首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到10条相似文献,搜索用时 125 毫秒
1.
The effects of changes in tropical land use on soil emissions of nitrous oxide (N2O) and nitric oxide (NO) are not well understood. We examined emissions of N2O and NO and their relationships to land use and forest composition, litterfall, soil nitrogen (N) pools and turnover, soil moisture, and patterns of carbon (C) cycling in a lower montane, subtropical wet region of Puerto Rico. Fluxes of N2O and NO were measured monthly for over 1 year in old (more than 60 years old) pastures, early- and mid-successional forests previously in pasture, and late-successional forests not known to have been in pasture within the tabonuco (Dacryodes excelsa) forest zone. Additional, though less frequent, measures were also made in an experimentally fertilized tabonuco forest. N2O fluxes exceeded NO fluxes at all sites, reflecting the consistently wet environment. The fertilized forest had the highest N oxide emissions (22.0 kg N · ha−1· y−1). Among the unfertilized sites, the expected pattern of increasing emissions with stand age did not occur in all cases. The mid-successional forest most dominated by leguminous trees had the highest emissions (9.0 kg N · ha−1· y−1), whereas the mid-successional forest lacking legumes had the lowest emissions (0.09 kg N · ha−1· y−1). N oxide fluxes from late-successional forests were higher than fluxes from pastures. Annual N oxide fluxes correlated positively to leaf litter N, net nitrification, potential nitrification, soil nitrate, and net N mineralization and negatively to leaf litter C:N ratio. Soil ammonium was not related to N oxide emissions. Forests with lower fluxes of N oxides had higher rates of C mineralization than sites with higher N oxide emissions. We conclude that (a) N oxide fluxes were substantial where the availability of inorganic N exceeded the requirements of competing biota; (b) species composition resulting from historical land use or varying successional dynamics played an important role in determining N availability; and (c) the established ecosystem models that predict N oxide loss from positive relationships with soil ammonium may need to be modified. Received 22 February 2000; accepted 6 September 2000.  相似文献   

2.
Soils are among the important sources of atmospheric nitric oxide (NO) and nitrous oxide (N2O), acting as a critical role in atmospheric chemistry. Updated data derived from 114 peer‐reviewed publications with 520 field measurements were synthesized using meta‐analysis procedure to examine the N fertilizer‐induced soil NO and the combined NO+N2O emissions across global soils. Besides factors identified in earlier reviews, additional factors responsible for NO fluxes were fertilizer type, soil C/N ratio, crop residue incorporation, tillage, atmospheric carbon dioxide concentration, drought and biomass burning. When averaged across all measurements, soil NO‐N fluxes were estimated to be 4.06 kg ha?1 yr?1, with the greatest (9.75 kg ha?1 yr?1) in vegetable croplands and the lowest (0.11 kg ha?1 yr?1) in rice paddies. Soil NO emissions were more enhanced by synthetic N fertilizer (+38%), relative to organic (+20%) or mixed N (+18%) sources. Compared with synthetic N fertilizer alone, synthetic N fertilizer combined with nitrification inhibitors substantially reduced soil NO emissions by 81%. The global mean direct emission factors of N fertilizer for NO (EFNO) and combined NO+N2O (EFc) were estimated to be 1.16% and 2.58%, with 95% confidence intervals of 0.71–1.61% and 1.81–3.35%, respectively. Forests had the greatest EFNO (2.39%). Within the croplands, the EFNO (1.71%) and EFc (4.13%) were the greatest in vegetable cropping fields. Among different chemical N fertilizer varieties, ammonium nitrate had the greatest EFNO (2.93%) and EFc (5.97%). Some options such as organic instead of synthetic N fertilizer, decreasing N fertilizer input rate, nitrification inhibitor and low irrigation frequency could be adopted to mitigate soil NO emissions. More field measurements over multiyears are highly needed to minimize the estimate uncertainties and mitigate soil NO emissions, particularly in forests and vegetable croplands.  相似文献   

3.
Tropical nitrogen (N) deposition is projected to increase substantially within the coming decades. Increases in soil emissions of the climate‐relevant trace gases NO and N2O are expected, but few studies address this possibility. We used N addition experiments to achieve N‐enriched conditions in contrasting montane and lowland forests and assessed changes in the timing and magnitude of soil N‐oxide emissions. We evaluated transitory effects, which occurred immediately after N addition, and long‐term effects measured at least 6 weeks after N addition. In the montane forest where stem growth was N limited, the first‐time N additions caused rapid increases in soil N‐oxide emissions. During the first 2 years of N addition, annual N‐oxide emissions were five times (transitory effect) and two times (long‐term effect) larger than controls. This contradicts the current assumption that N‐limited tropical montane forests will respond to N additions with only small and delayed increases in soil N‐oxide emissions. We attribute this fast and large response of soil N‐oxide emissions to the presence of an organic layer (a characteristic feature of this forest type) in which nitrification increased substantially following N addition. In the lowland forest where stem growth was neither N nor phosphorus (P) limited, the first‐time N additions caused only gradual and minimal increases in soil N‐oxide emissions. These first N additions were completed at the beginning of the wet season, and low soil water content may have limited nitrification. In contrast, the 9‐ and 10‐year N‐addition plots displayed instantaneous and large soil N‐oxide emissions. Annual N‐oxide emissions under chronic N addition were seven times (transitory effect) and four times (long‐term effect) larger than controls. Seasonal changes in soil water content also caused seasonal changes in soil N‐oxide emissions from the 9‐ and 10‐year N‐addition plots. This suggests that climate change scenarios, where rainfall quantity and seasonality change, will alter the relative importance of soil NO and N2O emissions from tropical forests exposed to elevated N deposition.  相似文献   

