生物地球化学的概念与方法——DNDC模型的发展

生物地球化学作为一个学科包含4个概念,即生物地球化学量、流、群和场.这4个概念从不同角度描述了生命与其环境的关系.生物地球化学量探索生命及其无机环境在元素丰度上的相似性,这种相似性决定了生命体对环境化学状态的依赖性.生物地球化学流描写化学元素在生态系统中的迁移,此迁移导致了生命体与环境间的物质和能量交换.生物地球化学群描述化学元素在迁移转化时的复杂组合关系及其生物效应.生物地球化学场是生态系统中控制生物地球化学反应的各种环境营力的总和.生物地球化学模型即是这一综合力场及其对化学元素迁移转化影响的数学描述,DNDC模型即基于这些概念发展起来.DNDC模型可用来预测陆地生态系统中碳和氮的生物地球化学行为,该模型已被一些国家用来预测农业土壤的长期肥力和温室气体排放.     

海洋沉积物—海水界面附近氮、磷、硅的生物地球化学

海洋沉积物中的N和P随沉积物的粒度由粗到细,含量逐渐升高,而Si则降低;积物中N、P、Si的含量还随海区、输入源、季节、动力学过程及生物生产过程不同而变化.控制海洋沉积物-海水界面N、P、Si沉积、释放及循环的因素,包括有机质和溶解氧的浓度、有机质中C、N、P、Si的相对比例、沉积物-海水界面附近的氧化还原环境、生物扰动、温度、水深、pH值、不同形态S的浓度、金属离子以及水动力条件等.一般其综合作用的表现是,沉积物-海水界面之间NH+4、PO3-4和Si(OH)4从沉积物向上覆水扩散转移,而硝酸盐和亚硝酸盐的通量方向相反,通量的大小随着海区的不同差别较大.     

Atmospheric Deposition Connected with Marine Primary Production,Nitrogen Cycle: A Review

Atmospheric Deposition (AD) provides external nutrients such as nitrogen (N), phosphorus (P) and iron (Fe) supporting the growth of phytoplankton in oceans and thereby exerts obvious impacts on carbon and nitrogen cycles and climate change associated. Specifically, the external nutrients derived from atmospheric deposition can promote the marine primary production and nitrogen fixation that enhance the ocean capacity in absorbing CO₂; AD may also change a few pathways of carbon and nitrogen cycles in oceans and increase the emissions of biogenic aerosol and radioactive gases such as N₂O, DMS, etc. Due to the underlying important impacts on climate and environmental change, AD and processes related have become the hot topics of multidisciplinary studies in the areas of ocean and atmospheric sciences, and the focus of some international core projects such as Surface Ocean Lower Atmosphere Study (SOLAS), an International Study of Marine Biogeochemical Cycles of Trace Elements and Their Isotopes (GEOTRACES) and Integrated Marine Biogeochemistry and Ecosystem Research (IMBER). With the severe air pollution and high frequencies of Asian dust events, as the downwind areas of big cities and dust sources, the East China Sea and adjacent North Pacific have received increasing influences of AD. Limited studies showed that the increase of AD indeed caused significant influence on carbon and nitrogen cycles in these immediately related oceanic areas and the study there would have a signature effect on global oceans. A multidisciplinary study on the impacts of AD in oceans, e.g., combing molecular biology and experimental ecology techniques to study primary production processes, utilizing isotopic techniques to trace the change of the nitrogen cycle, new evidences of ocean-biogenic aerosol emissions, etc. would be the focus in the future.     

