ enrichments. Among the surveyed streams, the fraction of NH4
+ uptake that was subsequently nitrified varied, and this variation was positively correlated with ambient streamwater NO3
− concentrations. We examined whether this variation in instream nitrification rates contributed significantly to the observed
variation in NO3
− concentrations across streams. In some cases, instream nitrification provided a substantial portion of instream NO3
− demand. However, because there was also substantial instream NO3
− uptake, the net effect of instream processing was to reduce rather than supplement the total amount of NO3
− exported from a watershed. Thus, instream rates of nitrification in conjunction with instream NO3
− uptake were too low to account for the wide range of streamwater NO3
−. The relationship between streamwater NO3
− concentration and rates of instream nitrification may instead be due to a shift in the competitive balance between heterotrophic
N uptake and nitrification when external inputs of NO3
− are relatively high.
Received 11 October 2000; accepted 14 December 2001. 相似文献
4.
A. Bryce Cooper 《Plant and Soil》1986,93(3):383-394
Summary Nitrate-N losses to stream waters and soil inorganic N pools, nitrifying potentials and NO3-N production rates were measured in 2 adjacent watersheds, one used as pasture and the other planted in exotic conifer forest
(Pinus radiata D. Don). Estimated NO3-N loss to stream waters draining the pine and pasture watersheds were 0.6kg ha−1 y−1 and 7.6 kg ha−1 y−1 respectively. Ammonium-N pool sizes were not significantly different between soils in the two watersheds but NO3−N pools and nitrifying potentials were always lower in the pine watershed soil samples. Laboratory incubation experiments
indicated that suppression of NO3−N formation in pine watershed soils required the presence of live tree roots and was not due to the direct action of allelopathic
chemicals on nitrifiers. 相似文献
5.
Nitrogen Dynamics in Ice Storm-Damaged Forest Ecosystems: Implications for Nitrogen Limitation Theory 总被引:3,自引:1,他引:2
Despite the widely recognized importance of disturbance in accelerating the loss of elements from land, there have been few empirical studies of the effects of natural disturbances on nitrogen (N) dynamics in forest ecosystems. We were provided the unusual opportunity for such study, partly because the intensively monitored watersheds at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, experienced severe canopy damage following an ice storm. Here we report the effects of this disturbance on internal N cycling and loss for watershed 1 (W1) and watershed 6 (W6) at the HBEF and patterns of N loss from nine other severely damaged watersheds across the southern White Mountains. This approach allowed us to test one component of N limitation theory, which suggests that N losses accompanying natural disturbances can lead to the maintenance of N limitation in temperate zone forest ecosystems. Prior to the ice storm, fluxes of nitrate (NO3
–) at the base of W1 and W6 were similar and were much lower than N inputs in atmospheric deposition. Following the ice storm, drainage water NO3
– concentrations increased to levels that were seven to ten times greater than predisturbance values. We observed no significant differences in N mineralization, nitrification, or denitrification between damaged and undamaged areas in the HBEF watersheds, however. This result suggests that elevated NO3
- concentrations were not necessarily due to accelerated rates of N cycling by soil microbes but likely resulted from decreased plant uptake of NO3
-. At the regional scale, we observed high variability in the magnitude of NO3
- losses: while six of the surveyed watersheds showed accelerated rates of NO3
– loss, three did not. Moreover, in contrast to the strong linear relationship between NO3
– loss and crown damage within HBEF watersheds [r
2: (W1 = 0.91, W6 = 0.85)], stream water NO3
– concentrations were weakly related to crown damage (r
2 = 0.17) across our regional sites. The efflux of NO3
– associated with the ice storm was slightly higher than values reported for soil freezing and insect defoliation episodes, but was approximately two to ten times lower than NO3
– fluxes associated with forest harvesting. Because over one half of the entire years worth of N deposition was lost following the ice storm, we conclude that catastrophic disturbances contribute synergistically to the maintenance of N limitation and widely observed delays of N saturation in northern, temperate zone forest ecosystems.
