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1.
A laboratory investigation was performed to compare the fluxes of dinitrogen (N2), N2O and carbon dioxide (CO2) from no-till (NT) and conventional till (CT) soils under the same water, mineral nitrogen and temperature status. Intact soil cores (0-10 cm) were incubated for 2 weeks at 25 °C at either 75% or 60% water-filled pore space (WFPS) with 15N-labeled fertilizers (100 mg N kg−1 soil). Gas and soil samples were collected at 1-4 day intervals during the incubation period. The N2O and CO2 fluxes were measured by a gas chromatography (GC) system while total N2 and N2O losses and their 15N mole fractions in the soil mineral N pool were determined by a mass spectrometer. The daily accumulative fluxes of N2 and N2O were significantly affected by tillage, N source and soil moisture. We observed higher (P<0.05) fluxes of N2+N2O, N2O and CO2 from the NT soils than from the CT soils. Compared with the addition of nitrate (NO3−), the addition of ammonium (NH4+) enhanced the emissions of these N and C gases in the CT and NT soils, but the effect of NH4+ on the N2 and/or N2O fluxes was evident only at 60% WFPS, indicating that nitrification and subsequent denitrification contributed largely to the gaseous N losses and N2O emission under the lower moisture condition. Total and fertilizer-induced emissions of N2 and/or N2O were higher (P<0.05) at 75% WFPS than with 60% WFPS, while CO2 fluxes were not influenced by the two moisture levels. These laboratory results indicate that there is greater potential for N2O loss from NT soils than CT soils. Avoiding wet soil conditions (>60% WFPS) and applying a NO3− form of N fertilizer would reduce potential N2O emissions from arable soils.  相似文献   

2.
Agricultural soils contribute significantly to atmospheric nitrous oxide (N2O). A considerable part of the annual N2O emission may occur during the cold season, possibly supported by high product ratios in denitrification (N2O/(N2+N2O)) and nitrification (N2O-N/(NO3−-N+NO2−-N)) at low temperatures and/or in response to freeze-thaw perturbation. Water-soluble organic materials released from frost-sensitive catch crops and green manure may further increase winter emissions. We conducted short-term laboratory incubations under standardized moisture and oxygen (O2) conditions, using nitrogen (N) tracers (15N) to determine process rates and sources of emitted N2O after freeze-thaw treatment of soil or after addition of freeze-thaw extract from clover. Soil respiration and N2O production was stimulated by freeze-thaw or addition of plant extract. The N2O emission response was inversely related to O2 concentration, indicating denitrification as the quantitatively prevailing process. Denitrification product ratios in the two studied soils (pH 4.5 and 7.0) remained largely unaltered by freeze-thaw or freeze-thaw-released plant material, refuting the hypothesis that high winter emissions are due to frost damage of N2O reductase activity. Nitrification rates estimated by nitrate (NO3−) pool enrichment were 1.5-1.8 μg NO3-N g−1 dw soil d−1 in freeze-thaw-treated soil when incubated at O2 concentrations above 2.3 vol% and one order of magnitude lower at 0.8 vol% O2. Thus, the experiments captured a situation with severely O2-limited nitrification. As expected, the O2 stress at 0.8 vol% resulted in a high nitrification product ratio (0.3 g g−1). Despite this high product ratio, only 4.4% of the measured N2O accumulation originated from nitrification, reaffirming that denitrification was the main N2O source at the various tested O2 concentrations in freeze-thaw-affected soil. N2O emission response to both freeze-thaw and plant extract addition appeared strongly linked to stimulation of carbon (C) respiration, suggesting that freeze-thaw-induced release of decomposable organic C was the major driving force for N2O emissions in our soils, both by fuelling denitrifiers and by depleting O2. The soluble C (applied as plant extract) necessary to induce a CO2 and N2O production rate comparable with that of freeze-thaw was 20-30 μg C g−1 soil dw. This is in the range of estimates for over-winter soluble C loss from catch crops and green manure plots reported in the literature. Thus, freeze-thaw-released organic C from plants may play a significant role in freeze-thaw-related N2O emissions.  相似文献   

