Alpine pastures have shaped the landscapes of the European Alps for millennia. However, more and more alpine pastures have been abandoned since the 1950s, e.g., due to the work intensiveness at high altitudes. Such abandonment of alpine pastures in the long term leads to natural reforestation. And despite ample potential surface in the mountains, the pressure to provide important ecosystem services by pastures under the auspices of climate change nowadays concentrates on the lowlands, because already abandoned alpine pastures are still very rarely re-established. Meanwhile, it has been widely acknowledged that alpine pastures fulfill important provisioning, regulating, and cultural ecosystem services, with particularly cultural landscape and plant and faunistic biodiversity being at risk due to reforestation.

     Cattle grazing during summer not only means a soil disturbance which can increase plant biodiversity, but also increases nutrient availability and has unclear effects on soil organic carbon and associated soil functions. However, the precise effects of grazing have only rarely been proven. To test whether the preservation of intact alpine pastures by re-introducing cattle grazing is worth supporting, it is important to evaluate the effects of re-grazing on the soil organic carbon (SOC) stocks, the soil nitrogen (N) cycling, and water contamination with nutrients.

     Within the SUSALPS (Sustainable use of alpine and pre-alpine grassland soils in a changing climate) project, a typical alpine pasture in the German Alps abandoned in 1955 (Brunnenkopf, Ammergauer Alpen) is being re-grazed since 2018 with the traditional robust old cow breed “Murnau-Werdenfelser”. Here, we compared non-grazed to different grazed areas (low grazing intensity, high grazing intensity, bare soil due to trampling) after five years of experimental re-grazing. The data show a non-significant effect of grazing on N cycling, with only the bare soil area (6% of the pasture) showing increased gross N mineralization and soil nitrate concentrations. The nitrate concentration in the drainage water stayed overall very low (range 0.3–2.2 mg N L^-1). What was striking, however, is a strong and statistically significant re-grazing-induced increase in the SOC stock by 11.8 t SOC ha^-1 in five years although we corrected for bulk density increases.

              Our results suggest that extensive grazing- and trampling-induced changes in belowground plant biomass, the soil microbiome, and overall productivity, are fostering soil functions of an alpine grassland soil. These findings are for the first time underpinning the presumed positive effects of grazed alpine pastures on soil functions with data.

How to cite: Ramm, E., Andrade-Linares, D. R., Garcia-Franco, N., Han, J., von Heßberg, A., Dashpurev, B., Jentsch, A., Krämer, A., Schloter, M., Wiesmeier, M., and Dannenmann, M.: Re-grazing of an alpine pasture sustains ecosystem services, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12037, https://doi.org/10.5194/egusphere-egu24-12037, 2024.

Ethiopia is experiencing severe loss of farmable soil due to high erosion and nutrient depletion. The loss of agricultural land puts pressure on Ethiopian farmers to produce more food on less land. The large number of livestock in Ethiopia further exacerbate the situation, since the high grazing intensity removes plant cover and crop residues and limits the return of organic matter to the soils. Introduction of perennial forage species could break the vicious circle by providing high-quality feed to animals while increasing the input of organic matter to the soil and stimulating soil life. We investigated the effect of four forage species, two grasses, Brachiaria hybrid Cayman and Panicum maximum, and two legumes, Desmodium intortum and Stylosanthes guianensis, in varying mixtures planted according to a simplex design at two locations in Southern and Northern Ethiopia. The field sites were established in six different locations, three in the Sidama and three in the Amhara region. In each region we had one large scale site, at Hawassa and Bahir Dar university respectively, and two smaller farm sites. To assess how the species mixtures affected the soil, we measured the above-ground biomass and took soil samples before and after 2 years of plant growth. We measured labile carbon, nitrification rate, and soil enzymatic activity involved in C, N and P acquisition. The effect of plant input will be compared to time-zero measurements to discern if there are any effects of the species mixtures on the soils. Currently, we see an effect of the total above ground yield of the plots on soil functions, indicating that a higher density of plant coverage influences the soil microbial activity and turnover in the soil.

How to cite: Wickander, N., Jørgensen, M., and Dörsch, P.: Can soil quality in subtropical agriculture be improved by selected forage species? , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10529, https://doi.org/10.5194/egusphere-egu24-10529, 2024.

For the past 17 years the Biodiversity Exploratories (BEs) in Germany have collected detailed data on land use intensity (LUI), climate as well as plant and microbial biodiversity for over 100 grassland sites in three different exploratories across Germany. Since these factors alongside physicochemical processes interactively drive soil nitrogen (SON) and carbon (SOC) cycling and storage, the BEs offer a unique opportunity to gain a mechanistic, process-based understanding of the interactions between soil type, climate, biodiversity and management that drive N and C turnover and storage in grasslands.