Tidal sands as biogeochemical reactors

Sandy sediments of continental shelves and most beaches are often thought of as geochemical deserts because they are usually poor in organic matter and other reactive substances. The present study focuses on analyses of dissolved biogenic compounds of surface seawater and pore waters of Aquitanian coastal beach sediments. To quantitatively assess the biogeochemical reactions, we collected pore waters at low tide on tidal cross-shore transects unaffected by freshwater inputs. We recorded temperature, salinity, oxygen saturation state, and nutrient concentrations. These parameters were compared to the values recorded in the seawater entering the interstitial environment during floods. Cross-shore topography and position of piezometric level at low tide were obtained from kinematics GPS records. Residence time of pore waters was estimated by a tracer approach, using dissolved silica concentration and kinetics estimate of quartz dissolution with seawater. Kinetics parameters were based on dissolved silica concentration monitoring during 20-day incubations of sediment with seawater. We found that seawater that entered the sediment during flood tides remained up to seven tidal cycles within the interstitial environment. Oxygen saturation of seawater was close to 100%, whereas it was as low as 80% in pore waters. Concentrations of dissolved nutrients were higher in pore waters than in seawater. These results suggest that aerobic respiration occurred in the sands. We propose that mineralised organic matter originated from planktonic material that infiltrated the sediment with water during flood tides. Therefore, the sandy tidal sediment of the Aquitanian coast is a biogeochemical reactor that promotes or accelerates remineralisation of coastal pelagic primary production. Mass balance calculations suggest that this single process supplies about 37 kmol of nitrate and 1.9 kmol of dissolved inorganic phosphorus (DIP) to the 250-km long Aquitanian coast during each semi-diurnal tidal cycle. It represents about 1.5% of nitrate and 5% of DIP supplied by the nearest estuary.     

Transient Eastern Mediterranean deep waters in response to the massive dense-water output of the Aegean Sea in the 1990s

We present a detailed account of the changing hydrography and the large-scale circulation of the deep waters of the Eastern Mediterranean (EMed) that resulted from the unique, high-volume influx of dense waters from the Aegean Sea during the 1990s, and of the changes within the Aegean that initiated the event, the so-called ‘Eastern Mediterranean Transient’ (EMT). The analysis uses repeated hydrographic and transient tracer surveys of the EMed in 1987, 1991, 1995, 1999, and 2001/2002, hydrographic time series in the southern Aegean and southern Adriatic Seas, and further scattered data. Aegean outflow averaged nearly 3 × 10⁶ m³ s⁻¹ between mid-1992 and late 1994, and was largest during 1993, when south and west of Crete Aegean-influenced deep waters extended upwards to 400 m depth. EMT-related Aegean outflow prior to 1992, confined to the region around Crete and to 1800 m depth-wise, amounted to about 3% of the total outflow. Outflow after 1994 up to 2001/2002, derived from the increasing inventory of the tracer CFC-12, contributed 20% to the total, of 2.8 × 10¹⁴ m³. Densities in the southern Aegean Sea deep waters rose by 0.2 kg/m³ between 1987 and 1993, and decreased more slowly thereafter. The Aegean waters delivered via the principal exit pathway in Kasos Strait, east of Crete, propagated westward along the Cretan slope, such that in 1995 the highest densities were observed in the Hellenic Trench west of Crete. Aegean-influenced waters also crossed the East Mediterranean Ridge south of Crete and from there expanded eastward into the southeastern Levantine Sea. Transfer into the Ionian mostly followed the Hellenic Trench, largely up to the trench’s northern end at about 37°N. From there the waters spread further west while mixing with the resident waters. Additional transfer occurred through the Herodotus Trough in the south. Levantine waters after 1994 consistently showed temperature–salinity (T–S) inversions in roughly 1000–1700 m depth, with amplitudes decreasing in time. The T–S distributions in the Ionian Sea were more diverse, one cause being added Aegean outflow of relatively lower density through the Antikithira Strait west of Crete. Spreading of the Aegean-influenced waters was quite swift, such that by early 1995 the entire EMed was affected. and strong mixing is indicated by near-linear T–S relationships observed in various places. Referenced to 2000 and 3000 dbar, the highest Aegean-generated densities observed during the event equaled those generated by Adriatic Sea outflow in the northern Ionian Sea prior to the EMT. A precarious balance between the two dense-water source areas is thus indicated. A feedback is proposed which helped triggering the change from a dominating Adriatic source to the Aegean source, but at the same time supported the previous long-year dominance of the Adriatic. The EMed deep waters will remain transient for decades to come.     

