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1.
Permafrost soils are a significant global store of carbon (C) with the potential to become a large C source to the atmosphere.
Climate change is causing permafrost to thaw, which can affect primary production and decomposition, therefore affecting ecosystem
C balance. To understand future responses of permafrost soils to climate change, we inventoried current soil C stocks, investigated
∆14C, C:N, δ13C, and δ15N depth profiles, modeled soil C accumulation rates, and calculated decadal net ecosystem production (NEP) in subarctic tundra
soils undergoing minimal, moderate, and extensive permafrost thaw near Eight Mile Lake (EML) in Healy, Alaska. We modeled
decadal and millennial soil C inputs, decomposition constants, and C accumulation rates by plotting cumulative C inventories
against C ages based on radiocarbon dating of surface and deep soils, respectively. Soil C stocks at EML were substantial,
over 50 kg C m−2 in the top meter, and did not differ much among sites. Carbon to nitrogen ratio, δ13C, and δ15N depth profiles indicated most of the decomposition occurred within the organic soil horizon and practically ceased in deeper,
frozen horizons. The average C accumulation rate for EML surface soils was 25.8 g C m−2 y−1 and the rate for the deep soil accumulation was 2.3 g C m−2 y−1, indicating these systems have been C sinks throughout the Holocene. Decadal net ecosystem production averaged 14.4 g C m−2 y−1. However, the shape of decadal C accumulation curves, combined with recent annual NEP measurements, indicates soil C accumulation
has halted and the ecosystem may be becoming a C source. Thus, the net impact of climate warming on tundra ecosystem C balance
includes not only becoming a C source but also the loss of C uptake capacity these systems have provided over the past ten
thousand years. 相似文献
2.
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
3.
Understanding what governs patterns of soil δ15N and δ13C is limited by the absence of these data assembled throughout the development of individual ecosystems. These patterns are important because stable isotopes of soil organic N and C are integrative indicators of biogeochemical processing of soil organic matter. We examined δ15N of soil organic matter (δ15NSOM) and δ13CSOM of archived soil samples across four decades from four depths of an aggrading forest in southeastern USA. The site supports an old-field pine forest in which the N cycle is affected by former agricultural fertilization, massive accumulation of soil N by aggrading trees over four decades, and small to insignificant fluxes of N via NH3 volatilization, nitrification, and denitrification. We examine isotopic data and the N and C dynamics of this ecosystem to evaluate mechanisms driving isotopic shifts over time. With forest development, δ13CSOM became depth-dependent. This trend resulted from a decline of ~2‰ in the surficial 15 cm of mineral soil to −26.0‰, due to organic matter inputs from forest vegetation. Deeper layers exhibited relatively little trend in δ13CSOM with time. In contrast, δ15NSOM was most dynamic in deeper layers. During the four decades of forest development, the deepest layer (35–60 cm) reached a maximum δ15N value of 9.1‰, increasing by 7.6‰. The transfer of >800 kg ha−1 of soil organic N into aggrading vegetation and the forest floor and the apparent large proportion of ectomycorrhizal (ECM) fungi in these soils suggest that fractionation via microbial transformations must be the major process changing δ15N in these soils. Accretion of isotopically enriched compounds derived from microbial cells (i.e., ECM fungi) likely promote isotopic enrichment of soils over time. The work indicates the rapid rate at which ecosystem development can impart δ15NSOM and δ13CSOM signatures associated with undisturbed soil profiles. 相似文献
4.