3.
Denitrification assays in soils spiked with zinc salt have shown inhibition of the N2O reduction resulting in increased soil N2O fluxes with increasing soil Zn concentration. It is unclear if the same is true for environmentally contaminated soils. Net production of N2O and N2 was monitored during anaerobic incubations (25 °C, He atmosphere) of soils freshly spiked with ZnCl2 and of corresponding soils that were gradually enriched with metals (mainly Zn) in the field by previous sludge amendments or by corrosion of galvanized structures. Total denitrification activity (i.e. the sum of N2O+N2 production rate) was not inhibited by freshly added Zn salts up to 1600 mg Zn kg−1, whereas N2O reduction decreased by 50% (EC50) at total Zn concentrations of 231 mg Zn kg−1 (ZEV soil) and 368 mg Zn kg−1 (TM soil). In contrast, N2O reduction was not reduced by soil Zn in any of the field contaminated soils, even at total soil Zn or soil solution Zn concentrations exceeding more than 5 times corresponding EC50's of the freshly spiked soil. The absence of adverse effects in the field contaminated soils was unrelated to soil NO3 or organic matter concentration. Ageing (2-8 weeks) and soil leaching after spiking reduced the toxicity of Zn on N2O reduction, either expressed as total Zn or soil solution Zn, suggesting adaptation reactions. However, no full recovery after spiking was identified at the largest incubation period in one soil. In addition, the denitrification assay performed with sewage sludge showed elevated N2O release in Zn contaminated sludges (>6000 mg Zn kg−1 dry matter) whereas this was not observed in low Zn sludge (<1000 mg Zn kg−1 dry matter) suggesting limits to adaptation reactions in the sludge particles. It is concluded that the use of soils spiked with Zn salts overestimates effects on N2O reduction. Field data on N2O fluxes in sludge amended soils are required to identify if metals indeed promote N2O emissions in sludge amended soils.  相似文献   

4.
The importance of subsoil denitrification on the fate of agriculturally derived nitrate (NO3) leached to groundwater is crucial for budgeting N in an ecosystem and for identifying areas where the risk of excess NO3 is reduced. However, the high atmospheric background of di-nitrogen (N2) causes difficulties in assessing denitrification enzyme activity (DEA) and denitrification potential (DP) in soils directly. Here, we apply Membrane Inlet Mass Spectrometry (MIMS) technique to investigate indirectly DEA and DP in soils by measuring N2/Ar ratio changes in headspace water over soil. Soils were collected from 0-10, 15-25 and 60-70 cm depths of a grazed ryegrass and grass-clover. The samples were amended with helium-flushed deionized water containing ranges of NO3 and carbon (glucose-C) and were incubated for six hours in the dark at 21 °C. The peaks for N2/Ar ratio, declined with increasing soil depth, indicating a reduced substrate requirements to initiate DEA en-masse (15-30 mg NO3-N alone or with 60-120 mg glucose-C, kg−1 soil). The dissolved N2O concentrations were very small (0.004-0.269 μg N kg−1 soil) but responded well to the added N and C, showing a reduction in DEA with soil depth. In three separate studies, only subsoils were incubated for 3 days at 12 °C with 20-30 mg NO3-N ± 40-60 mg glucose-C, kg−1 soil. Denitrification capacity (DC, NO3 only treatment) was not statistically different to the control (no amendment) within a land use (0.03-0.05 vs. 0.07-0.22 mg N kg−1 soil d−1), the highest being in ryegrass subsoils receiving groundwater. The DP was significantly (P < 0.0001) higher in subsoils under ryegrass than under grass-clover (0.50-0.71 vs. 1.15 mg N kg−1 soil d−1). The rates of DP (NO3 + glucose-C) increased significantly (P < 0.0001) in unsaturated and saturated subsoils (0.92 and 2.19 mg N kg−1 soil d−1, respectively) of grass-clover, due to the higher reductive state resulting from the 10 day pre-incubation. Available C accelerated denitrification in soils and superseded the temporary elevation in oxidative state due to NO3 addition. The substrates load differences between the land uses regulated the degree of denitrification rates. Results suggest that both dissolved N2O measured by gas chromatography and N2/Ar ratio measured by MIMS to indirectly determine DEA, and the latter to quantify total DC/DP in soils can be used. However, interference of oxygen in the MIMS system should be considered if available C is added or is naturally elevated in soil or groundwater.  相似文献   