Investigation of runoff generation in a pristine,poorly gauged catchment in the Chilean Andes I: A multi‐method experimental study

Catchment scale hydrological process studies in southern Chile are of special interest as little research at this scale has been carried out in this region. In particular, the young volcanic ash soils, which are typical for this area, are not well understood in their hydrological behaviour. In addition, extensive land use changes require detailed knowledge of hydrological processes in disturbed as well as undisturbed catchments in order to estimate resulting risks of erosion, eutrophication, floods and droughts. This study focuses on data collection and experimental determination of relevant processes in an undisturbed forested catchment in the Andes of southern Chile. The here gained understanding of runoff generation can serve as a reference for comparison with sites subject to human intervention, improving estimation of the effects of land use change. Owing to the lack of long‐term data for this catchment it was necessary to replace long time series by a multitude of experimental methods covering as many aspects of the runoff generation process as possible. The methods used in this investigation include: measurements of streamflow, rainfall, throughfall, water chemistry, soil water dynamics, groundwater dynamics, soil physics, soil mineralogy, geo‐electrical sounding, and tracer techniques. Methods and equipment used during field campaigns are described and evaluated for usefulness versus expenditure (labour and financial costs). Selected results and the hypotheses developed from these findings are presented. The results suggest the importance of fast processes for rainfall runoff response on the one hand as well as considerable dampening effects of a large subsurface storage on the other hand. Copyright © 2008 John Wiley & Sons, Ltd.     

Testing and comparison of four ionic tracers to measure stream flow loss by multiple tracer injection

The ionic tracers lithium, sodium, chloride and bromide were used to measure flow loss in a small stream (≈? 10 ls^?1). An injectate containing all four tracers was added continuously at five sites along a 507 m study reach of St Kevin Gulch, Lake County, Colorado to determine which sections of the stream were losing water to the stream bed and to ascertain how well the four tracers performed. The acidity of the stream (pH 3.6) made it possible for lithium and sodium, which are normally adsorbed by ion exchange with stream bed sediment, to be used as conservative tracers. Net flow losses as low as 0.8 ls^?1, or 8% of flow, were calculated between measuring sites. By comparing the results of simultaneous injection it was determined whether subsections of the study reach were influent or effluent. Evaluation of tracer concentrations along 116 m of stream indicated that all four tracers behaved conservatively. Discharges measured by Parshall flumes were 4–18% greater than discharges measured by tracer dilution.     

Assessment of magnetite as a magnetic tracer for sediments in the study of ephemeral gully erosion: Conditioning factors of magnetic susceptibility

In gully erosion, the detached soil can be transported over long distances along the landscape. The eroded material can be redistributed and/or deposited on the soil surface along the landscape and then eventually be buried by newly eroded and deposited sediment. There can be significant variability of the soil conditions (e.g., texture and moisture content) over which the eroded material travels. The eroded material can be detected through the use of magnetic tracers attached to or mixed with the eroded soil. In this study we evaluated the degree to which the magnetic signal of the magnetite is conditioned by (i) burial depth of tracer, (ii) condition of soil covering the tracer and (iii) tracer concentration. In the laboratory containers were filled with a specific soil. In the filling process, a 0.5-cm layer of a soil–magnetite mixture was interspersed in the soil profile at a certain depth. Experiments encompassed three different soil–tracer concentrations (1000:1, 200:1, 100:1), four burial depths of tracer (0 cm, 3 cm, 5 cm and 10 cm from soil surface), and two different soils. In each case, the magnetic susceptibility was measured with a susceptometer. Experiments were repeated with different soil moisture contents. If the tracer is located under the soil surface, a minimum soil–tracer concentration of 200:1 is required for its correct detection. The intensity of the magnetic signal decreases dramatically with the vertical distance of the tracer from the soil surface. The maximum detection depth for the tracer's magnetic signal is strongly dependent on the natural magnetic susceptibility of the soil, which masks the tracer's signal. Variation in soil moisture content does not significantly affect the magnetic signal. For extensive field studies, the soil–tracer volume to be handled would be very high and therefore, it is necessary to explore new tracer application techniques.     

