55 Geowissenschaften, Geologie
Jahresbericht 2010-11 zum Förderprojekt 01879 Untersuchung der Veränderung der Konzentration von Luftbeimengungen und Treibhausgasen im hohen Fichtelgebirge 2007 – 2014
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Flow and transport processes as affected by tillage management under monsoonal conditions in South Korea
- A sustainable agriculture, which provides enough yields to satisfy the food demand and minimizes the impacts on ecosystem services such as provision of high water quality, is challenging especially in monsoon regions. In this thesis, plastic mulched ridge cultivation (RTpm) under monsoonal conditions and its impact on flow processes and nitrate transport was investigated.
On hillslopes, we monitored surface and subsurface flow processes in four plastic mulched potato fields using a network of tensiometers, FDR sensors, runoff collectors and flow dividers as well as Brilliant Blue FCF tracer experiments. The obtained datasets were used to calibrate the process-based models HYDRUS 2/3D and EROSION 3D in order to quantify drainage water fluxes, surface runoff and erosion rates of RTpm compared to ridge tillage without coverage (RT) and conventional flat tillage (CT). In a flat terrain, N fate under fertilizer rates at 50, 150, 250 and 350 kg NO3− ha−1 was investigated in a plastic mulched radish field using suction lysimeters, tensiometers and a 15N tracer experiment. We used datasets of nitrate concentrations and matric potentials to calibrate a water flow and solute transport model using the numerical code HydroGeoSphere.
RTpm affects soil water dynamics dominantly during dry periods, when ridge soil was drier compared to furrow soil caused by the protective plastic coverage and root water uptake in ridges. Hence, pressure head gradients induced lateral flow from furrows to ridges. Under monsoonal conditions, soil was fully saturated and down slope lateral flow occurred in the coarse textured topsoil. The dye tracer experiments showed that matrix flow dominated in the sandy topsoil. Lateral funnel flow above the tillage pan was the prominent preferential flow path. Unexpectedly, macropore flow in deeper soil horizons was not detected. The field and modeling studies revealed that surface runoff was substantially increased under RTpm compared to RT and CT. However, the field topography primarily controlled surface runoff and erosion rates. The concavity of the field led to flow accumulation and high erosion losses in the center of the field, while a convex shape resulted in less soil erosion.
NO3− leaching was found to be the prominent pathway especially during the early season. Furthermore, the biomass production did not significantly differ between NO3− fertilizer rates of 150 to 350 kg ha−1. Hence, we recommend NO3− fertilizer application of 150 kg ha−1, a better fertilizer placement and split applications. We simulated whether the given recommendations on fertilizer best management practices (FBMPs) decreased NO3− leaching amounts. Compared to RT under conventional fertilization in ridges and furrows, the simulations showed that NO3− leaching can be considerably reduced up to 82% by combining RTpm, fertilizer placement only in ridges and split applications with a total fertilizer NO3− amount of 150 kg ha−1.
Based on these findings, the impact of RTpm on flow and transport processes has to be evaluated differently depending on terrain complexity. In a flat terrain, where surface runoff processes are absent or minimal, RTpm has several advantages. Beside functions such as weed control, and earlier plant emergence due to higher temperatures, plastic mulching decreases drainage water and NO3− leaching. Thus, RTpm enhances nutrient retention below the plastic mulch and reduces NO3− groundwater contamination risk. On slopes, where precipitation contributes substantially to surface runoff, RTpm even increases runoff, soil erosion and surface leaching of agrochemicals into aquatic systems.
This thesis provides several recommendations, aiming to minimize environmental impacts and to decrease costs of fertilizer and herbicide inputs. To reduce surface runoff and soil erosion at sloped fields, we suggest applying perforated plastic mulch and a ridge configuration following contours of the field. Furthermore, we recommend omitting application of herbicides in furrows to allow weed growth, which slows down runoff processes. These suggestions would increase infiltration and subsurface flow processes automatically become more important. However, absent preferential flow to deeper soil layers indicated a low groundwater contamination risk. Since funnel flow above the tillage pan was found to be the most important preferential flow path, we propose the establishment of riparian buffer zones. This would also help to reduce the discharge of sediments and fertilizers via surface runoff into the streams. Finally, FBMPs such as fertilizer placement only in ridges and split applications were found to decrease nitrate leaching considerably. Hence, we suggest applying FBMPs with impermeable plastic mulch in flat terrain, while on hill slopes FBMPs should be applied with perforated plastic mulch. To reduce the leaching and erosion risk after harvest when the plastic mulched ridges are removed, we recommend cultivating cover crops.
Tibet Plateau Atmosphere-Ecology-Glaciology Cluster Joint Kobresia Ecosystem Experiment: Documentation of the second Intensive Observation Period, Summer 2012 in KEMA, Tibet
- Experiment documentation of the second joined Kobresia ecosystem experiment conducted by the Atmosphere-Ecology-Glaciology Cluster within DFG SPP 1372 (Tibetan Plateau)in Kema, Tibet, China. The report provides background information about the field side, conducted measurements and participants.