To identify the controls on dissolved organic carbon (DOC) production, we incubated soils from 18 sites, a mixture of 52 forest
floor and peats and 41 upper mineral soil samples, at three temperatures (3, 10, and 22°C) for over a year and measured DOC
concentration in the leachate and carbon dioxide (CO2) production from the samples. Concentrations of DOC in the leachate were in the range encountered in field soils (<2 to >50 mg l−1). There was a decline in DOC production during the incubation, with initial rates averaging 0.03–0.06 mg DOC g−1 soil C day−1, falling to averages of 0.01 mg g−1 soil C day−1; the rate of decline was not strongly related to temperature. Cumulative DOC production rates over the 395 days ranged from
less than 0.01 to 0.12 mg g−1 soil C day−1 (0.5–47.6 mg g−1 soil C), with an average of 0.021 mg g−1 soil C day−1 (8.2 mg g−1 soil C). DOC production rate was weakly related to temperature, equivalent to Q10 values of 0.9 to 1.2 for mineral samples and 1.2 to 1.9 for organic samples. Rates of DOC production in the organic samples
were correlated with cellulose (positively) and lignin (negatively) proportion in the organic matter, whereas in the mineral
samples C and nitrogen (N) provided positive correlations. The partitioning of C released into CO2–C and DOC showed a quotient (CO2–C:DOC) that varied widely among the samples, from 1 to 146. The regression coefficient of CO2–C:DOC production (log10 transformed) ranged from 0.3 to 0.7, all significantly less than 1. At high rates of DOC production, a smaller proportion
of CO2 is produced. The CO2–C:DOC quotient was dependent on incubation temperature: in the organic soil samples, the CO2–C:DOC quotient rose from an average of 6 at 3 to 16 at 22°C and in the mineral samples the rise was from 7 to 27. The CO2–C:DOC quotient was related to soil pH in the organic samples and C and N forms in the mineral samples. 相似文献
5.
Patterns of net nitrogen mineralization and nitrification in 0–7.5 cm deep mineral soils of different stages (seral ages 1,
6 and 20 years) of a post-fire coastal fynbos succession were assayed using laboratory andin situ incubations. No evidence of increasing allelopathic inhibition of nitrification with successional development was found as
NO3−N was the predominant product at all seral stages and the NO3−N∶NH4−N ratio remained constant. Rather the results of field incubations of soils beneathProtea repens stands of different successional ages showed that increased mineralization and nitrification appeared to be associated with
increased soil total N content rather than with successional age. Further, the incubation of soilsin situ during the dry summer months showed that NO3−N production appears to be closely related to temperature and soil moisture content, both of which are variables that vary
throughout succession due to the changing structure of the vegetation. 相似文献
6.
Soil nitrogen cycling and nitrous oxide flux in a Rocky Mountain Douglas-fir forest: effects of fertilization,irrigation and carbon addition 总被引:6,自引:4,他引:2
Pamela A. Matson Stith Thoma Gower Carol Volkmann Christine Billow Charles C. Grier 《Biogeochemistry》1992,18(2):101-117
Nitrous oxide fluxes and soil nitrogen transformations were measured in experimentally-treated high elevation Douglas-fir
forests in northwestern New Mexico, USA. On an annual basis, forests that were fertilized with 200 kg N/ha emitted an average
of 0.66 kg/ha of N2O-N, with highest fluxes occurring in July and August when soils were both warm and wet. Control, irrigated, and woodchip
treated plots were not different from each other, and annual average fluxes ranged from 0.03 to 0.23 kg/ha. Annual net nitrogen
mineralization and nitrate production were estimated in soil and forest floor usingin situ incubations; fertilized soil mineralized 277 kg ha−1 y−1 in contrast to 18 kg ha−1 y−1 in control plots. Relative recovery of15NH4-N applied to soil in laboratory incubations was principally in the form of NO3-N in the fertilized soils, while recovery was mostly in microbial biomass-N in the other treatments. Fertilization apparently
added nitrogen that exceeded the heterotrophic microbial demand, resulting in higher rates of nitrate production and higher
nitrous oxide fluxes. Despite the elevated nitrous oxide emission resulting from fertilization, we estimate that global inputs
of nitrogen into forests are not currently contributing significantly to the increasing concentrations of nitrous oxide in
the atmosphere. 相似文献
7.
We used the natural abundance of 15N in soils in forests, pastures and cultivated lands in the Menagesha and Wendo-Genet areas of Ethiopia to make inferences
about the N cycles in these ecosystems. Since we have described the history of these sites based on variations in 13C natural abundance, patterns of δ15N and δ13C values were compared to determine if shifts of 15N correlate with shifts of vegetation. At Menagesha, a > 500-yr-old planted forest, we found δ15N values from −8.8 to +3.5‰ in litter, from −3.5 to +4.5‰ in 0–10 cm soil layer, and from −1.5 to +6.8‰ at >20 cm soil depth.