5.
The accurate measurement of nitrous oxide (N2O) and dinitrogen (N2) during the denitrification process in soils is a challenge which will help to estimate the contribution of soil N2O emissions to global warming. Oxygen concentration, nitrate concentration and carbon availability are generally the main factors that control soil denitrification rate and the amount of N2O or N2 emitted. The aim of this paper is to present a database of the N2O mole fraction measured at the field scale, and to test hypotheses concerning its regulation. A 15N-nitrate tracer solution was added to 36 undisturbed soil cores on a 20 m×20 m cultivated field plot. Fluxes of CO2, N2O and N2 from the soil surface were monitored for 24 h. Soil moisture, bulk density, carbon, nitrogen and mineral nitrogen concentration were also measured to investigate possible spatial relationships between their variations and those of N2O, N2 and nitrous oxide mole fraction. Under high water content, nitrous oxide and N2 emissions were highly variable with variation coefficients of 70-140%. N2O emission rates were about twice as high as those of N2, with a total denitrification rate ranging from 269 to 3843 g N ha−1 d−1. After 24 h of incubation, the values of nitrous oxide mole fraction ranged from 0.15 to 0.94 and no significant decline during incubation time was observed. Spatial variability of N2O, N2 and nitrous oxide mole fraction was high and no spatial dependence was observed at the scale of the experimental plot. Only tenuous relationships between gaseous nitrogen emissions and soil properties (mainly nitrate concentration and moisture content) were found. Meanwhile, a positive correlation was observed between N2 and CO2 emissions. This result supports the hypothesis that an increase in soil available organic carbon leads to N2 emissions as the end product of denitrification.  相似文献   

6.
Urine deposition by grazing livestock causes an immediate increase in nitrous oxide (N2O) emissions, but the responsible mechanisms are not well understood. A nitrogen-15 (15N) labelling study was conducted in an organic grass-clover sward to examine the initial effect of urine on the rates and N2O loss ratio of nitrification (i.e. moles of N2O-N produced per moles of nitrate produced) and denitrification (i.e. moles of N2O produced per moles of N2O+N2 produced). The effect of artificial urine (52.9 g N m−2) and ammonium solution (52.9 g N m−2) was examined in separate experiments at 45% and 35% water-filled pore space (WFPS), respectively, and in each experiment a water control was included. The N2O loss derived from nitrification or denitrification was determined in the field immediately after application of 15N-labelled solutions. During the next 24 h, gross nitrification rates were measured in the field, whereas the denitrification rates were measured in soil cores in the laboratory. Compared with the water control, urine application increased the N2O emission from 3.9 to 42.3 μg N2O-N m−2 h−1, whereas application of ammonium increased the emission from 0.9 to 6.1 μg N2O-N m−2 h−1. In the urine-affected soil, nitrification and denitrification contributed equally to the N2O emission, and the increased N2O loss resulted from a combination of higher rates and higher N2O loss ratios of the processes. In the present study, an enhanced nitrification rate seemed to be the most important factor explaining the high initial N2O emission from urine patches deposited on well-aerated soils.  相似文献   

7.
Nitrous oxide, nitric oxide and denitrification losses from an irrigated soil amended with organic fertilizers with different soluble organic carbon fractions and ammonium contents were studied in a field study covering the growing season of potato (Solanum tuberosum). Untreated pig slurry (IPS) with and without the nitrification inhibitor dicyandiamide (DCD), digested thin fraction of pig slurry (DTP), composted solid fraction of pig slurry (CP) and composted municipal solid waste (MSW) mixed with urea were applied at a rate of 175 kg available N ha−1, and emissions were compared with those from urea (U) and a control treatment without any added N fertilizer (Control). The cumulative denitrification losses correlated significantly with the soluble carbohydrates, dissolved N and total C added. Added dissolved organic C (DOC) and dissolved N affected the N2O/N2 ratio, and a lower ratio was observed for organic fertilizers than from urea or unfertilized controls. The proportion of N2O produced from nitrification was higher from urea than from organic fertilizers. Accumulated N2O losses during the crop season ranged from 3.69 to 7.31 kg N2O-N ha−1 for control and urea, respectively, whereas NO losses ranged from 0.005 to 0.24 kg NO-N ha−1, respectively. Digested thin fraction of pig slurry compared to IPS mitigated the total N2O emission by 48% and the denitrification rate by 33%, but did not influence NO emissions. Composted pig slurry compared to untreated pig slurry increased the N2O emission by 40% and NO emission by 55%, but reduced the denitrification losses (34%). DCD partially inhibited nitrification rates and reduced N2O and NO emissions from pig slurry by at least 83% and 77%, respectively. MSW+U, with a C:N ratio higher than that of the composted pig slurry, produced the largest denitrification losses (33.3 kg N ha−1), although N2O and NO emissions were lower than for the U and CP treatments.This work has shown that for an irrigated clay loam soil additions of treated organic fertilizers can mitigate the emissions of the atmospheric pollutants NO and N2O in comparison with urea.  相似文献   