4.
Rapid climate change and intensified human activities have resulted in water table lowering (WTL) and enhanced nitrogen (N) deposition in Tibetan alpine wetlands. These changes may alter the magnitude and direction of greenhouse gas (GHG) emissions, affecting the climate impact of these fragile ecosystems. We conducted a mesocosm experiment combined with a metagenomics approach (GeoChip 5.0) to elucidate the effects of WTL (?20 cm relative to control) and N deposition (30 kg N ha?1 yr?1) on carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) fluxes as well as the underlying mechanisms. Our results showed that WTL reduced CH4 emissions by 57.4% averaged over three growing seasons compared with no‐WTL plots, but had no significant effect on net CO2 uptake or N2O flux. N deposition increased net CO2 uptake by 25.2% in comparison with no‐N deposition plots and turned the mesocosms from N2O sinks to N2O sources, but had little influence on CH4 emissions. The interactions between WTL and N deposition were not detected in all GHG emissions. As a result, WTL and N deposition both reduced the global warming potential (GWP) of growing season GHG budgets on a 100‐year time horizon, but via different mechanisms. WTL reduced GWP from 337.3 to ?480.1 g CO2‐eq m?2 mostly because of decreased CH4 emissions, while N deposition reduced GWP from 21.0 to ?163.8 g CO2‐eq m?2, mainly owing to increased net CO2 uptake. GeoChip analysis revealed that decreased CH4 production potential, rather than increased CH4 oxidation potential, may lead to the reduction in net CH4 emissions, and decreased nitrification potential and increased denitrification potential affected N2O fluxes under WTL conditions. Our study highlights the importance of microbial mechanisms in regulating ecosystem‐scale GHG responses to environmental changes.  相似文献   

5.
Agricultural activities have greatly altered the global nitrogen (N) cycle and produced nitrogenous gases of environmental significance. More than half of all chemical N fertilizer produced globally is used in crop production in East, Southeast and South Asia, where rice is central to nutrition. Emissions of nitrous oxide (N2O), nitric oxide (NO) and ammonia (NH3) from croplands in this region were estimated by considering background emission and emissions resulting from N added to croplands, including chemical N, animal manure, biologically fixed N and N in crop residues returned to fields. Background emission fluxes of N2O and NO from croplands were estimated to be 1.22 and 0.57 kg N ha?1 yr?1, respectively. Separate fertilizer‐induced emission factors were estimated for upland fields and rice fields. Total N2O emission from croplands in the study region was estimated to be 1.19 Tg N yr?1, with 43% contributed by background emissions. The average fertilizer‐induced N2O emission, however, accounts for only 0.93% of the applied N, which is less than the default IPCC value of 1.25%, because of the low emission factor from paddy fields. Total NO emission was 591 Gg N yr?1 in the study region, with 40% from background emissions. The average fertilizer‐induced NO emission factor was 0.48%. Total NH3 emission was estimated to be 11.8 Tg N yr?1. The use of urea and ammonium bicarbonate and the cultivation of rice led to a high average NH3 loss rate from chemical N fertilizer in the study region. Emissions were displayed at a 0.5° × 0.5° resolution with the use of a global landuse database.  相似文献   

6.
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.
The relationship between nitrous oxide (N2O) flux and N availability in agricultural ecosystems is usually assumed to be linear, with the same proportion of nitrogen lost as N2O regardless of input level. We conducted a 3‐year, high‐resolution N fertilizer response study in southwest Michigan USA to test the hypothesis that N2O fluxes increase mainly in response to N additions that exceed crop N needs. We added urea ammonium nitrate or granular urea at nine levels (0–292 kg N ha?1) to four replicate plots of continuous maize. We measured N2O fluxes and available soil N biweekly following fertilization and grain yields at the end of the growing season. From 2001 to 2003 N2O fluxes were moderately low (ca. 20 g N2O‐N ha?1 day?1) at levels of N addition to 101 kg N ha?1, where grain yields were maximized, after which fluxes more than doubled (to >50 g N2O‐N ha?1 day?1). This threshold N2O response to N fertilization suggests that agricultural N2O fluxes could be reduced with no or little yield penalty by reducing N fertilizer inputs to levels that just satisfy crop needs.  相似文献   