We quantified SON and SOC stocks as well as δ15N and δ13C isotopic signatures for 25 grassland plots in each of the three exploratories, thereby covering a wide range of LUI. Currently, the results for the Schwäbische Alb exploratory are available. These data clearly show the importance to distinguish for the individual effects of LUI components (fertilization, mowing and grazing). For example, SOC and TN concentrations and stocks in the top 30 cm of soil tend to increase with LUI, but this increase is largely driven by the individual effect of the grazing component of LUI. The C:N ratio on the other hand was largely impacted by mowing and fertilization, possibly because mowing was a relatively important C loss pathway while fertilization was relatively important for N inputs. The narrower C:N ratio with increasing LUI negatively affected plant biodiversity.

Both δ15N and δ13C were related largely to the overall LUI and plant biodiversity. Topsoil δ15N increased with higher LUI and lower plant biodiversity, likely due to the high δ15N of added organic fertilizer and reduced importance of biological N fixation with its low δ15N signature.

Currently, our results indicate that grazing is the dominating management factor regulating SON and SOC stocks in calcareous grasslands of Southwest Germany, with grazing increasing SOC and SON stocks and associated soil functions. Further measurements and data evaluation will show whether this finding is of more universal importance for grasslands across Germany.

How to cite: Kepp, J., Dannenmann, M., Kiese, R., Pacay, N., Schulz, S., Schweizer, S., Schloter, M., and Schwärzler, T.: Grazing dominantly regulates top soil organic carbon and nitrogen stocks in grasslands, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10411, https://doi.org/10.5194/egusphere-egu24-10411, 2024.

Enhanced rock weathering (ERW) is the process of spreading basalt rock on agricultural soils to absorb carbon dioxide (CO₂). If rolled out globally models predict it could sequester gigatonnes of atmospheric CO₂ by the end of the century (Beerling et al., 2020). Extensive research is being carried out on identifying potential co-benefits of ERW and validating carbon dioxide sequestration within tilled arable soils; the annual nature of crops grown in these soils and their associated fertiliser use causes plants to have limited total root surface areas and disturbed mycorrhizal networks. ERW’s potential in grasslands, where perennial plants dominate with stable root biomass and mycorrhizal partners, is yet to be tested thoroughly. It does offer potential, as previous studies have shown that mycorrhizal fungal association to plant roots enhances mineral weathering (Quirk et al., 2012). To characterise whether grasslands carry any potential as an ERW system 50 tonnes per hectare of basalt was applied in March 2022 to 5 plots, with 5 adjacent plots being left un-treated within a traditional mildly acidic hay meadow in the Peak District. Over the course of the growing season soil pH, cations, phosphorus, and plant available silicon was assessed at regular intervals for treated and untreated samples. Furthermore, plant yield, nutrition data, and biodiversity were also analysed. Results indicate that rock weathering did occur, with significant increases in soil magnesium, silicon, and pH noted over the course of the experiment. There were no significant changes to plant yield, biodiversity, or nutrition in most cases; however, in basalt treated samples there were significant increases in the plant concentrations of silicon, magnesium, strontium, and sodium. Results indicate that ERW could benefit acidic grasslands through its pH and soil nutrient effects, while also potentially resulting in the absorption of CO₂.

How to cite: Bell, D., Leake, J., Beerling, D., and Epihov, D.: Enhanced Rock Weatherings Effects on Soil and Plant Chemistry in Acidic Biodiverse Grassland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20750, https://doi.org/10.5194/egusphere-egu24-20750, 2024.

We applied ¹⁵N labelled cattle slurry over one year to pre-alpine grassland in order to study its importance for plant N nutrition not only in the year of application, but also in the four following years. This five-year ¹⁵N tracing study was combined with a space for time climate change experiment in order to assess long-term fertilizer N cycling under current and future climatic conditions. In the year of ¹⁵N fertilizer application, the recovery of ¹⁵N in harvested aboveground plant biomass was as low as 7-17%, while fertilizer ¹⁵N retention in the soil nitrogen pool was considerably higher (32-42%). In the year after its application, fertilizer was of equal importance for plant N nutrition compared to the year of application, as illustrated by a plant ¹⁵N recovery of 9-14%. ¹⁵N recovery in mowed plant biomass then only slowly declined in the following years and stayed significant over the entire 5 years monitored in this study.