Present address: Department of Ecology and Evolutionary Biology, Princeton University, Guyot Hall, Princeton, New Jersey 08544, USA. 相似文献
6.
Nitrogen (N) pollution is a problem in many large temperate zone rivers, and N retention in river channels is often small
in these systems. To determine the potential for floodplains to act as N sinks during overbank flooding, we combined monitoring,
denitrification assays, and experimental nitrate (NO3− -N) additions to determine how the amount and form of N changed during flooding and the processes responsible for these changes
in the Wisconsin River floodplain (USA). Spring flooding increased N concentrations in the floodplain to levels equal to the
river. As discharge declined and connectivity between the river and floodplain was disrupted, total dissolved N decreased
over 75% from 1.41 mg l−1, equivalent to source water in the Wisconsin River on 14 April 2001, to 0.34 mg l−1 on 22 April 2001. Simultaneously NO3− -N was attenuated almost 100% from 1.09 to <0.002 mg l−1. Unamended sediment denitrification rates were moderate (0–483 μg m−2 h−1) and seasonally variable, and activity was limited by the availability of NO
3− -N on all dates. Two experimental NO3− -N pulse additions to floodplain water bodies confirmed rapid NO3− -N depletion. Over 80% of the observed NO
3− -N decline was caused by hydrologic export for addition #1 but only 22% in addition #2. During the second addition, a significant
fraction (>60%) of NO3− -N mass loss was not attributable to hydrologic losses or conversion to other forms of N, suggesting that denitrification
was likely responsible for most of the NO3− -N disappearance. Floodplain capacity to decrease the dominant fraction of river borne N within days of inundation demonstrates
that the Wisconsin River floodplain was an active N sink, that denitrification often drives N losses, and that enhancing connections
between rivers and their floodplains may enhance overall retention and reduce N exports from large basins. 相似文献
7.
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
8.
Spatial variability in nutrient concentration and biofilm nutrient limitation in an urban watershed 总被引:1,自引:0,他引:1
Nutrient enrichment threatens river ecosystem health in urban watersheds, but the influence of urbanization on spatial variation
in nutrient concentrations and nutrient limitation of biofilm activity are infrequently measured simultaneously. In summer
2009, we used synoptic sampling to measure spatial patterns of nitrate (NO3
−), ammonium (NH4
+), and soluble reactive phosphorus (SRP) concentration, flux, and instantaneous yield throughout the Bronx River watershed
within New York City and adjacent suburbs. We also quantified biofilm response to addition of NO3
−, phosphate (PO4
3−), and NO3
− + PO4
3− on organic and inorganic surfaces in the river mainstem and tributaries. Longitudinal variation in NO3
− was low and related to impervious surface cover across sub-watersheds, but spatial variation in NH4
+ and SRP was higher and unrelated to sub-watershed land-use. Biofilm respiration on organic surfaces was frequently limited
by PO4
3− or NO3
− + PO4
3−, while primary production on organic and inorganic surfaces was nutrient-limited at just one site. Infrequent NO3
− limitation and low spatial variability of NO3
− throughout the watershed suggested saturation of biological N demand. For P, both higher biological demand and point-sources
contributed to greater spatial variability. Finally, a comparison of our data to synoptic studies of forested, temperate watersheds
showed lower spatial variation of N and P in urban watersheds. Reduced spatial variation in nutrients as a result of biological
saturation may represent an overlooked effect of urbanization on watershed ecology, and may influence urban stream biota and
downstream environments. 相似文献
9.