The low δ15N in litter and surface mineral soils suggests that a closed N cycle has operated for a long time. At this site, the low δ13C of the surface horizon and the high δ13C of the lower soil horizons is clear evidence of a long phase of C4 grass dominance or cultivation of C4 crops before the establishment of the forest >500 years ago. In contrast, at Wendo-Genet, high δ13C of soils reveals that most of the land has been uncovered by forests until recently. Soil δ15N was high throughout (3.4–9.8‰), and there were no major differences between forested, cultivated and pasture soils in δ15N values of surface mineral soils. The high δ15N values suggest that open N cycles operate in the Wendo-Genet area. From the points of view of soil fertility management,
it is interesting that tall forest ecosystems with relatively closed N cycling could be established on the fairly steep slopes
at Menagesha after a long period of grass vegetation cover or cultivation.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
8.
Christine L. Goodale Steven A. Thomas Guinevere Fredriksen Emily M. Elliott Kathryn M. Flinn Thomas J. Butler M. Todd Walter 《Biogeochemistry》2009,93(3):197-218
Atmospheric deposition contributes a large fraction of the annual nitrogen (N) input to the basin of the Susquehanna River,
a river that provides two-thirds of the annual N load to the Chesapeake Bay. Yet, there are few measurements of the retention
of atmospheric N in the Upper Susquehanna’s forested headwaters. We characterized the amount, form (nitrate, ammonium, and
dissolved organic nitrogen), isotopic composition (δ15N- and δ18O-nitrate), and seasonality of stream N over 2 years for 7–13 catchments. We expected high rates of N retention and seasonal
nitrate patterns typical of other seasonally snow-covered catchments: dormant season maxima and growing season minima. Coarse
estimates of N export indicated high rates of inorganic N retention (>95%), yet streams had unexpected seasonal nitrate patterns,
with summer peaks (14–96 μmol L−1), October crashes (<1 μmol L−1), and modest rebounds during the dormant season (<1–20 μmol L−1). Stream δ18O-nitrate values indicated microbial nitrification as the primary source of stream nitrate, although snowmelt or other atmospheric
source contributed up to 47% of stream nitrate in some March samples. The autumn nitrate crash coincided with leaffall, likely
due to in-stream heterotrophic uptake of N. Hypothesized sources of the summer nitrate peaks include: delayed release of nitrate
previously flushed to groundwater, weathering of geologic N, and summer increases in net nitrate production. Measurements
of shale δ15N and soil-, well-, and streamwater nitrate within one catchment point toward a summer increase in soil net nitrification
as the driver of this pattern. Rather than seasonal plant demand, processes governing the seasonal production, retention,
and transport of nitrate in soils may drive nitrate seasonality in this and many other systems. 相似文献
9.
Dissolved organic carbon affects soil microbial activity and nitrogen dynamics in a Mexican tropical deciduous forest 总被引:1,自引:0,他引:1
Seasonal variation of dissolved organic C (DOC) and its effects on microbial activity and N dynamics were studied during two
consecutive years in soils with different organic C concentrations (hilltop and hillslope) in a tropical deciduous forest
of Mexico. We found that DOC concentrations were higher at the hilltop than at the hillslope soils, and in both soils generally
decreased from the dry to the rainy season during the two study years. Microbial biomass and potential C mineralization rates,
as well as dissolved organic N (DON) and NH4+ concentrations and net N immobilization were higher in soils with higher DOC than in soils with lower DOC. In contrast, net
N immobilization and NH4+ concentration were depleted in the soil with lowest DOC, whereas NO3− concentrations and net nitrification increased. Negative correlations between net nitrification and DOC concentration suggested
that NH4+ was transformed to NO3− by nitrifiers when the C availability was depleted. Taken together, our results suggest that available C appears to control
soil microbial activity and N dynamics, and that microbial N immobilization is facilitated by active heterotrophic microorganisms
stimulated by high C availability. Soil autotrophic nitrification is magnified by decreases in C availability for heterotrophic
microbial activity. This study provides an experimental data set that supports the conceptual model to show and highlight
that microbial dynamics and N transformations could be functionally coupled with DOC availability in the tropical deciduous
forest soils.
