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1.
Nitrogen (N) is a key nutrient that shapes cycles of other essential elements in forests, including calcium (Ca). When N availability exceeds ecosystem demands, excess N can stimulate Ca leaching and deplete Ca from soils. Over the long term, these processes may alter the proportion of available Ca that is derived from atmospheric deposition vs. bedrock weathering, which has fundamental consequences for ecosystem properties and nutrient supply. We evaluated how landscape variation in soil N, reflecting long‐term legacies of biological N fixation, influenced plant and soil Ca availability and ecosystem Ca sources across 22 temperate forests in Oregon. We also examined interactions between soil N and bedrock Ca using soil N gradients on contrasting basaltic vs. sedimentary bedrock that differed 17‐fold in underlying Ca content. We found that low‐N forests on Ca‐rich basaltic bedrock relied strongly on Ca from weathering, but that soil N enrichment depleted readily weatherable mineral Ca and shifted forest reliance toward atmospheric Ca. Forests on Ca‐poor sedimentary bedrock relied more consistently on atmospheric Ca across all levels of soil N enrichment. The broad importance of atmospheric Ca was unexpected given active regional uplift and erosion that are thought to rejuvenate weathering supply of soil minerals. Despite different Ca sources to forests on basaltic vs. sedimentary bedrock, we observed consistent declines in plant and soil Ca availability with increasing N, regardless of the Ca content of underlying bedrock. Thus, traditional measures of Ca availability in foliage and soil exchangeable pools may poorly reflect long‐term Ca sources that sustain soil fertility. We conclude that long‐term soil N enrichment can deplete available Ca and cause forests to rely increasingly on Ca from atmospheric deposition, which may limit ecosystem Ca supply in an increasingly N‐rich world.  相似文献   
2.
Variability of entrainment of cohesive sediments in freshwater   总被引:6,自引:0,他引:6  
Estimates of sediment entrainment are required for models of particle transport in lakes and estuaries but are difficult to make because of the multiplicity of factors affecting cohesiveness of surficial sediments. We present results of sediment resuspension studies performed in an annular flume calibrated with laser-Doppler velocimetry. In our experiments, using sediments collected from two sites in the R. Raisin which flows into L. Erie and from one site in the western basin of L. Erie near the mouth of the R. Raisin, we applied shear stresses at the sediment-water interface in steps from 2 to 12 dyne/cm2. Percent water content at the surface of the sediments was either 77 or 74%, and trials were run with and without oxygenating the water overlying the sediments. Entrainment rates as a function of shear stress at the sediment-water interface were best described by a power-law relationship. All but 14% of the variability in the power law expression was due to shear stress and percent water content; the variability not accounted for was due to differences in particle size distributions, chemical properties, and biological activity in the sediments.  相似文献   
3.
Understanding changes in terrestrial carbon balance is important to improve our knowledge of the regional carbon cycle and climate change. However, evaluating regional changes in the terrestrial carbon balance is challenging due to the lack of surface flux measurements. This study reveals that the terrestrial carbon uptake over the Republic of Korea has been enhanced from 1999 to 2017 by analyzing long‐term atmospheric CO2 concentration measurements at the Anmyeondo Station (36.53°N, 126.32°E) located in the western coast. The influence of terrestrial carbon flux on atmospheric CO2 concentrations (ΔCO2) is estimated from the difference of CO2 concentrations that were influenced by the land sector (through easterly winds) and the Yellow Sea sector (through westerly winds). We find a significant trend in ΔCO2 of ?4.75 ppm per decade (p < .05) during the vegetation growing season (May through October), suggesting that the regional terrestrial carbon uptake has increased relative to the surrounding ocean areas. Combined analysis with satellite measured normalized difference vegetation index and gross primary production shows that the enhanced carbon uptake is associated with significant nationwide increases in vegetation and its production. Process‐based terrestrial model and inverse model simulations estimate that regional terrestrial carbon uptake increases by up to 18.9 and 8.0 Tg C for the study period, accounting for 13.4% and 5.7% of the average annual domestic carbon emissions, respectively. Atmospheric chemical transport model simulations indicate that the enhanced terrestrial carbon sink is the primary reason for the observed ΔCO2 trend rather than anthropogenic emissions and atmospheric circulation changes. Our results highlight the fact that atmospheric CO2 measurements could open up the possibility of detecting regional changes in the terrestrial carbon cycle even where anthropogenic emissions are not negligible.  相似文献   
4.