8.
Hippuric acid (HA) in cattle urine acts as a natural inhibitor of soil N2O emissions. As HA concentration varies with diet, we determined critical HA levels. We also tested the hypothesis that the inhibition occurs because the HA breakdown product benzoic acid (BA) inhibits denitrification rates. During a 64-day incubation, we quantified emissions from artificial urine varying in HA, BA and glycine (Gly) concentrations, added to a sandy pasture soil. Increasing HA concentration from 0.4 to 5.6 mmol kg−1 soil significantly decreased the average N2O flux by 54%. At 3.9 mmol kg−1 soil, denitrification levels were 50% reduced for BA as compared to Gly. We conclude that HA inhibits both denitrification and N2O emission, at least partly through a BA mechanism.  相似文献   

9.
Nitrogen (N) fertilizer application and grazing are known to induce nitrous oxide (N2O) emissions from grassland soils. In a field study, general information on rates of N2O emission, the effect of cattle grazing and the type (mineral fertilizer, cattle slurry) and amount of N supply on the flux of N2O from a sandy soil were investigated. N2O emissions from permanent grassland managed as a mixed system (two cuts followed by two grazing cycles) were monitored over 11 months during 2001-2002 in northern Germany using the closed chamber method. The field experiment consisted of four regionally relevant fertilizer combinations, i.e. two mineral N application rates (0 and 100 kg N ha−1 yr−1) and two slurry levels (0 and 74 kg N ha−1 yr−1).Mean cumulative N2O-N loss was 3.0 kg ha−1 yr−1, and the cumulative 15N-labelled N2O emissions varied from 0.03% to 0.19% of the 15N applied. 15N labelling indicated that more N2O was emitted from mineral N than from slurry treated plots, and in all treatments the soil N pool was always clearly the major source of N2O. Regarding the total cumulative N2O losses, differences among treatments were not significant, which was caused by: (i) a high variance in emissions during and after cattle grazing due to the random distribution of excrements and by (ii) high N2 fixation of white clover in the 0 kg N ha−1 treatments, which resulted in similar N status of all treatments. However before grazing started, treatments showed significant differences. After cattle grazing in summer, N2O emission rates were higher than around the time of spring fertilizer application, or in winter. Grazing resulted in N2O flux rates up to 489 μg N2O-N m−2 h−1 and the grazing period contributed 31-57% to the cumulative N2O emission. During freeze-thaw cycles in winter (December-February) N2O emission rates of up to 147 μg N2O-N m−2 h−1 were measured, which contributed up to 26% to the annual N2O flux. The results suggest that N fertilizer application and grazing caused only short-term increases of N2O flux rates whereas the major share of annual N2O emission emitted from the soil N pool. The significantly increased N2O fluxes during freeze-thaw cycles show the importance of emission events in winter which need to be covered by measurements for obtaining reliable estimates of annual N2O emissions.  相似文献   

