TY  - GEN
A1  - Walther, Sophia
A1  - Guanter, Luis
A1  - Heim, Birgit
A1  - Jung, Martin
A1  - Duveiller, Gregory
A1  - Wolanin, Aleksandra
A1  - Sachs, Torsten
T1  - Assessing the dynamics of vegetation productivity in circumpolar regions with different satellite indicators of greenness and photosynthesis
T2  - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe
N2  - High-latitude treeless ecosystems represent spatially highly heterogeneous landscapes with small net carbon fluxes and a short growing season. Reliable observations and process understanding are critical for projections of the carbon balance of the climate-sensitive tundra. Space-borne remote sensing is the only tool to obtain spatially continuous and temporally resolved information on vegetation greenness and activity in remote circumpolar areas. However, confounding effects from persistent clouds, low sun elevation angles, numerous lakes, widespread surface inundation, and the sparseness of the vegetation render it highly challenging. Here, we conduct an extensive analysis of the timing of peak vegetation productivity as shown by satellite observations of complementary indicators of plant greenness and photosynthesis. We choose to focus on productivity during the peak of the growing season, as it importantly affects the total annual carbon uptake. The suite of indicators are as follows: (1) MODIS-based vegetation indices (VIs) as proxies for the fraction of incident photosynthetically active radiation (PAR) that is absorbed (fPAR), (2) VIs combined with estimates of PAR as a proxy of the total absorbed radiation (APAR), (3) sun-induced chlorophyll fluorescence (SIF) serving as a proxy for photosynthesis, (4) vegetation optical depth (VOD), indicative of total water content and (5) empirically upscaled modelled gross primary productivity (GPP). Averaged over the pan-Arctic we find a clear order of the annual peak as APAR ≦ GPP<SIF<VIs/VOD. SIF as an indicator of photosynthesis is maximised around the time of highest annual temperatures. The modelled GPP peaks at a similar time to APAR. The time lag of the annual peak between APAR and instantaneous SIF fluxes indicates that the SIF data do contain information on light-use efficiency of tundra vegetation, but further detailed studies are necessary to verify this. Delayed peak greenness compared to peak photosynthesis is consistently found across years and land-cover classes. A particularly late peak of the normalised difference vegetation index (NDVI) in regions with very small seasonality in greenness and a high amount of lakes probably originates from artefacts. Given the very short growing season in circumpolar areas, the average time difference in maximum annual photosynthetic activity and greenness or growth of 3 to 25 days (depending on the data sets chosen) is important and needs to be considered when using satellite observations as drivers in vegetation models.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1025 
KW  - induced chlorophyll Fluorescence
KW  - net ecosystem exchange
KW  - Arctic tundra ecosystems
KW  - gross primary production
KW  - carbon dioxide exchange
KW  - earth system models
KW  - growing season
KW  - climate-change
KW  - alaskan tundra
KW  - high latitudes
Y1  - 2020
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-446205
SN  - 1866-8372
IS  - 1025
SP  - 6221
EP  - 6256
ER  - 
TY  - GEN
A1  - Hoffmann, Mathias
A1  - Jurisch, Nicole
A1  - Alba, Juana Garcia
A1  - Borraz, Elisa Albiac
A1  - Schmidt, Marten
A1  - Huth, Vytas
A1  - Rogasik, Helmut
A1  - Rieckh, Helene
A1  - Verch, Gernot
A1  - Sommer, Michael
A1  - Augustin, Jürgen
T1  - Detecting small-scale spatial heterogeneity and temporal dynamics of soil organic carbon (SOC) stocks
BT  - a comparison between automatic chamber-derived C budgets and repeated soil inventories
T2  - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe
N2  - Carbon (C) sequestration in soils plays a key role in the global C cycle. It is therefore crucial to adequately monitor dynamics in soil organic carbon (Delta SOC) stocks when aiming to reveal underlying processes and potential drivers. However, small-scale spatial (10-30 m) and temporal changes in SOC stocks, particularly pronounced in arable lands, are hard to assess. The main reasons for this are limitations of the well-established methods. On the one hand, repeated soil inventories, often used in long-term field trials, reveal spatial patterns and trends in Delta SOC but require a longer observation period and a sufficient number of repetitions. On the other hand, eddy covariance measurements of C fluxes towards a complete C budget of the soil-plant-atmosphere system may help to obtain temporal Delta SOC patterns but lack small-scale spatial resolution.