张蕊  赵钰  何红波  张旭东 《生态学杂志》2017,28(7):2379-2388
大气CO2浓度升高影响植物光合作用过程和生物量积累,改变植物地上和地下生物量的动态分配.土壤有机质的形成和周转依赖于植物组分的输入,因此,CO2浓度升高所造成的植物生理和代谢的变化对土壤碳库收支平衡具有重要影响.采用稳定碳同位素(13C)技术研究土壤-植物系统的碳循环可阐明大气CO2浓度升高条件下光合碳在植物各器官的分配特征和时间动态,明确光合碳在土壤中的积累、分解与迁移转化过程以及对土壤有机碳库周转的影响.本文综述了基于13C自然丰度法或13C示踪技术研究大气CO2浓度升高对土壤-植物系统碳循环的影响,主要包括:1)对植物光合作用的同位素分馏的影响;2)对植物光合碳(新碳)分配动态的影响;3)对土壤有机碳新老碳库动态以及微生物转化过程的影响.明确上述过程及其调控机制可为预测CO2浓度升高对陆地生态系统碳循环及源汇效应的长期影响奠定基础.  相似文献   
5.
Risk assessments often rely on deterministic models using long-term averages or “steady-state” values of input variables. Such models do not provide the information needed to estimate acute exposures. This study uses extreme value theory to examine the frequency and magnitude of daily pollutant concentrations in surface soils predicted at six U.S. locations. Concentrations are predicted using a deposition-leaching model and 50 years of historical precipitation data. A stochastic model also is used to generate 1000 years of precipitation data as modeling inputs for each location. The annual maximum concentrations at each site are fitted to a Gumbel type I distribution to estimate occurrence probability. For soluble pollutants, the predicted concentration varied substantially with precipitation, and the maximum daily concentrations exceeded annual averages by 4 to 8 times. Observed and synthetic precipitation data produced similar results at most study locations, though the synthetic data provided a slightly better fit to the Gumbel type I distribution. The precipitation model allows the generation of representative precipitation data that extend limited historical records. The extreme value analysis facilitates the evaluation of maximum pollutant concentrations, return periods, and other statistics that are important in evaluating acute exposures.  相似文献   
6.
The input and output flux data of total Hg (THg) and methylmercury (MeHg) from three catchments located in different geographical regions in Sweden and one catchment in southern Finland were compared to elucidate the role of current atmospheric Hg/MeHg deposition with regard to waterborne Hg/MeHg output.There was a negative co-variaton between the open field THg inputs and the ratio of THg output to open field input. The highest ratio (and lowest input) occurring in N. Sweden and S. Finland, while the lowest output ratio (and highest inputs) occurred in southwest Sweden. A much larger variation was found in the ratio of output to open field input for MeHg (14 to 160%). Examinations of MeHg input/output data in relation to catchment charateristics suggest that riparian peat, mires and wet organic soil contributed to the large MeHg output from certain catchments, probably due to in situ production of MeHg. This finding is consistent with other studies which have found that catchment characteristics such as wetland area, flow pathways, seasonal temperature and water flow are important in controlling the output of MeHg. These catchment characteristics govern the fate of the contemporary input of Hg and MeHg as well as the mobilization of the soil pools.  相似文献   
7.