8.
Ion concentrations and fluxes in seepage water (below the main rooting zone) were compared before and after clear cutting at two similar long-term experimental Norway spruce forest plots. While Ballyhooly (Ireland) was influenced by sea salt deposition, Höglwald (Germany) received high nitrogen (N) deposition. These differences were reflected in seepage water concentrations with sodium (Na+) and chloride (Cl) dominating at Ballyhooly and high nitrate (NO3 ?) and aluminium concentrations at Höglwald. Following clear cutting of the forest plots, NO3 ? concentrations peaked (Ballyhooly: 2018 μmolc L?1, Höglwald: 2595 μmolc L?1). Moreover, at Ballyhooly, NO3 ? concentrations and fluxes were continuously elevated for ~1.5 years. At Höglwald, the clear cut plot, which was replanted with spruce and beech saplings, exhibited periodically elevated NO3 ? concentrations with two distinct peaks. However, low concentrations, compared to the control (uncut) plot, were also observed. Further, at Höglwald a plot with a pre-existing dense natural regeneration of Norway spruce exhibited much lower NO3 ? concentrations before and after clear cutting. Nonetheless, NO3 ? concentrations following clear cut at both sites were elevated at least periodically above European drinking water standards (50 mg L?1). An important prerequisite for NO3 ? leaching is that forests are N saturated or at least not N-limited; consequently chronic elevated N deposition may lead to increased deterioration of seepage water quality across Europe following forest disturbances (harvesting, windthrow, insect attacks). Clear cutting at Ballyhooly was responsible for significant element loss, especially of potassium, N and calcium, while magnesium loss was compensated by high sea salt inputs. At Höglwald the contamination of seepage water with NO3 ? has been the main problem for more than 20 years at the mature stand. A pre-existing regeneration can help to reduce NO3 ? and cation leaching after cutting.  相似文献   

9.
Fine root dynamics have the potential to contribute significantly to ecosystem‐scale biogeochemical cycling, including the production and emission of greenhouse gases. This is particularly true in tropical forests which are often characterized as having large fine root biomass and rapid rates of root production and decomposition. We examined patterns in fine root dynamics on two soil types in a lowland moist Amazonian forest, and determined the effect of root decay on rates of C and N trace gas fluxes. Root production averaged 229 (±35) and 153 (±27) g m?2 yr?1 for years 1 and 2 of the study, respectively, and did not vary significantly with soil texture. Root decay was sensitive to soil texture with faster rates in the clay soil (k=?0.96 year?1) than in the sandy loam soil (k=?0.61 year?1), leading to greater standing stocks of dead roots in the sandy loam. Rates of nitrous oxide (N2O) emissions were significantly greater in the clay soil (13±1 ng N cm?2 h?1) than in the sandy loam (1.4±0.2 ng N cm?2 h?1). Root mortality and decay following trenching doubled rates of N2O emissions in the clay and tripled them in sandy loam over a 1‐year period. Trenching also increased nitric oxide fluxes, which were greater in the sandy loam than in the clay. We used trenching (clay only) and a mass balance approach to estimate the root contribution to soil respiration. In clay soil root respiration was 264–380 g C m?2 yr?1, accounting for 24% to 35% of the total soil CO2 efflux. Estimates were similar using both approaches. In sandy loam, root respiration rates were slightly higher and more variable (521±206 g C m2 yr?1) and contributed 35% of the total soil respiration. Our results show that soil heterotrophs strongly dominate soil respiration in this forest, regardless of soil texture. Our results also suggest that fine root mortality and decomposition associated with disturbance and land‐use change can contribute significantly to increased rates of nitrogen trace gas emissions.  相似文献   

10.
Sulfur dioxide (SO2) in the atmosphere has been demonstrated to have many adverse impacts on the environment and human health. In this study, deposition of SO2 ranging from 9.0 to 127.8 mg kg?1 with an average of 35.7 mg S kg?1 was found to substantially stimulate NO and N2O emissions from soils in the humid subtropical areas of Hainan, Fujian, Jiangxi, and Yunnan provinces of China under field conditions. Laboratory tests indicated that the stimulations were mediated biologically as the effects were not observed in sterilized soils. Acidification of soil resulting from SO2 deposition was not responsible for the stimulated NO and N2O emissions alone as the stimulation did not occur by acidifying soil with HNO3 treatment. By using the 15N tracing method, we found that the N2O emissions stimulated by SO2 deposition were from either denitrification, heterotrophic nitrification or both, but not from autotrophic nitrification. Therefore, atmospheric SO2 deposition would most likely stimulate NO and N2O emissions in acidic soils in which heterotrophic nitrification dominates NO and N2O production and waterlogged soils in which denitrification dominates NO and N2O production.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号