After five years, the cumulative ¹⁵N recovery rate in mowed biomass was 33 to 41 %. Considering ¹⁵N recovery in soil and roots after 5 years revealed a total ¹⁵N tracer recovery of 66% for the climate change treatment and 77% for the climate control treatment. These results show a rapid cycling of nitrogen through soil organic matter until remineralization and plant uptake. Furthermore, we reveal a minimal contribution of recent fertilizer nitrogen to plant nutrition and the dominance of soil organic nitrogen over fertilizer nitrogen for plant nutrition in such grasslands. The findings reinforce the concept that fertilizing such grasslands is largely a fertilization of soils rather than a fertilization of plants, thereby replenishing mineralized soil organic nitrogen (SON) stocks that is exported by harvests. Particularly under climate change conditions, the low N recovery rates of plant nitrogen, high plant N export and the rapid remineralization of soil organic nitrogen led to negative nitrogen balances.

How to cite: Han, J., Kiese, R., Gasche, R., and Dannenmann, M.: High importance of organic fertilizer N for grassland plant N nutrition in the years following fertilization, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12121, https://doi.org/10.5194/egusphere-egu24-12121, 2024.

Grazed and fertilized pastures are considerable sources of the greenhouse gas N₂O. While fertilizer applications usually lead to short emission pulses, animal excreta lead to small-scaled emission hotspots resulting in a non-homogeneous source distribution. The strong spatial and temporal source variability represents an inherent problem for the quantification of gaseous emissions from pastures with chamber techniques. The eddy covariance method, integrating emissions over a larger footprint domain, is well suited to quantify total field-scale N₂O emissions, but the partitioning of emissions for different sources and the determination of source-specific emission factors (according to the IPCC guidelines) is still a challenge.

We present results of two multiple-year field experiments carried out in different regions of Switzerland. The investigated pastures were grazed by dairy cows in an intensive rotational management. The fields were additionally fertilized with organic and/or mineral fertilizer. The field-scale N₂O fluxes were quantified with the eddy covariance technique using a fast response Quantum cascade laser spectrometer for N₂O concentration measurements. The management and environmental conditions resulted in high temporal and spatial dynamics of the N₂O fluxes with highest values typically occurring after fertilization events in the summer months. Total annual N₂O emissions amounted to between 2.5 and 5 kg N ha^-1 y^-1. Data-based partitioning methods of different complexity were used to attribute the observed field-scale emissions to the main source classes (grazing excreta, fertilizer application, and background) and to derive annual N₂O emission factors. Using random forest and other regression methods the effect of environmental parameters on grazing-related emissions were analyzed.

How to cite: Ammann, C. and Barczyk, L.: Pasture N2O emission fluxes by eddy covariance - partitioning and driver analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11377, https://doi.org/10.5194/egusphere-egu24-11377, 2024.

Introduction

Wet grassland sites have accumulated large amounts of carbon in their soil over millennia. While under excessive drainage and intensive agriculture, these sites have shown substantial losses of the stored carbon to the atmosphere, the current climate mitigation strategies see a conversion of this process through rewetting. Situated in the fringe of ascending groundwater, these sites respond very sensitively to changes in the water table, both in the vegetation and the turn-over processes in the soil. While researchers have extensively investigated how wet grasslands respond to changes in environmental conditions or management practices from various perspectives, there is a lack of a comprehensive simultaneous study addressing the intricate interplay of water, carbon, and nitrogen cycles in these ecosystems. This study aimed at addressing the water, carbon, and nitrogen dynamics in a wet grassland site using a process-based agroecosystem model to prepare the model for future scenario simulations under various options for rewetting.

Material and method

The study site is situated in the Spreewald wetlands, where a lysimeter station featuring four lysimeters with different groundwater level management practices has been installed. Within the lysimeter station, a weather station records the meteorological conditions. Above-ground biomass is measured after each cut, and various parameters including evapotranspiration, gross primary productivity, ecosystem respiration, nitrogen amount in biomass, nitrate leaching, and N₂O are monitored. The process-based model employed in this study is the MONICA model (Model for Nitrogen and Carbon in Agro-ecosystems). The SPOTPY algorithm was used for optimising the model.

Results

Presented in three categories are the results: firstly, evapotranspiration as a vital component of the water cycle; followed by gross primary productivity and ecosystem respiration, offering insights into the carbon cycle. Additionally, nitrogen content in biomass, nitrate leachate, and N₂O are examined, providing information related to the nitrogen balance.

Within a wet grassland ecosystem, MONICA has effectively reproduced these essential variables, showcasing remarkable performance with rRMSE ranging between 0.05 to 0.81, and Willmott’s Refined Index of Agreement d_r ≥ 0.3 in all cases. This substantiates its capability to simulate the impacts of environmental or management changes, particularly those associated with modifications in surface-near groundwater conditions. The model's robust replication of crucial variables emphasizes its suitability for comprehensive assessments in dynamic ecological scenarios.