J. Alan Yeakley David C. Coleman Bruce L. Haines Brian D. Kloeppel Judy L. Meyer Wayne T. Swank Barry W. Argo James M. Deal Sharon F. Taylor 《Ecosystems》2003,6(2):0154-0167
We investigated the effects of removing near-stream Rhododendron and of the natural blowdown of canopy trees on nutrient export to streams in the southern Appalachians. Transects were instrumented
on adjacent hillslopes in a first-order watershed at the Coweeta Hydrologic Laboratory (35°03′N, 83°25′W). Dissolved organic
carbon (DOC), K+, Na+, Ca2+, Mg2+, NO3
−-N, NH4
+-N, PO4
3−-P, and SO4
2− were measured for 2 years prior to disturbance. In August 1995, riparian Rhododendron on one hillslope was cut, removing 30% of total woody biomass. In October 1995, Hurricane Opal uprooted nine canopy trees
on the other hillslope, downing 81% of the total woody biomass. Over the 3 years following the disturbance, soilwater concentrations
of NO3
−-N tripled on the cut hillslope. There were also small changes in soilwater DOC, SO4
2−, Ca2+, and Mg2+. However, no significant changes occurred in groundwater nutrient concentrations following Rhododendron removal. In contrast, soilwater NO3
−-N on the storm-affected hillslope showed persistent 500-fold increases, groundwater NO3
−-N increased four fold, and streamwater NO3
−-N doubled. Significant changes also occurred in soilwater pH, DOC, SO4
2−, Ca2+, and Mg2+. There were no significant changes in microbial immobilization of soil nutrients or water outflow on the storm-affected hillslope.
Our results suggest that Rhododendron thickets play a relatively minor role in controlling nutrient export to headwater streams. They further suggest that nutrient
uptake by canopy trees is a key control on NO3
−-N export in upland riparian zones, and that disruption of the root–soil connection in canopy trees via uprooting promotes
significant nutrient loss to streams.
Received 30 January 2001; accepted 25 July 2002. 相似文献
10.
Late-successional forests in the upper Great Lakes region are susceptible to nitrogen (N) saturation and subsequent nitrate
(NO3−) leaching loss. Endemic wind disturbances (i.e., treefall gaps) alter tree uptake and soil N dynamics; and, gaps are particular
susceptible to NO3− leaching loss. Inorganic N was measured throughout two snow-free periods in throughfall, forest floor leachates, and mineral
soil leachates in gaps (300–2,000 m2, 6–9 years old), gap-edges, and closed forest plots in late-successional northern hardwood, hemlock, and northern hardwood–hemlock
stands. Differences in forest water inorganic N among gaps, edges, and closed forest plots were consistent across these cover
types: NO3− inputs in throughfall were significantly greater in undisturbed forest plots compared with gaps and edges; forest floor leachate
NO3− was significantly greater in gaps compared to edges and closed forest plots; and soil leachate NO3− was significantly greater in gaps compared to the closed forest. Significant differences in forest water ammonium and pH
were not detected. Compared to suspected N-saturated forests with high soil NO3− leaching, undisturbed forest plots in these late-successional forests are not losing NO3− (net annual gain of 2.8 kg ha−1) and are likely not N-saturated. Net annual NO3− losses were observed in gaps (1.3 kg ha−1) and gap-edges (0.2 kg ha−1), but we suspect these N leaching losses are a result of decreased plant uptake and increased soil N mineralization associated
with disturbance, and not N-saturation. 相似文献
11.