Responsible Editor: Chris Neill 相似文献
10.
Fungal control of nitrous oxide production in semiarid grassland 总被引:2,自引:0,他引:2
Fungi are capable of both nitrification and denitrification and dominate the microbial biomass in many soils. Recent work
suggests that fungal rather than bacterial pathways dominate N transformation in desert soils. We evaluated this hypothesis
by comparing the contributions of bacteria and fungi to N2O production at control and N fertilized sites within a semiarid grassland in central New Mexico (USA). Soil samples were
taken from the rhizosphere of blue grama (B. gracilus) and the microbiotic crusts that grow in open areas between the bunch grasses. Soils incubated at 30% or 70% water holding
capacity, were exposed to one of three biocide treatments (control, cycloheximide or streptomycin). After 48 h, N2O and CO2 production were quantified along with the activities of several extracellular enzymes. N2O production from N fertilized soils was higher than that of control soils (165 vs. 41 pmol h−1 g−1), was higher for crust soil than for rhizosphere soil (108 vs. 97 pmol h−1 g−1), and increased with soil water content (146 vs. 60 pmol h−1 g−1). On average, fungicide (cycloheximide) addition reduced N2O production by 85% while increasing CO2 production by 69%; bactericide (streptomycin) reduced N2O by 53% with mixed effects on CO2 production. N2O production was significantly correlated with C and N mineralization potential as measured by assays for glycosidic and proteolytic
enzymes, and with extractable nitrate and ammonium. Our data indicate that fungal nitrifier denitrification and bacterial
autotrophic nitrification dominate N transformation in this ecosystem and that N2O production is highly sensitive to soil cover, N deposition and moisture. 相似文献
11.
Isotopic signature of nitrate in two contrasting watersheds of Brush Brook,Vermont, USA 总被引:2,自引:0,他引:2
We used the dual isotope method to study differences in nitrate export in two subwatersheds in Vermont, USA. Precipitation,
soil water and streamwater samples were collected from two watersheds in Camels Hump State Forest, located within the Green
Mountains of Vermont. These samples were analyzed for the δ15N and δ18O of NO3−. The range of δ15N–NO3− values overlapped, with precipitation −4.5‰ to +2.0‰ (n = 14), soil solution −10.3‰ to +6.2‰ (n = 12) and streamwater +0.3‰ to +3.1‰ (n = 69). The δ18O of precipitation NO3− (mean 46.8 ± 11.5‰) was significantly different (P < 0.001) from that of the stream (mean 13.2 ± 4.3‰) and soil waters (mean 14.5 ± 4.2‰) even during snowmelt periods. Extracted
soil solution and streamwater δ18O of NO3− were similar and within the established range of microbially produced NO3−, demonstrating that NO3− was formed by microbial processes. The δ15N and δ18O of NO3− suggests that although the two tributaries have different seasonal NO3− concentrations, they have a similar NO3− source. 相似文献
12.
The effects of forest management (thinning) on gross and net N conversion, the balance of inorganic N production and consumption, inorganic N concentrations and on soil microbial biomass in the Ah layer were studied in situ during eight intensive field measuring campaigns in the years 2002–2004 at three beech (Fagus sylvatica L.) forest sites. At all sites adjacent thinning plots (“T”) and untreated control plots (“C”) were established. Since the sites are characterized either by cool-moist microclimate (NE site and NW site) or by warm-dry microclimate (SW site) and thinning took place in the year 1999 at the NE and SW sites and in the year 2003 at the NW site the experimental design allowed to evaluate (1) short-term effects (years 1–2) of thinning at the NW site and (2) medium-term effects (years 4–6) of thinning under different microclimate at the SW and NE site. Microbial biomass N was consistently higher at the thinning plots of all sites during most of the field campaigns and was overall significantly higher at the SWT and NWT plots as compared to the corresponding untreated control plots. The size of the microbial biomass N pool was found to correlate positively with both gross ammonification and gross nitrification as well as with extractable soil NO3− concentrations. At the SW site neither gross ammonification, gross nitrification, gross ammonium (NH4+) immobilization and gross nitrate (NO3−) immobilization nor net ammonification, net nitrification and extractable NH4+ and NO3− contents were significantly different between control and thinning plot. At the NET plot lower gross ammonification and gross NH4+ immobilization in conjunction with constant nitrification rates