10.
Arsenic (As), lead (Pb), copper (Cu) and zinc (Zn) can be found in large concentrations in mine spills of central and northern Mexico. Interest in these heavy metals has increased recently as they contaminate drinking water and aquifers in large parts of the world and severely affect human health, but little is known about how they affect biological functioning of soil. Soils were sampled in seven locations along a gradient of heavy metal contamination with distance from a mine in San Luis Potosí (Mexico), active since about 1800 AD. C mineralization and N2O production were monitored in an aerobic incubation experiment. Concentrations of As in the top 0-10 cm soil layer ranged from 8 to 22,992 mg kg−1, from 31 to 1845 mg kg−1 for Pb, from 27 to 1620 mg kg−1 for Cu and from 81 to 4218 mg kg−1 for Zn. There was a significant negative correlation between production rates of CO2 and concentrations of As, Pb, Cu and Zn, and there was a significant positive correlation with pH, water holding capacity (WHC), total N and soil organic C. There was a significant negative correlation (P<0.05) between production rate of nitrous oxide (N2O) attributed to nitrification by the inhibition method in soil incubated at 50% WHC and total concentrations of Pb and Zn, and there was a significant positive correlation (P<0.05) with pH and total N content. There was a significant negative correlation (P<0.05) between the production rate of N2O attributed to denitrification by the inhibition method in soil incubated at 100% WHC and total concentrations of Pb, Cu and Zn, and a significant positive correlation (P<0.01) with pH; there was a significant positive correlation (P<0.05) between the production of N2O attributed to other processes by the inhibition method and WHC, inorganic C and clay content. A negative value for production rate of N2O attributed to nitrifier denitrification by the inhibition method was obtained at 100% WHC. The large concentrations of heavy metals in soil inhibited microbial activity and the production rate of N2O attributed to nitrification by the inhibition method when soil was incubated at 50% WHC and denitrification when soil was incubated at 100% WHC. The inhibitor/suppression technique used appeared to be flawed, as negative values for nitrifier denitrification were obtained and as the production rate of N2O through denitrification increased when soil was incubated with C2H2.  相似文献   

11.
Global warming potential (GWP) of sandy paddy soils may be reduced by trade-offs between N2O, CH4 and CO2 emissions. Laboratory experiments using either rice straw (1% or 0.5%) or together with urea-N (25 or 50 mg N kg−1 soil) at various levels of soil water were carried out for 30 days each, to test this assumption. Waterlogging combined with urea-N increased total N2O emissions, with greater release upon rewaterlogging (7.4 mg N kg−1 soil) than experienced by removing waterlogging only. Rice straw±urea-N either emitted small amounts of N2O or resulted in negative values at all water levels, including saturated and aerobic. Total CH4 fluxes declined with the decreased water levels and amount of rice straw (<193 mg C kg−1 soil), and also for CO2 with the latter (<1340 mg C kg−1 soil), and rewaterlogging had little influence on both. N2O under rewaterlogged and waterlogged±urea-N, CH4 under waterlogged with rice straw, and CO2 for the remainder were the major contributors to GWP. Results show that waterlogging following aerobic decomposition of rice straw (1%) with urea-N, applied either at the beginning or at the end of the aerobic conditions, could decrease GWP by 56-64% and 32-42% over the sole addition of rice straw (1% and 0.5%) under waterlogged and saturated conditions, respectively.  相似文献   

12.
Nitrous oxide emitted by soils can be produced either by denitrification in anoxic conditions or by nitrification in presence of O2. The relative importance of the two processes, particularly under varied partial pressures of O2, is not always known. This paper focuses on the influence of O2 concentration on N2O production by nitrification and denitrification in an arable Orthic Luvisol. Soil aggregates (2-3 mm size), water unsaturated, received 116 mg N kg−1 as ammonium sulphate labelled with 15N and were incubated during 14 days at different O2 partial pressures: 0, 0.35, 0.76, 1.5, 4.3 and 20.4 kPa. A 15N tracing technique was used to quantify nitrification and denitrification rates. 15N2O and 15N2 were measured. Oxygen pressure appeared to strongly influence both nitrification and denitrification rates and also N2O emissions. Nitrification rates were reduced by a factor of 6-9 when O2 decreased from 20.4 to 0.35 kPa. They were highly correlated with O2 consumption rates. Denitrification mainly occurred in complete anoxic conditions. The proportion of N2O emitted by denitrification was estimated by two independent methods: one based on 15N tracing using isotope composition of NH4, NO3 and N2O, the other based on the measurement of the 15N2O:15N2 ratio. The two methods gave close results. The highest N2O emissions were obtained under complete anoxic conditions and were due to denitrification. However, N2O emissions almost as important were obtained at day 14 with 1.5 kPa O2 pressure, and they were due to nitrification. Nitrification was the main source of N2O at O2 concentrations greater than 0.35 kPa. The amounts of N2O-N emitted by nitrification were linearly related to the amounts of N nitrified, but the slope of the regression was highly dependent on O2 concentration: it varied from 0.16 to 1.48% when O2 concentration was reduced from 20.4 to 0.76 kPa. Emissions of N2O by nitrification may then be quite significant if nitrification occurs at a reduced O2 concentration.  相似文献   