Mark S. Castro Keith N. Eshleman Louis F. Pitelka Geoff Frech Molly Ramsey William S. Currie Karen Kuers Jeffrey A. Simmons Bob R. Pohlad Carolyn L. Thomas David M. Johnson 《Biogeochemistry》2007,84(3):333-348
The objective of this study was to evaluate the nitrogen (N) biogeochemistry of an 18–22 year old forested watershed in western
Maryland. We hypothesized that this watershed should not exhibit symptoms of N saturation. This watershed was a strong source
of nitrate (NO3
−) to the stream in all years, with a mean annual export of 9.5 kg N ha−1 year−1 and a range of 4.4–18.4 kg N ha−1 year−1. During the 2001 and 2002 water years, wet deposition of inorganic N was 9.0 kg N ha−1 year−1 and 6.3 kg N ha−1 year−1, respectively. Watershed N export rates in 2001 and 2002 water years were 4.2 kg N ha−1 year−1 and 5.3 kg N ha−1 year−1, respectively. During the wetter water years of 2003 and 2004, the watershed exported 15.0 kg N ha−1 year−1 and 18.4 kg N ha−1 year−1, rates that exceeded annual wet deposition of N by a factor of two (7.5 kg N ha−1 year−1 in 2003) and three (5.5 kg N ha−1 year−1 in 2004). Consistent with the high rates of N export, were high concentrations (2.1–3.3%) of N in foliage, wood (0.3%) and
fine roots, low C:N ratios in the forest floor (17–24) and mineral soil (14), high percentages (83–96%) of the amount of mineralized
N that was nitrified and elevated N concentrations (up to 3 mg N l−1) in soil solution. Although this watershed contained a young aggrading forest, it exhibited several symptoms of N saturation
commonly observed in more mature forests. 相似文献
12.
Since 1987 we have studied weekly change in winter (December–April) precipitation, snowpack, snowmelt, soil water, and stream
water solute flux in a small (176-ha) Northern Michigan watershed vegetated by 65–85 year-old northern hardwoods. Our primary
study objective was to quantify the effect of change in winter temperature and precipitation on watershed hydrology and solute
flux. During the study winter runoff was correlated with precipitation, and forest soils beneath the snowpack remained unfrozen.
Winter air temperature and soil temperature beneath the snowpack increased while precipitation and snowmelt declined. Atmospheric
inputs declined for H+, NO3−, NH4+, dissolved inorganic nitrogen (DIN), and SO42−. Replicated plot-level results, which could not be directly extrapolated to the watershed scale, showed 90% of atmospheric
DIN input was retained in surface shallow (<15 cm deep) soils while SO42− flux increased 70% and dissolved organic carbon (DOC) 30-fold. Most stream water base cation (CB), HCO3−, and Cl− concentrations declined with increased stream water discharge, K+, NO3−, and SO42− remained unchanged, and DOC and dissolved organic nitrogen (DON) increased. Winter stream water solute outputs declined or
were unchanged with time except for NO3− and DOC which increased. DOC and DIN outputs were correlated with the percentage of winter runoff and stream discharge that
occurred when subsurface flow at the plot-level was shallow (<25 cm beneath Oi). Study results suggest that the percentage
of annual runoff occurring as shallow lateral subsurface flow may be a major factor regulating solute outputs and concentrations
in snowmelt-dominated ecosystems. 相似文献
13.
Donald R. Zak William E. Holmes Matthew J. Tomlinson Kurt S. Pregitzer Andrew J. Burton 《Ecosystems》2006,9(2):242-253
Sugar maple (Acer saccharum Marsh.)-dominated northern hardwood forests in the upper Lakes States region appear to be particularly sensitive to chronic
atmospheric NO3− deposition. Experimental NO3− deposition (3 g NO3− N m−2 y−1) has significantly reduced soil respiration and increased the export of DOC/DON and NO3− across the region. Here, we evaluate the possibility that diminished microbial activity in mineral soil was responsible for
these ecosystem-level responses to NO3− deposition. To test this alternative, we measured microbial biomass, respiration, and N transformations in the mineral soil
of four northern hardwood stands that have received 9 years of experimental NO3− deposition. Microbial biomass, microbial respiration, and daily rates of gross and net N transformations were not changed
by NO3− deposition. We also observed no effect of NO3− deposition on annual rates of net N mineralization. However, NO3− deposition significantly increased (27%) annual net nitrification, a response that resulted from rapid microbial NO3− assimilation, the subsequent turnover of NH4+, and increased substrate availability for this process. Nonetheless, greater rates of net nitrification were insufficient
to produce the 10-fold observed increase in NO3− export, suggesting that much of the exported NO3− resulted directly from the NO3− deposition treatment. Results suggest that declines in soil respiration and increases in DOC/DON export cannot be attributed
to NO3−-induced physiological changes in mineral soil microbial activity. Given the lack of response we have observed in mineral
soil, our results point to the potential importance of microbial communities in forest floor, including both saprotrophs and
mycorrhizae, in mediating ecosystem-level responses to chronic NO3− deposition in Lake States northern hardwood forests. 相似文献
14.