Linkages between bedload displacements and topographic change

Changes in bed topography that build and maintain channel morphology are driven by the displacements of individual particles, either though their entrainment or deposition. However, the linkages between these topographic changes and individual grain displacements have not been comprehensively addressed, as many historical tracer studies have not included coincident topographic data. In this study, we compare the movements of bedload tracers to the differences in repeat topographic surveys across four gravel-bed river reaches. To do this, we apply a 1-D Bayesian survival process model to the starting and ending locations of tracers. This model estimates downstream trapping probabilities, which represent the likelihood that a given segment of channel will “trap” an entrained particle. We then adapt this model to estimate downstream trapping probabilities using digital elevation models of difference and compare the results. The estimates from the tracer and topographic trapping models showed general alignment, meaning that tracers were preferentially trapped in segments that experienced deposition along the channel. Thus, tracers in this study were able to identify downstream differences in bedload transport. The comparison also highlighted that tracer-estimated trapping probabilities were larger than topographically estimated ones. This supports previous observations that sediment travel distances estimated using tracers are shorter than those estimated using morphological methods. We find that the differences between these two estimates vary systematically across study environments. These variations are attributable to either study design (i.e., tracers being larger than the median size of the sediment that deforms the bed) or differences in compensating scour and fill. We explore potential causes for differences in compensating scour and fill, including hydrograph shape, sediment delivery regime, channel deformation style, and channel width, highlighting that morphodynamics needs to be considered in designing bedload tracer studies.     

Impacts of forest conversion and agriculture practices on water pathways in Southern Brazil

Land‐use/cover change (LUCC), and more specifically deforestation and multidecadal agriculture, is one of the various controlling factors of water fluxes at the hillslope or catchment scale. We investigated the impact of LUCC on water pathways and stream stormflow generation processes in a subtropical region in southern Brazil. We monitored, sampled and analysed stream water, pore water, subsurface water, and rainwater for dissolved silicon concentration (DSi) and ¹⁸O/¹⁶O (δ¹⁸O) signature to identify contributing sources to the streamflow under forest and under agriculture. Both forested and agricultural catchments were highly responsive to rainfall events in terms of discharge and shallow groundwater level. DSi versus δ¹⁸O scatter plots indicated that for both land‐use types, two run‐off components contributed to the stream discharge. The presence of a dense macropore network, combined with the presence of a compact and impeding B‐horizon, led to rapid subsurface flow in the forested catchment. In the agricultural catchment, the rapid response to rainfall was mostly due to surface run‐off. A 2‐component isotopic hydrograph separation indicated a larger contribution of rainfall water to run‐off during rainfall event in the agricultural catchments. We attributed this higher contribution to a decrease in topsoil hydraulic conductivity associated with agricultural practices. The chemical signature of the old water component in the forested catchment was very similar to that of the shallow groundwater and the pore soil water: It is therefore likely that the shallow groundwater was the main source of old water. This is not the case in the agricultural catchments where the old water component had a much higher DSi concentration than the shallow groundwater and the soil pore water. As the agricultural catchments were larger, this may to some extent simply be a scale effect. However, the higher water yields under agriculture and the high DSi concentration observed in the old water under agriculture suggest a significant contribution of deep groundwater to catchment run‐off under agriculture, suggesting that LUCC may have significant effects on weathering rates and patterns.