To overcome these limitations, this study presents a reliable method to detect both short-term temporal dynamics as well as small-scale spatial differences of Delta SOC using measurements of the net ecosystem carbon balance (NECB) as a proxy. To estimate the NECB, a combination of automatic chamber (AC) measurements of CO2 exchange and empirically modeled aboveground biomass development (NPPshoot / were used. To verify our method, results were compared with Delta SOC observed by soil resampling.

Soil resampling and AC measurements were performed from 2010 to 2014 at a colluvial depression located in the hummocky ground moraine landscape of northeastern Germany. The measurement site is characterized by a variable groundwater level (GWL) and pronounced small-scale spatial heterogeneity regarding SOC and nitrogen (Nt) stocks. Tendencies and magnitude of Delta SOC values derived by AC measurements and repeated soil inventories corresponded well. The period of maximum plant growth was identified as being most important for the development of spatial differences in annual Delta SOC. Hence, we were able to confirm that AC-based C budgets are able to reveal small-scale spatial differences and short-term temporal dynamics of Delta SOC.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 666 
KW  - net ecosystem exchange
KW  - North Central region
KW  - no-till ecosystem
KW  - eddy covariance
KW  - CO2 fluxes
KW  - dioxide exchange
KW  - United States
KW  - gas-exchange
KW  - agricultural landscapes
KW  - monitoring networks
Y1  - 2019
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-417118
SN  - 1866-8372
IS  - 666
ER  - 
TY  - GEN
A1  - Pohl, Madlen
A1  - Hoffmann, M.
A1  - Hagemann, Ulrike
A1  - Giebels, M.
A1  - Albiac Borraz, E.
A1  - Sommer, Michael
A1  - Augustin, Jürgen
T1  - Dynamic C and N stocks
BT  - key factors controlling the C gas exchange of maize in heterogenous peatland
T2  - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe
N2  - The drainage and cultivation of fen peatlands create complex small-scale mosaics of soils with extremely variable soil organic carbon (SOC) stocks and groundwater levels (GWLs). To date, the significance of such sites as sources or sinks for greenhouse gases such as CO2 and CH4 is still unclear, especially if the sites are used for cropland. As individual control factors such as GWL fail to account for this complexity, holistic approaches combining gas fluxes with the underlying processes are required to understand the carbon (C) gas exchange of drained fens. It can be assumed that the stocks of SOC and N located above the variable GWL - defined as dynamic C and N stocks - play a key role in the regulation of the plant- and microbially mediated CO2 fluxes in these soils and, inversely, for CH4. To test this assumption, the present study analysed the C gas exchange (gross primary production - GPP; ecosystem respiration - R-eco; net ecosystem exchange - NEE; CH4) of maize using manual chambers for 4 years. The study sites were located near Paulinenaue, Germany, where we selected three soil types representing the full gradient of GWL and SOC stocks (0-1 m) of the landscape: (a) Haplic Arenosol (AR; 8 kg C m(-2)); (b) Mollic Gleysol (GL; 38 kg C m(-2)); and (c) Hemic Histosol (HS; 87 kg C m(-2)). Daily GWL data were used to calculate dynamic SOC (SOCdyn) and N (N-dyn) stocks.

Average annual NEE differed considerably among sites, ranging from 47 +/- 30 g C m(-2) yr(-1) in AR to -305 +/- 123 g C m(-2) yr(-1) in GL and -127 +/- 212 g C m(-2) yr(-1) in HS. While static SOC and N stocks showed no significant effect on C fluxes, SOCdyn and N-dyn and their interaction with GWL strongly influenced the C gas exchange, particularly NEE and the GPP : R-eco ratio. Moreover, based on nonlinear regression analysis, 86% of NEE variability was explained by GWL and SOCdyn. The observed high relevance of dynamic SOC and N stocks in the aerobic zone for plant and soil gas exchange likely originates from the effects of GWL-dependent N availability on C formation and transformation processes in the plant-soil system, which promote CO2 input via GPP more than CO2 emission via R-eco.

The process-oriented approach of dynamic C and N stocks is a promising, potentially generalisable method for system-oriented investigations of the C gas exchange of groundwater-influenced soils and could be expanded to other nutrients and soil characteristics. However, in order to assess the climate impact of arable sites on drained peatlands, it is always necessary to consider the entire range of groundwater-influenced mineral and organic soils and their respective areal extent within the soil landscape.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 496 
KW  - soil organic-matter
KW  - Zea-mays L.
KW  - term management impacts
KW  - carbon-dioxide exchange
KW  - net ecosystem exchange
KW  - nitrogen-fertilization
KW  - water-table
KW  - northeast germany
KW  - CO2 fluxes
KW  - mineral fertilization
Y1  - 2019
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-408184
SN  - 1866-8372
IS  - 496
ER  -