Soil-mixing effects on inorganic nitrogen production
and consumption in forest and shrubland soils 总被引:1,自引:0,他引:1
Soils that are physically disturbed are often reported to show net nitrification and NO3− loss. To investigate the response of soil N cycling rates to soil mixing, we assayed gross rates of mineralization, nitrification, NH4+ consumption, and NO3− consumption in a suite of soils from eleven woody plant communities in Oregon, New Mexico, and Utah. Results suggest that the common response of net NO3− flux from disturbed soils is not a straightforward response of increased gross nitrification, but instead may be due to the balance of several factors. While mineralization and NH4+ assimilation were higher in mixed than intact cores, NO3− consumption declined. Mean net nitrification was 0.12 mg N kg−1 d−1 in disturbed cores, which was significantly higher than in intact cores (−0.19 mg N kg−1 d−1). However, higher net nitrification rates in disturbed soils were due to the suppression of NO3− consumption, rather than an increase in nitrification. Our results suggest that at least in the short term, disturbance may significantly increase NO3− flux at the ecosystem level, and that N cycling rates measured in core studies employing mixed soils may not be representative of rates in undisturbed soils. 相似文献
15.
An Unexpected Nitrate Decline in New Hampshire Streams 总被引:7,自引:2,他引:5
Theories of forest nitrogen (N) cycling suggest that stream N losses should increase in response to chronic elevated N deposition
and as forest nutrient requirements decline with age. The latter theory was supported initially by measurements of stream
NO3
− concentration in old-growth and successional stands on Mount Moosilauke, New Hampshire (Vitousek and Reiners 1975; Bioscience 25:376–381). We resampled 28 of these and related streams to evaluate their response to 23 years of forest aggradation and
chronic N deposition. Between 1973–74 and 1996–97, mean NO3
− concentration in quarterly samples from Mount Moosilauke decreased by 71% (25 μmol/L), Ca2+ decreased by 24% (8 μmol/L), and Mg2+ decreased by 22% (5 μmol/L). Nitrate concentrations decreased in every stream in every season, but spatial patterns among
streams persisted: Streams draining old-growth stands maintained higher NO3
− concentrations than those draining successional stands. The cause of the NO3
− decline is not evident. Nitrogen deposition has changed little, and although mechanisms such as insect defoliation and soil
frost may contribute to the temporal patterns of nitrate loss, they do not appear to fully explain the NO3
− decline across the region. Although the role of climate remains uncertain, interannual climate variation and its effects
on biotic N retention may be responsible for the synchronous decrease in NO3
− across all streams, overriding expected increases due to chronic N deposition and forest aging.
Received 4 December 2001; accepted 30 May 2002. 相似文献
16.