coincided with higher net nitrification and significantly higher extractable NO3− concentrations. Thus, the medium-term effects of thinning varied with different microclimate. The most striking thinning effects were found at the newly thinned NW site, where gross ammonification and gross NH4+ immobilization were dramatically higher immediately after thinning. However, they subsequently tended to decrease in favor of gross nitrification, which was significantly higher at the NWT plot as compared to␣the␣NWC plot during all field campaigns after␣thinning except for April 2004. This increase␣in␣gross nitrification at the NWT plot (1.73 mg N kg−1 sdw day−1 versus 0.48 mg N kg−1 sdw day−1 at the NWC plot) coincided with significantly higher extractable NO3− concentrations (4.59 mg N kg−1 sdw at the NWT plot versus 0.96 mg N kg−1 sdw at the NWC plot). Pronounced differences in relative N retention (the ratio of gross NH4+ immobilization + gross NO3− immobilization to gross ammonification + gross nitrification) were found across the six research plots investigated and could be positively correlated to the soil C/N ratio (R = 0.94; p = 0.005). In sum, the results obtained in this study show that (1) thinning can lead to a shift in the balance of microbial inorganic N production and consumption causing a clear decrease in the N retention capacity in the monitored forest soils especially in the first two years after thinning, (2)␣the resistance of the investigated forest ecosystems to disturbances of N cycling by thinning may vary with different soil C contents and C/N ratios, e. g. caused by differences in microclimate, (3) thinning effects tend to decline with the growth of understorey vegetation in the years 4–6 after thinning. 相似文献
13.
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
14.
Katja Pörtl Sophie Zechmeister-Boltenstern Wolfgang Wanek Per Ambus Torsten W. Berger 《Plant and Soil》2007,295(1-2):79-94
Natural 15N abundance measurements of ecosystem nitrogen (N) pools and 15N pool dilution assays of gross N transformation rates were applied to investigate the potential of δ15N signatures of soil N pools to reflect the dynamics in the forest soil N cycle. Intact soil cores were collected from pure
spruce (Picea abies (L.) Karst.) and mixed spruce-beech (Fagus sylvatica L.) stands on stagnic gleysol in Austria. Soil δ15N values of both forest sites increased with depth to 50 cm, but then decreased below this zone. δ15N values of microbial biomass (mixed stand: 4.7 ± 0.8‰, spruce stand: 5.9 ± 0.9‰) and of dissolved organic N (DON; mixed stand:
5.3 ± 1.7‰, spruce stand: 2.6 ± 3.3‰) were not significantly different; these pools were most enriched in 15N of all soil N pools. Denitrification represented the main N2O-producing process in the mixed forest stand as we detected a significant 15N enrichment of its substrate NO3− (3.6 ± 4.5‰) compared to NH4+ (−4.6 ± 2.6‰) and its product N2O (−11.8 ± 3.2‰). In a 15N-labelling experiment in the spruce stand, nitrification contributed more to N2O production than denitrification. Moreover, in natural abundance measurements the NH4+ pool was slightly 15N-enriched (−0.4 ± 2.0 ‰) compared to NO3− (−3.0 ± 0.6 ‰) and N2O (−2.1 ± 1.1 ‰) in the spruce stand, indicating nitrification and denitrification operated in parallel to produce N2O. The more positive δ15N values of N2O in the spruce stand than in the mixed stand point to extensive microbial N2O reduction in the spruce stand. Combining natural 15N abundance and 15N tracer experiments provided a more complete picture of soil N dynamics than possible with either measurement done separately. 相似文献
15.
Lars Ola Nilsson Håkan Wallander Erland Bååth Ursula Falkengren-Grerup 《Biogeochemistry》2006,80(1):43-55
Anthropogenic N deposition may change soil conditions in forest ecosystems as demonstrated in many studies of coniferous forests,
whereas results from deciduous forests are relatively scarce. Therefore the influence of N deposition on several variables
was studied in situ in 45 oak-dominated deciduous forests along a N deposition gradient in southern Sweden, where the deposition ranged from
10 to 20 kg N ha−1 year−1. Locally estimated NO
−
3
deposition, as measured with ion-exchange resins (IER) on the soil surface, and grass N concentration (%) were positively
correlated with earlier modelled regional N deposition. Furthermore, the δ15N values of grass and uppermost soil layers were negatively correlated with earlier modelled N deposition. The data on soil
NO
−
3
, measured with IER in the soil, and grass N concentration suggest increased soil N availability as a result of N deposition.
