• Energy Research

While emissions of nitric oxide (NO), ammonia (NH₃) and nitrous oxide (N₂O) from grassland soils have been increasingly well constrained, soil dinitrogen (N₂) emissions are poorly understood. However, N₂ losses might dominate total gaseous nitrogen (N) losses. Knowledge on N losses is key for the development of climate-adapted management that balances agronomic and environmental needs. Hence, we quantified all gaseous N losses from a montane grassland in Southern Germany both for ambient climatic conditions and for a climate change treatment (+ 2°C MAT, - 300 mm MAP). Monthly measurements of soil N₂ emissions of intact soil cores revealed that those exceeded by far soil N₂O emissions and averaged at 350 ± 101 (ambient climate) and 738 ± 197 lg N m¯²h¯¹ (climate change). Because these measurements did not allow to quantify emission peaks after fertilization, an additional laboratory experiment was deployed to quantify the response of NH₃, NO, N₂O, and N₂ emissions in sub daily temporal resolution to a typical slurry fertilization event (51 kg N ha¯¹). Our results revealed that total N gas losses amounted to roughly half of applied slurry-N. Surprisingly, N₂ but not NH₃ dominated fertilizer N losses, with N₂ emissions accounting for 16–21 kg or 31–42% of the applied slurry-N, while NH₃ volatilization (3.5 kg), N2O (0.2–0.5 kg) and NO losses (0–0.2 kg) were of minor importance. Though constraining annual N₂ loss remained uncertain due to high spatiotemporal variability of fluxes, we show that N₂ losses are a so far overlooked key component of the N balance in montane grasslands, which needs to be considered for developing improved grassland management strategies targeted at increasing N use efficiency.

AbstractClimate warming and management will likely affect carbon (C) fluxes of montane grassland ecosystems. In this study, we assessed the effect of simultaneous warming (+2°C) and decreased precipitation (−25%) on carbon exchange of montane grasslands in S‐Germany by translocating large intact plant‐soil cores from a high altitude to a low altitude site. Cores received two common grassland management regimes: intensive (4–5 cuts and slurry application) and extensive (1–2 cuts and slurry application). Diurnal patterns of net ecosystem exchange (NEE) and total ecosystem respiration (Reco) were measured over 1.5 years in 2–3 weeks intervals during the snow free period. Additional data on environmental controls, that is, photosynthetic active radiation, grass height and soil moisture and temperature, were used to develop empirical models to estimate daily and annual fluxes of gross primary production (GPP) and Reco. Considering the 2 years period (2014 and 2015), we found that, under warmer and slightly drier conditions, both GPP and Reco significantly (p < 0.01) increased (up to 20%) but with a higher temperature sensitivity of Reco, particularly in intensive managed grassland. The higher temperature sensitivity of Reco reduced the NEE by 0.7 t C ha−1 yr−1 for both extensive and intensive management, respectively. Considering additional carbon inputs via slurry and exports via harvest (i.e., annual net ecosystem carbon budget), our results showed that managed grasslands are already a source of C under current climate conditions (1.7–1.8 t ha−1 yr−1) which significantly (p < 0.05) increased under climate warming (2.3–2.9 t ha−1 yr−1).

Traditional irrigated double-rice cropping systems have to cope with reduced water availability due to changes of climate and economic conditions. To quantify the shift in CH4 and N2O emissions when changing from traditional to diversified double cropping-systems, an experiment including flooded rice, non-flooded “aerobic” rice and maize was conducted during the dry season (February–June 2012) in the Philippines. Two automated static chamber–GC systems were used to continuously measure CH4 and N2O emissions in the three cropping systems of which each included three different nitrogen fertilization regimes. Turning away from flooded cropping systems leads to shifts in greenhouse gas emissions from CH4 under wet soil to N2O emissions under drier soil conditions. The global warming potential (GWP) of the non-flooded crops was lower compared to flooded rice, whereas high CH4 emissions under flooded conditions still override enhanced N2O emissions in the upland systems. The yield-scaled GWP favored maize over aerobic rice, due to lower yields of aerobic rice. However, the lower GHG emissions of upland systems are only beneficial if they are not overwhelmed by enhanced losses of soil organic carbon.

The data package compiles vegetation data of ten temperate pre-Alpine managed grasslands in southern Germany. The first dataset originates from a sampling campaign in April 2018. A 30 m x 30 m homogenous flat plot was selected at each of the ten grasslands and sampled at nine to twelve 0.25 m x 0.25 m subplots. After determining the bulk canopy height of the subplot, the vegetation was cut at 0.03 m. Afterwards the following parameters of the cut vegetation were determined in the laboratory: weight of the fresh and dry biomass; weight percentage of the plant functional types (PFT) non-green vegetation, legumes, graminoids, other forbs; total green area index (GAI) and PFT-specific GAI; carbon (C) and nitrogen (N) content of the PFT. Water content and mean C and N contents were calculated from measured values. Additional samples around the subplots were taken to determine the leaf mass per area (LMA) of PFTs that occur in the plot. The second data set is based on a vegetation survey from June 2020 at exactly the same ten plots and includes grassland type, plant species richness and species coverage of the plots. The data package was obtained within the framework of the SUSALPS project (https://www.susalps.de/) to provide in-situ data for the estimation of grassland parameters with unmanned aircraft system (UAS)-based and satellite-based remote sensing data.