We investigated the possible effect of recent (1927-1995) increases in the concentration of atmospheric CO2 on the stomatal densities of leaves of a wide range of tree, shrub, and herb species (N = 60) by making new measurements for comparison with corresponding data reported by E. J. Salisbury in 1927--a time when ice core studies indicate CO2 concentrations ~55 mL/L lower than present. A detailed intraspecific study of the herb Mercurialis perennis showed plants of M. perennis in a Cambridgeshire woodland in 1994 had significantly lower stomatal densities, irrespective of leaf insertion point, compared with their 1927 counterparts. Comparisons made across species using evolutionary comparative methods (independent contrasts) revealed a significant (P 2 increases have influenced leaf morphology in a manner consistent with recent experiments and the palaeoecological record. Further analyses suggested that the strength of the stomatal density response was independent of life form but dependent on "exposure" and the initial leaf stomatal density. Consequently, firmer predictions for future changes in stomatal density across all species, expected as a possible result of anthropogenically related CO2 increases, may now be possible.  相似文献   
8.
潮间带海藻光合作用总是处于水生(高潮时)与气生(低潮时)两种连续变化的环境状态下进行.对汕头沿岸常见的潮间带海藻石莼(Ulva lactuca L.)在水生和气生不同状态下光合作用对无机碳的响应特性进行了比较研究.在水生状态下,现有海水中溶解性无机碳浓度能充分饱和(10 ℃和20 ℃时)或接近饱和(30 ℃时)石莼的光合作用;而在气生状态下,石莼光合作用受大气CO2浓度的限制,且这种限制作用在较高温度(20-30 ℃)下比在低温(10 ℃)下更严重.在10 ℃和20 ℃时,石莼在气生状态下比在水生状态下具有更高的碳饱和最大光合速率;而在30 ℃时,石莼在这两种状态下的碳饱和光合速率相似.石莼光合作用的Km (CO2)值在气生状态下比在水生状态下高;而在气生状态下石莼对CO2的表观光合导度远小于其在水生状态下的值.认为大气CO2浓度升高将通过促进石莼在气生状态下的光合作用而增加其初级生产力.  相似文献   
9.
10.
The relative importance of nitrogen inputs from atmospheric deposition and biological fixation is reviewed in a number of diverse, non-agricultural terrestrial ecosystems. Bulk precipitation inputs of N (l–l2 kg N ha–1 yr–1) are the same order of magnitude as, or frequently larger than, the usual range of inputs from nonsymbiotic fixation (< 1="" –="" 5="" kg="" n="">–1 yr–1), especially in areas influenced by industrial activity. Bulk precipitation measurements may underestimate total atmospheric deposition by 30–40% because they generally do not include all forms of wet and dry deposition. Symbiotic fixation generally ranges from 10–160 kg N ha–1 yr–1) in ecosystems where N-fixing species are present during early successional stages, and may exceed the range under unusual conditions.Rates of both symbiotic and nonsymbiotic fixation appear to be greater during early successional stages of forest development, where they have major impacts on nitrogen dynamics and ecosystem productivity. Fates and impacts of these nitrogen inputs are important considerations that are inadequately understood. These input processes are highly variable in space and time, and few sites have adequate comparative information on both nitrogen deposition and fixation.
–  - more intensive studies of total atmospheric deposition, especially of dry deposition, are needed over a wide range of ecosystems;
–  - additional studies of symbiotic fixation are needed that carefully quantify variation over space and time, examine more factors regulating fixation, and focus upon the availability of N and its effects upon productivity and other nutrient cycling processes;
–  - process-level studies of associative N-fixation should be conducted over a range of ecosystems to determine the universal importance of rhizosphere fixation;
–  - further examination of the role of free-living fixation in wood decomposition and soil organic matter genesis is needed, with attention upon spatial and temporal variation; and
–  - investigations of long-term biogeochemical impacts of these inputs must be integrated with process-level studies using modern modelling techniques.
  相似文献   
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