The δ15N values of grass and uppermost soil layers indicate increased nitrification rates in high N deposition sites, but no large
downward movements of NO
−
3
in these soils. Only a few sites had NO
−
3
concentrations exceeding 1 mg N l−1 in soil solution at 50 cm depth, which showed that N deposition to these acid oak-dominated forests has not yet resulted
in extensive leaching of N. The δ15N enrichment factor was the variable best correlated with NO
−
3
concentrations at 50 cm and is thus a variable that potentially may be used to predict leaching of NO
−
3
from forest soils. 相似文献
16.
During microbial breakdown of leaf litter a fraction of the C lost by the litter is not released to the atmosphere as CO2 but remains in the soil as microbial byproducts. The amount of this fraction and the factors influencing its size are not
yet clearly known. We performed a laboratory experiment to quantify the flow of C from decaying litter into the soil, by means
of stable C isotopes, and tested its dependence on litter chemical properties. Three sets of 13C-depleted leaf litter (Liquidambar styraciflua L., Cercis canadensis L. and Pinus taeda L.) were incubated in the laboratory in jars containing 13C-enriched soil (i.e. formed C4 vegetation). Four jars containing soil only were used as a control. Litter chemical properties
were measured using thermogravimetry (Tg) and pyrolysis–gas chromatography/mass spectrometry–combustion interface–isotope
ratio mass spectrometry (Py–GC/MS–C–IRMS). The respiration rates and the δ13C of the respired CO2 were measured at regular intervals. After 8 months of incubation, soils incubated with both L. styraciflua and C. canadensis showed a significant change in δ13C (δ13Cfinal = −20.2 ± 0.4‰ and −19.5 ± 0.5‰, respectively) with respect to the initial value (δ13Cinitial = −17.7 ± 0.3‰); the same did not hold for soil incubated with P. taeda (δ13Cfinal:−18.1 ± 0.5‰). The percentages of litter-derived C in soil over the total C loss were not statistically different from one
litter species to another. This suggests that there is no dependence of the percentage of C input into the soil (over the
total C loss) on litter quality and that the fractional loss of leaf litter C is dependent only on the microbial assimilation
efficiency. The percentage of litter-derived C in soil was estimated to be 13 ± 3% of total C loss. 相似文献
17.
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
18.
Steven Sleutel Peter Leinweber Shamim Ara Begum Mohammed Abdul Kader Patrick Van Oostveldt Stefaan De Neve 《Biogeochemistry》2008,89(2):253-271
Unusually high SOC levels have been reported for sandy cropland soils in North-Western Europe. A potential link with their
general heathland land-use history was investigated by comparing two soil pairs of relict heathland and cultivated former
heathland in the Belgian sandy region. A sequential chemical fractionation yielded similar sizes in corresponding SOM fractions
between the heathland and cropland soils (i.e. NaOCl resistant: 12.3–15.0 g C kg−1 and NaOCl + HF resistant: 2.6–5.3 g C kg−1). Higher amounts of clay sized N in the cropland plots can be attributed to N additions from mineral fertilizers and animal
manure. Temperature resolved Pyrolysis Field Ionization Mass Spectroscopy analysis showed that the composition of both relict
heathland and cultivated soils was surprisingly similar, in spite of over 60 years of intense cropland management. The mass
spectra of SOM in both heathland-cropland soil pairs investigated was dominated by signals from lipids, alkylaromatics and
sterols. The accumulation of this SOM rich in aliphatics was logically linked to the high input of lipids, long-chain aliphatics
and sterols from heathland vegetation and the low soil pH and microbial activity. Based on the relatively high OC surface
loadings of HF-extractable OM (13–44 mg C m−2 Fe and 1.2–2.3 mg C m−2 clay), direct organo-mineral bonds between OM and Fe-oxides or clay minerals seem to be only partly involved as a stabilization
mechanism in these soils. The distinct bimodal shape of the thermograms indicates that OM-crosslinking could furthermore contribute
substantially to SOM stabilization in these soils. This study therefore corroborates the previously proposed view that lipids
may be bound in networks of alkylaromatics, the structural building blocks of OM macromolecules. We hypothesize that such
binding is able to explain the measured retention of these OM components, even under several decades of cropland management. 相似文献
19.