AbstractClimate warming will affect terrestrial ecosystems in many ways, and warming‐induced changes in terrestrial carbon (C) cycling could accelerate or slow future warming. So far, warming experiments have shown a wide range of C flux responses, across and within biome types. However, past meta‐analyses of C flux responses have lacked sufficient sample size to discern relative responses for a given biome type. For instance grasslands contribute greatly to global terrestrial C fluxes, and to date grassland warming experiments provide the opportunity to evaluate concurrent responses of both plant and soil C fluxes. Here, we compiled data from 70 sites (in total 622 observations) to evaluate the response of C fluxes to experimental warming across three grassland types (cold, temperate, and semi‐arid), warming methods, and short (≤3 years) and longer‐term (>3 years) experiment lengths. Overall, our meta‐analysis revealed that experimental warming stimulated C fluxes in grassland ecosystems with regard to both plant production (e.g., net primary productivity (NPP) 15.4%; aboveground NPP (ANPP) by 7.6%, belowground NPP (BNPP) by 11.6%) and soil respiration (Rs) (9.5%). However, the magnitude of C flux stimulation varied significantly across cold, temperate and semi‐arid grasslands, in that responses for most C fluxes were larger in cold than temperate or semi‐arid ecosystems. In semi‐arid and temperate grasslands, ecosystem respiration (Reco) was more sensitive to warming than gross primary productivity (GPP), while the opposite was observed for cold grasslands, where warming produced a net increase in whole‐ecosystem C storage. However, the stimulatory effect of warming on ANPP and Rs observed in short‐term studies (≤3 years) in both cold and temperate grasslands disappeared in longer‐term experiments (>3 years). These results highlight the importance of conducting long‐term warming experiments, and in examining responses across a wide range of climate.

In this study water balance components as well as nitrogen and dissolved organic carbon leaching were quantified by means of large weighable grassland lysimeters at three sites (860, 770 and 600 m a.s.l.) for both intensive and extensive management. Our results show that at E600, the site with highest air temperature (8.6 °C) and lowest precipitation (981.9 mm), evapotranspiration losses were 100.7 mm higher as at the site (E860) with lowest mean annual air temperature (6.5 °C) and highest precipitation (1359.3 mm). Seepage water formation was substantially lower at E600 (-440.9 mm) as compared to E860. Compared to climate, impacts of management on water balance components were negligible. However, intensive management significantly increased total nitrogen leaching rates across sites as compared to extensive management from 2.6 kg N ha-1 year-1 (range: 0.5-6.0 kg N ha-1 year-1) to 4.8 kg N ha-1 year-1 (range: 0.9-12.9 kg N ha-1 year-1). N leaching losses were dominated by nitrate (64.7%) and less by ammonium (14.6%) and DON (20.7%). The low rates of N leaching (0.8-6.9% of total applied N) suggest a highly efficient nitrogen uptake by plants as measured by plant total N content at harvest. Moreover, plant uptake was often exceeding slurry application rates, suggesting further supply of N due to soil organic matter decomposition. The low risk of nitrate losses via leaching and surface runoff of cut grassland on non-sandy soils with vigorous grass growth may call for a careful site and region specific re-evaluation of fixed limits of N fertilization rates as defined by e.g. the German Fertilizer Ordinance following requirements set by the European Water Framework and Nitrates Directive.

We present a new model system, which facilitates scaling of ecosystem processes from the site to regional simulation domains. The new framework LandscapeDNDC—partly based on the biogeochemical site scale model DNDC—inherits a series of new features with regard to process descriptions, model structure and data I/O functionality. LandscapeDNDC incorporates different vegetation types and management systems for simulating carbon, nitrogen and water related biosphere–atmosphere–hydrosphere fluxes in forest, arable and grassland ecosystems and allows the dynamic simulation of land use changes. The modeling concept divides ecosystems into six substates (canopy air chemistry, microclimate, physiology, water cycle, vegetation structure, and soil biogeochemistry) and provides alternative modules dealing with these substates. The model can be applied on the site scale, as well as for three-dimensional regional simulations. For regional applications LandscapeDNDC integrates all grid cells synchronously forward in time. This allows easy coupling to other spatially distributed models (e.g. for hydrology or atmospheric chemistry) and efficient two-way exchange of states. This paper describes the fundamental design concept of the model and its object-oriented software implementation. Two example applications are presented. First, calculation of a nitrous oxide emission inventory from agricultural soils for the State of Saxaony (Germany), including data preprocessing of the regional model input data. The computational effort for the LandscapeDNDC preprocessing and simulation could be speed up by a factor of almost 100 compared to the approach using the original DNDC version 9.3. Calculated N2O emissions for Saxony with LandscapeDNDC (2693 t N2O–N/a) were compared with the original DNDC model (2725 t N2O–N/a), the IPCC Tier I methodology (1107 t N2O–N/a), and the German National Inventory Report (equal to IPCC Tier II, 2100 t N2O–N/a). The second example illustrates the capabilities of LandscapeDNDC for building a fully coupled three-dimensional model system on the landscape scale. Therefore we coupled the biogeochemical and plant growth calculations to a hydrological transport model and demonstrate the transport of nitrogen along a virtual hillslope and associated formation of indirect nitrous oxide emissions.