Decadal-scale Dynamics of Water, Carbon and Nitrogen in a California Chaparral Ecosystem: DAYCENT Modeling Results 总被引:1,自引:1,他引:0
Xuyong Li Thomas Meixner James O. Sickman Amy E. Miller Joshua P. Schimel John M. Melack 《Biogeochemistry》2006,77(2):217-245
The Mediterranean climate, with its characteristic of dry summers and wet winters, influences the hydrologic and microbial
processes that control carbon (C) and nitrogen (N) biogeochemical processes in chaparral ecosystems. These biogeochemical
processes in turn determine N cycling under chronic N deposition. In order to examine connections between climate and N dynamics,
we quantified decadal-scale water, C and N states and fluxes at annual, monthly and daily time steps for a California chaparral
ecosystem in the Sierra Nevada using the DAYCENT model. The daily output simulations of net mineralization, stream flow and
stream nitrate (NO3−) export were developed for DAYCENT in order to simulate the N dynamics most appropriate for the abrupt rewetting events characteristic
of Mediterranean chaparral ecosystems. Overall, the magnitude of annual modeled net N mineralization, soil and plant biomass
C and N, nitrate export and gaseous N emission agreed with those of observations. Gaseous N emission was a major N loss pathway
in chaparral ecosystems, in which nitric oxide (NO) is the dominant species. The modeled C and N fluxes of net primary production
(NPP), N uptake and N mineralization, NO3− export and gaseous N emission showed both high inter-annual and intra-annual variability. Our simulations also showed dramatic
fire effects on NPP, N uptake, N mineralization and gaseous N emission for three years of postfire. The decease in simulated
soil organic C and N storages was not dramatic, but lasted a longer time. For the seasonal pattern, the predicted C and N
fluxes were greatest during December to March, and lowest in the summer. The model predictions suggested that an increase
in the N deposition rate would increase N losses through gaseous N emission and stream N export in the chaparral ecosystems
of the Sierra Nevada due to changes in N saturation status. The model predictions could not capture stream NO3− export during most rewetting events suggesting that a dry-rewetting mechanism representing the increase in N mineralization
following soil wetting needs to be incorporated into biogeochemical models of semi-arid ecosystems. 相似文献
17.
Jean-Michel Harmand Hector Ávila Etienne Dambrine Ute Skiba Sergio de Miguel Reina Vanessa Renderos Robert Oliver Francisco Jiménez John Beer 《Biogeochemistry》2007,85(2):125-139
Nitrogen fertilization is a key factor for coffee production but creates a risk of water contamination through nitrate (NO3−) leaching in heavily fertilized plantations under high rainfall. The inclusion of fast growing timber trees in these coffee
plantations may increase total biomass and reduce nutrient leaching. Potential controls of N loss were measured in an unshaded
coffee (Coffea arabica L.) plot and in an adjacent coffee plot shaded with the timber species Eucalyptus deglupta Blume (110 trees ha−1), established on an Acrisol that received 180 kg N ha−1 as ammonium-nitrate and 2,700 mm yr−1 rainfall. Results of the one year study showed that these trees had little effect on the N budget although some N fluxes
were modified. Soil N mineralization and nitrification rates in the 0–20 cm soil layer were similar in both systems (≈280 kg N ha−1 yr−1). N export in coffee harvest (2002) was 34 and 25 kg N ha−1 yr−1 in unshaded and shaded coffee, and N accumulation in permanent biomass and litter was 25 and 45 kg N ha−1 yr−1, respectively. The losses in surface runoff (≈0.8 kg mineral N ha−1 yr−1) and N2O emissions (1.9 kg N ha−1 yr−1) were low in both cases. Lysimeters located at 60, 120, and 200 cm depths in shaded coffee, detected average concentrations
of 12.9, 6.1 and 1.2 mg NO3−-N l−1, respectively. Drainage was slightly reduced in the coffee-timber plantation. NO3− leaching at 200 cm depth was about 27 ± 10 and 16 ± 7 kg N ha−1 yr−1 in unshaded and shaded coffee, respectively. In both plots, very low NO3− concentrations in soil solution at 200 cm depth (and in groundwater) were apparently due to NO3− adsorption in the subsoil but the duration of this process is not presently known. In these conventional coffee plantations,
fertilization and agroforestry practices must be refined to match plant needs and limit potential NO3− contamination of subsoil and shallow soil water. 相似文献
18.