Conventionally Tilled and Permanent Raised Beds with Different Crop Residue Management: Effects on Soil C and N Dynamics 总被引:1,自引:0,他引:1
B. Govaerts K. D. Sayre J. M. Ceballos-Ramirez M. L. Luna-Guido A. Limon-Ortega J. Deckers L. Dendooven 《Plant and Soil》2006,280(1-2):143-155
Conservation tillage in its version of permanent bed planting under zero-tillage with crop residue retention has been proposed
as an alternative wheat production system for northwest Mexico. However, little is known about the dynamics of C and N in
soils under wheat/maize on permanent beds (PB) where straw was burned, removed, partly removed or retained, as opposed to
conventionally tilled beds (CTB) where straw was incorporated. We investigated the dynamics of soil C and N and normalized
difference vegetative index (NDVI) crop values in zero-tilled PB and CTB after 26 successive maize and wheat crops. Organic
C and total N were respectively, 1.15 and 1.17 times greater in PB with straw partly removed and with straw retained on the
surface, than in CTB with straw incorporated. Organic C and total N were 1.10 times greater in soils with 300 kg N ha−1 added than in unfertilized soil. Cumulative production of CO2 was lower under CTB with straw incorporated than under PB treatments, and CO2 production increased with increments in inorganic fertilizer. The N-mineralization rate was 1.18 times greater than in unamended
soils when 150 kg inorganic N ha−1 was applied, and 1.48 times greater when 300 kg inorganic N ha−1 was added. The N-mineralization rate was significantly (1.66 times) greater in PB where the straw was burned or retained
on the surface than in CTB where the straw was incorporated, but significantly (1.25 times) lower than in PB with straw partly
removed. The NDVI values reached a maximum 56 days after planting and decreased thereafter. The NDVI for unfertilized soil
were similar for CTB with straw incorporated, PB with straw partly removed, and PB with straw retained on the surface, but
significantly lower for PB with straw burned and PB with straw removed. In soils to which 150 or 300 kg N ha−1 was added, NDVI was significantly lower for PB with straw burned than for other treatments. Among other things, this suggests
the utility of rotating maize or wheat with crops whose residues have lower C–N ratios, thus avoiding immobilization of large
amounts of N for extended periods. PB with residue burning, however, is an unsustainable practice leading to low crop performance
and soil and environmental degradation. 相似文献
20.
Richard D. Bowden Mark S. Castro Jerry M. Melillo Paul A. Steudler John D. Aber 《Biogeochemistry》1993,21(2):61-71
Fluxes of nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) between soils and the atmosphere were measured monthly for one year in a 77-year-old temperate hardwood forest following
a simulated hurricane blowdown. Emissions of CO2 and uptake of CH4 for the control plot were 4.92 MT C ha−1 y−1 and 3.87 kg C ha−1 y−1, respectively, and were not significantly different from the blowdown plot. Annual N2O emissions in the control plot (0.23 kg N ha−1 y−1) were low and were reduced 78% by the blowdown. Net N mineralization was not affected by the blowdown. Net nitrification
was greater in the blowdown than in the control, however, the absolute rate of net nitrification, as well as the proportion
of mineralized N that was nitrified, remained low. Fluxes of CO2 and CH4 were correlated positively to soil temperature, and CH, uptake showed a negative relationship to soil moisture. Substantial
resprouting and leafing out of downed or damaged trees, and increased growth of understory vegetation following the blowdown,
were probably responsible for the relatively small differences in soil temperature, moisture, N availability, and net N mineralization
and net nitrification between the control and blowdown plots, thus resulting in no change in CO2 or CH4 fluxes, and no increase in N2O emissions. 相似文献