13.
Soil compaction and soil moisture are important factors influencing denitrification and N2O emission from fertilized soils. We analyzed the combined effects of these factors on the emission of N2O, N2 and CO2 from undisturbed soil cores fertilized with (150 kg N ha−1) in a laboratory experiment. The soil cores were collected from differently compacted areas in a potato field, i.e. the ridges (ρD=1.03 g cm−3), the interrow area (ρD=1.24 g cm−3), and the tractor compacted interrow area (ρD=1.64 g cm−3), and adjusted to constant soil moisture levels between 40 and 98% water-filled pore space (WFPS).High N2O emissions were a result of denitrification and occurred at a WFPS≥70% in all compaction treatments. N2 production occurred only at the highest soil moisture level (≥90% WFPS) but it was considerably smaller than the N2O-N emission in most cases. There was no soil moisture effect on CO2 emission from the differently compacted soils with the exception of the highest soil moisture level (98% WFPS) of the tractor-compacted soil in which soil respiration was significantly reduced. The maximum N2O emission rates from all treatments occurred after rewetting of dry soil. This rewetting effect increased with the amount of water added. The results show the importance of increased carbon availability and associated respiratory O2 consumption induced by soil drying and rewetting for the emissions of N2O.  相似文献   

14.
Earthworm activity may have an effect on nitrous oxide (N2O) emissions from crop residue. However, the importance of this effect and its main controlling variables are largely unknown. The main objective of this study was to determine under which conditions and to what extent earthworm activity impacts N2O emissions from grass residue. For this purpose we initiated a 90-day (experiment I) and a 50-day (experiment II) laboratory mesocosm experiment using a Typic Fluvaquent pasture soil with silt loam texture. In all treatments, residue was applied, and emissions of N2O and carbon dioxide (CO2) were measured. In experiment I the residue was applied on top of the soil surface and we tested (a) the effects of the anecic earthworm species Aporrectodea longa (Ude) vs. the epigeic species Lumbricus rubellus (Hoffmeister) and (b) interactions between earthworm activity and bulk density (1.06 vs. 1.61 g cm−3). In experiment II we tested the effect of L. rubellus after residue was artificially incorporated in the soil. In experiment I, N2O emissions in the presence of earthworms significantly increased from 55.7 to 789.1 μg N2O-N kg−1 soil (L. rubellus; p<0.001) or to 227.2 μg N2O-N kg−1 soil (A. longa; p<0.05). This effect was not dependent on bulk density. However, if the residue was incorporated into the soil (experiment II) the earthworm effect disappeared and emissions were higher (1064.2 μg N2O-N kg−1 soil). At the end of the experiment and after removal of earthworms, a drying/wetting and freezing/thawing cycle resulted in significantly higher emissions of N2O and CO2 from soil with prior presence of L. rubellus. Soil with prior presence of L. rubellus also had higher potential denitrification. We conclude that the main effect of earthworm activity on N2O emissions is through mixing residue into the soil, switching residue decomposition from an aerobic and low denitrification pathway to one with significant denitrification and N2O production. Furthermore, A. longa activity resulted in more stable soil organic matter than L. rubellus.  相似文献   