Jonathan M. O’Brien Walter K. Dodds Kymberly C. Wilson Justin N. Murdock Jessica Eichmiller 《Biogeochemistry》2007,84(1):31-49
We conducted 15NO3− stable isotope tracer releases in nine streams with varied intensities and types of human impacts in the upstream watershed
to measure nitrate (NO3−) cycling dynamics. Mean ambient NO3− concentrations of the streams ranged from 0.9 to 21,000 μg l−1 NO3−–N. Major N-transforming processes, including uptake, nitrification, and denitrification, all increased approximately two
to three orders of magnitude along the same gradient. Despite increases in transformation rates, the efficiency with which
stream biota utilized available NO3−-decreased along the gradient of increasing NO3−. Observed functional relationships of biological N transformations (uptake and nitrification) with NO3− concentration did not support a 1st order model and did not show signs of Michaelis–Menten type saturation. The empirical
relationship was best described by a Efficiency Loss model, in which log-transformed rates (uptake and nitrification) increase
with log-transformed nitrate concentration with a slope less than one. Denitrification increased linearly across the gradient
of NO3− concentrations, but only accounted for ∼1% of total NO3− uptake. On average, 20% of stream water NO3− was lost to denitrification per km, but the percentage removed in most streams was <5% km−1. Although the rate of cycling was greater in streams with larger NO3− concentrations, the relative proportion of NO3− retained per unit length of stream decreased as NO3− concentration increased. Due to the rapid rate of NO3− turnover, these streams have a great potential for short-term retention of N from the landscape, but the ability to remove
N through denitrification is highly variable. 相似文献
19.
T. Rütting D. Huygens C. Müller O. Van Cleemput R. Godoy P. Boeckx 《Biogeochemistry》2008,90(3):243-258
Nitrite (NO2
−) is an intermediate in a variety of soil N cycling processes. However, NO2
− dynamics are often not included in studies that explore the N cycle in soil. Within the presented study, nitrite dynamics
were investigated in a Nothofagus betuloides forest on an Andisol in southern Chile. We carried out a 15N tracing study with six 15N labeling treatments, including combinations of NO3
−, NH4
+ and NO2
−. Gross N transformation rates were quantified with a 15N tracing model in combination with a Markov chain Monte Carlo optimization routine. Our results indicate the occurrence of
functional links between (1) NH4
+ oxidation, the main process for NO2
− production (nitritation), and NO2
− reduction, and (2) oxidation of soil organic N, the dominant NO3
− production process in this soil, and dissimilatory NO3
− reduction to NH4
+ (DNRA). The production of NH4
+ via DNRA was approximately ten times higher than direct mineralization from recalcitrant soil organic matter. Moreover, the
rate of DNRA was several magnitudes higher than the rate of other NO3
− reducing processes, indicating that DNRA is able to outcompete denitrification, which is most likely not an important process
in this ecosystem. These functional links are most likely adaptations of the microbial community to the prevailing pedo-climatic
conditions of this Nothofagus ecosystem. 相似文献
20.
Biogeochemical theory emphasizes nitrogen (N) limitation and the many factors that can restrict N accumulation in temperate
forests, yet lacks a working model of conditions that can promote naturally high N accumulation. We used a dynamic simulation
model of ecosystem N and δ15N to evaluate which combination of N input and loss pathways could produce a range of high ecosystem N contents characteristic
of forests in the Oregon Coast Range. Total ecosystem N at nine study sites ranged from 8,788 to 22,667 kg ha−1 and carbon (C) ranged from 188 to 460 Mg ha−1, with highest values near the coast. Ecosystem δ15N displayed a curvilinear relationship with ecosystem N content, and largely reflected mineral soil, which accounted for 96–98%
of total ecosystem N. Model simulations of ecosystem N balances parameterized with field rates of N leaching required long-term
average N inputs that exceed atmospheric deposition and asymbiotic and epiphytic N2-fixation, and that were consistent with cycles of post-fire N2-fixation by early-successional red alder. Soil water δ15NO3
− patterns suggested a shift in relative N losses from denitrification to nitrate leaching as N accumulated, and simulations
identified nitrate leaching as the primary N loss pathway that constrains maximum N accumulation. Whereas current theory emphasizes
constraints on biological N2-fixation and disturbance-mediated N losses as factors that limit N accumulation in temperate forests, our results suggest
that wildfire can foster substantial long-term N accumulation in ecosystems that are colonized by symbiotic N2-fixing vegetation. 相似文献