Interactions between hydrology and biogeochemistry within riparian wetlands
- Interactions between hydrology and biogeochemistry at various spatio-temporal scales are important control mechanisms within terrestrial and aquatic ecosystems and exist among different compartments and transition interfaces. Understanding the fundamental mechanistic couplings between hydrological and biogeochemical processes and how these couplings feed back into ecosystem services and functions is an interdisciplinary challenge that must be addressed especially in the context of humanly mediated climate change. Riparian wetlands, as a transition zone between terrestrial and aquatic ecosystems, occupy large fractions of terrestrial ecosystems and provide important ecohydrological services. Due to their anoxic environments, riparian wetlands are able to store significant amounts of carbon as peat and act as an effective nutrient sink e.g. for sulfur, phosphorous and nitrogen. Riparian wetlands are characterized by highly dynamical interactions between hydrologically controlled transport mechanisms and biogeochemically controlled substrate availability, which governs nutrient cycling as well as the sink and source functions of wetlands. Generally, these interactions and their potential implications on ecosystem functions are only poorly understood. The representation of the tight couplings between hydrology and biogeochemistry in mechanistic models is a very challenging task because they have revealed a complexity which is often beyond the capabilities of current models. The objective of this thesis is to investigate interactions between hydrology and biogeochemistry in riparian wetlands and to understand their potential implications for internal biogeochemical process distributions and solute mobilization. Additionally, one major focus of the thesis is the attempt to represent such fundamental couplings in a process-based, hydrological/biogeochemical modeling approach. To this end, this thesis uses a combination of field and virtual experiments, as well as catchment-scale numerical modeling, performed for the Lehstenbach catchment, which was exemplarily chosen as main study site.
Results from the virtual experiments show very complex small-scale hydrological dynamics within the riparian areas. Here, runoff generation processes are strongly influenced by the spatial structure of the wetland-typical micro-topography (hummocks and hollows). Surface flow is episodically generated by a highly dynamical, threshold-controlled process where extended surface flow networks drain large fractions of the wetland's area. During intensive rainstorm events these surface flow networks, which contribute to stream discharge due to a fill and spill mechanism, dominate runoff generation. These fast flow components are characterized by very low residence times (minutes to hours) and once they are activated, the surface flow networks are able to rapidly mobilize large amounts of solutes, like nitrate or dissolved organic carbon (DOC), out of the wetlands by bypassing deeper anoxic layers. The importance of fast flow components for the catchment-scale mobilization of DOC was further confirmed by field investigations and catchment-scale numerical modeling. High frequency measurements of DOC in runoff of the Lehstenbach catchment revealed that DOC export is subject to substantial short term variations at an hourly to daily timescale. During intense rainstorms, DOC concentrations are up to ten times higher (up to 40 mg/L) compared to low flow conditions (~3-5 mg/L). Short term variations together with the dramatic rise of DOC concentrations in runoff during rainstorms can be explained by the episodically activation of fast flow components in the wetland areas. At the catchment-scale, application of a hydraulic mixing-cell (HMC) methodology in combination with numerical modeling has revealed that fast flow components like saturated overland flow are exclusively generated in the wetland areas during intensive rainstorm events. On an annual basis, exemplarily for the hydrological year 2001, the HMC analysis quantified the relative contribution of saturated overland flow related to the total discharge with 19.5%, which highlights the importance of riparian wetlands for catchment-scale runoff generation.
Virtual experiments, additionally show that distinct shifts between surface and subsurface flow dominance, as a result of small-scale micro-topographic driven runoff generation in the wetlands, are responsible for very complex three-dimensional subsurface flow patterns showing a wide range of subsurface residence times. To investigate how these micro-topography induced subsurface flow patterns, together with the non-uniform hydrological and biogeogeochemical boundary conditions, affect the internal re-distribution and transformation of redox-sensitive species (like nitrate, sulfate or iron) a coupled hydrological/biogeogeochemical model was developed. In the model, wetland-typical biogeochemical processes are represented in a sequential stream tube approach where redox-sensitive processes are implemented as kinetic reactions. Simulations show the formation of local hot spots for redox-sensitive processes within the subsurface as a result of the complex subsurface flow paths and the transport-limited availability of electron acceptors and donors. Formation of hot spots was simulated for all key reduction processes including iron(III)-/sulfate reduction and denitrification as well as for the corresponding re-oxidation processes. These results offer a new perspective on hydrologically controlled biogeochemical transformation processes in riparian wetlands, which provides a dynamic framework to explain process heterogeneity in wetland soils and variability in process rates over space and time.
Findings from this thesis clearly prove how useful interdisciplinary approaches are in understanding processes and mechanisms in ecosystems and how important functions of ecosystems are affected by couplings among those. However, a lot of knowledge gaps still exist in understanding the nature of dependency between water and nutrient cycles across scales and how these interacting cycles feed back into humanly-mediated climate change in ecosystems. Development of new interdisciplinary methodologies and frameworks as well as an integrated way of thinking across the boundaries of the different environmental disciplines is necessary to address the grand challenges associated with climate change.