15.
Emissions of N2O and N2 were measured from Lolium perenne L. swards under ambient (36 Pa) and elevated (60 Pa) atmospheric CO2 at the Swiss free air carbon dioxide enrichment experiment following application of 11.2 g N m−2 as 15NH415NO3 or 14NH415NO3 (1 at.% excess 15N). Total denitrification (N2O+N2) was increased under elevated pCO2 with emissions of 6.2 and 19.5 mg 15N m−2 measured over 22 d from ambient and elevated pCO2 swards, respectively, supporting the hypothesis that increased belowground C allocation under elevated pCO2 provides the energy for denitrification. Nitrification was the predominant N2O producing process under ambient pCO2 whereas denitrification was predominant under elevated pCO2. The N2-to-N2O ratio was often higher under elevated pCO2 suggesting that previous estimates of gaseous N losses based only on N2O emissions have greatly underestimated the loss of N by denitrification.  相似文献   

16.
There is conflicting evidence about toxic effects of heavy metals in soil on symbiotic nitrogen fixation. This study was set-up to assess the general occurrence of such effects. Soils with metal concentration gradients were sampled from six established field trials, where sewage sludge or metal salts have been applied, or from a transect in a sludge treated soil. Additional contaminated soils were sampled near metal smelters, in floodplains, in sludge amended arable land and in a metalliferous area. Symbiotic nitrogen fixation was measured with 15N isotope dilution in white clover (Trifolium repens L.) grown in potted soil that was not re-inoculated, and using ryegrass (Lolium perenne L.) as reference crop. The fraction nitrogen in clover derived from fixation (Ndff) varied from 0 to 88% depending on soil. Pronounced metal toxicity on Ndff was only confirmed in a sludge treated soil where nitrogen fixation was halved from the control value at soil total metal concentration of 737 mg Zn kg−1, 428 mg Cu kg−1 and 10 mg Cd kg−1. The Ndff was significantly reduced by increasing metal concentration in soils from two other sites where Ndff was low throughout and where these effects might be attributed to confounding factors. No significant effects of metals on Ndff were identified in all other gradients even up to elevated total metal concentration (e.g. 55 mg Cd kg−1). The variation of Ndff among all soils (n=48), is mainly explained by the number of rhizobia in the soil (log MPN, log (cells g−1 soil)), whereas correlations with total or soil solution metal concentrations were weak (R2<0.25). The is significantly affected by the presence or absence of the host plant at the sampling site. No effects of metals were identified at even at total Zn concentrations of about 2000 mg Zn kg−1, whereas metal toxicity could be identified at lower most probable number (MPN) values. This survey shows that the metal toxicity on symbiotic nitrogen fixation cannot be generalized and that survival of a healthy population of the microsymbiont is probably the critical factor.  相似文献   

17.
In temperate regions, a majority of N2O is emitted during spring soil thawing. We examined the influence of two winter field covers, snow and winter rye, on soil temperature and subsequent spring N2O emissions from a New York corn field over two years. The first season (2006-07) was a cold winter (2309 h below 0 °C at 8 cm soil depth), historically typical for the region. The snow removal treatment resulted in colder soils and higher N2O fluxes (73.3 vs. 57.9 ng N2O-N cm−2 h−1). The rye cover had no effect on N2O emissions. The second season (2007-08) was a much milder winter (1271 h below freezing at 8 cm soil depth), with lower N2O fluxes overall. The winter rye cover resulted in lower N2O fluxes (5.9 vs. 33.7 ng N2O-N cm−2 h−1), but snow removal had no effect. Climate scenarios predict warmer temperature and less snow cover in the region. Under these conditions, spring N2O emissions can be expected to decrease and could be further reduced by winter rye crops.  相似文献   

18.
Quantifying the nitrous oxide (N2O) and nitric oxide (NO) fluxes emitted from croplands remains a major challenge. Field measurements in different climates, soil and agricultural conditions are still scarce and emissions are generally assessed from a small number of measurements. In this study, we report continuously measured N2O and NO fluxes with a high temporal resolution over a 2-year crop sequence of barley and maize in northern France. Measurements were carried out using 6 automatic chambers at a rate of 16 mean flux measurements per day. Additional laboratory measurements on soil cores were conducted to study the response of NO and N2O emissions to environmental conditions.The detection limit of the chamber setup was found to be 3 ng N m−2 s−1 for N2O and 0.1 ng N m−2 s−1 for NO. Nitrous oxide fluxes were higher than the threshold 37% of the time, while they were 72% of the time for NO fluxes.The cumulated annual NO and N2O emissions were 1.7 kg N2O-N ha−1 and 0.5 kg NO-N ha−1 in 2007, but 2.9 kg N2O-N ha−1 and 0.7 kg NO-N ha−1 in 2008. These inter-annual differences were largely related to crop types and to their respective management practices. The forms, amounts and timing of nitrogen applications and the mineralization of organic matter by incorporation of crop residues were found to be the main factor controlling the emissions peaks. The inter-annual variability was also due to different weather conditions encountered in 2007 and 2008. In 2007, the fractioned N inputs applied on barley (54 kg ha−1 in March and in April) did not generate N2O emissions peaks because of the low rainfall during the spring. However, the significant rainfall observed in the summer and fall of 2007, promoted rapid decomposition of barley residues which caused high levels of N2O emissions. In 2008, the application of dairy cattle slurry and mineral fertilizer before the emergence of maize (107 kg Nmin ha−1 or 130 kg Ntot ha−1 in all) coincided with large rainfalls promoting both NO and N2O emissions, which remained high until early summer.Laboratory measurements corroborated the field observations: NO fluxes were maximum at a water-filled pore space (WFPS) of around 27% while N2O fluxes were optimal at 68% WFPS, with a maximum potentially 14 times larger than for NO.  相似文献   

19.
Emissions of N2O were measured following addition of 15N-labelled (2.6-4.7 atom% excess 15N) agroforestry residues (Sesbania sesban, mixed Sesbania/Macroptilium atropurpureum, Crotalaria grahamiana and Calliandra calothyrsus) to a Kenyan oxisol at a rate of 100 mg N kg soil−1 under controlled environment conditions. Emissions were increased following addition of residues, with 22.6 mg N m−2 (124.4 mg N m−2 kg biomass−1; 1.1 mg 15N m−2; 1.03% of 15N applied) emitted as N2O over 29 d after addition of both Sesbania and Macroptilium residues in the mixed treatment. Fluxes of N2O were positively correlated with CO2 fluxes, and N2O emissions and available soil N were negatively correlated with residue lignin content (r=−0.49;P<0.05), polyphenol content (r=−0.94;P<0.05), protein binding capacity (r=−0.92;P<0.05) and with (lignin+polyphenol)-to-N ratio (r=−0.55;P<0.05). Lower emission (13.6 mg N m−2 over 29 d; 94.5 mg N m−2 kg biomass−1; 0.6 mg 15N m−2; 0.29% of 15N applied) after addition of Calliandra residue was attributed to the high polyphenol content (7.4%) and high polyphenol protein binding capacity (383 μg BSA mg plant−1) of this residue binding to plant protein and reducing its availability for microbial attack, despite the residue having a N content of 2.9%. Our results indicate that residue chemical composition, or quality, needs to be considered when proposing mitigation strategies to reduce N2O emissions from systems relying on incorporation of plant biomass, e.g. improved-fallow agroforestry systems, and that this consideration should extend beyond the C-to-N ratio of the residue to include polyphenol content and their protein binding capacity.  相似文献   

20.
We examined the effects of forest clearfelling on the fluxes of soil CO2, CH4, and N2O in a Sitka spruce (Picea sitchensis (Bong.) Carr.) plantation on an organic-rich peaty gley soil, in Northern England. Soil CO2, CH4, N2O as well as environmental factors such as soil temperature, soil water content, and depth to the water table were recorded in two mature stands for one growing season, at the end of which one of the two stands was felled and one was left as control. Monitoring of the same parameters continued thereafter for a second growing season. For the first 10 months after clearfelling, there was a significant decrease in soil CO2 efflux, with an average efflux rate of 4.0 g m−2 d−1 in the mature stand (40-year) and 2.7 g m−2 d−1 in clearfelled site (CF). Clearfelling turned the soil from a sink (−0.37 mg m−2 d−1) for CH4 to a net source (2.01 mg m−2 d−1). For the same period, soil N2O fluxes averaged 0.57 mg m−2 d−1 in the CF and 0.23 mg m−2 d−1 in the 40-year stand. Clearfelling affected environmental factors and lead to higher daily soil temperatures during the summer period, while it caused an increase in the soil water content and a rise in the water table depth. Despite clearfelling, CO2 remained the dominant greenhouse gas in terms of its greenhouse warming potential.  相似文献   

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