Keywords: Wet grasslands, Carbon, Nitrogen, MONICA, SPOTPY algorithm

How to cite: Khaledi, V., Baatz, R., Antonijević, D., Hoffmann, M., Dietrich, O., Lischeid, G., Davies, M. F., Merz, C., and Nendel, C.: Using MONICA model to investigate the water, carbon, and nitrogen dynamics in wet grasslands for future rewetting plans., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3836, https://doi.org/10.5194/egusphere-egu24-3836, 2024.

Nitrous oxide (N₂O) is a significant greenhouse gas that contributes to climate change, with one of the major sources being agricultural fertilizer application. In Europe, grass-clover ley plays a crucial role in the agricultural ecosystem. However, N₂O emissions and emission factors (EFs) associated with it are not well documented. We therefore monitored N₂O emissions in grass-clover ley and assessed the feasibility of using the process-based model DayCent to simulate N₂O emissions and the underlying N cycling. The experiment was undertaken in a long-term fertilization experiment on a ley–arable rotation in Switzerland. We compared N₂O emissions and EFs from organic fertilizer (slurry), mineral fertilizer and control (unfertilized) plots over three years (2021–2023). The results showed average N₂O emissions of 0.50 ± 0.08, 0.51 ± 0.03 and 0.01 ± 0.04 kg N ha^-1 yr^-1 from organic, mineral and control treatments, respectively. The N₂O EF, which was determined from measured emissions of the fertilized treatments after subtracting of the control treatment, was much lower than the IPCC default of 1%, with values of 0.22% and 0.40% for organic and mineral fertilizer treatments, respectively. DayCent was adjusted for plant C/N ratio parameters and the biological N fixation parameter. It accurately predicted soil moisture, soil temperature, and aboveground N yield, with deviations of 2.2%, 4.4%, and 6.0% from the measured values. Concerning N₂O emissions, DayCent simulated average EFs of 0.24% and 0.28% for organic and mineral fertilizers, respectively, suggesting a good agreement with the measurements. Our field observation and model simulation results indicate that using IPCC default EF may overestimate the N₂O emission from grass-clover ley, and that, DayCent is able to reproduce the comparatively low N₂O emissions. 

How to cite: Wang, Y. and Ammann, C.: Assessing N cycling and N2O emissions from ley within a crop rotation using measurements and modeling , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10897, https://doi.org/10.5194/egusphere-egu24-10897, 2024.

The aim of this paper was to compare effects of organic and mineral fertilizers on greenhouse gas (GHG) emissions from legume grasslands in Finland. We invoke DNDC, a process-based model that integrates effects of agricultural practices, soil characteristics, nitrogen mass balance and climate change on GHG emissions from soil-plant ecosystems. Data measured in the field were collected from 2017 to 2020 using an eddy covariance site cultivated with legume grass species (Phleum pratense L., Festuca pratensis Huds, Trifolium pratense L., Hordeum vulgare L.) at Anttila, Maaninka, eastern Finland. The focus of the modelling was to evaluate the performance of DNDC heat exchange version under two distinct management practices: organic input, utilizing digestate residue (slurry), and mineral input (NPK) with chemical fertilizer. The primary emphasis was on understanding the model's accuracy in simulating greenhouse gas emissions and comparing the total annual greenhouse gas exchanges between these two management approaches. The DNDC heat exchange model version was calibrated and validated for key processes, including Gross Primary Productivity (GPP), Net Ecosystem Exchange (NEE), Ecosystem Respiration (Reco), Soil Temperature, and Water-Filled Pore Space (WFPS) at 5 cm and 20 cm depths. The model demonstrated satisfactory performance in estimating the total annual GHG exchanges during validation years under both management practices. For the mineral treatment, the model demonstrated fair performance (Spearman's correlation (ρ) for GPP (0.81), NEE (0.72), and Reco (0.85)). Root Mean Square Error (RMSE) and Mean Absolute Error (MAE) values indicated reasonable agreement between model predictions and measured data. Notably, soil temperature simulations demonstrated an excellent correlation (ρ=0.99) with low RMSE and MAE. Water-Filled Pore Space (WFPS) at both 5 cm and 20 cm depths exhibited good correlations, with acceptable RMSE and MAE values. Similarly, for organic inputs, the DNDC model had fair correlation (ρ) for GPP (0.81), NEE (0.72), and Reco (0.85). Soil temperature and WFPS at 5 cm presented high positive correlations (ρ=0.98 and 0.55), accompanied by low RMSE and MAE. WFPS at 20cm, while exhibiting good correlation (ρ=0.065), displayed a slightly elevated RMSE and MAE. Overall, we conclude that the model offered valuable insights into GHG dynamics associated with organic and mineral fertilization practices. Overestimation of biomass yield for some of the data by DNDC suggests that future work would be well placed targeting physiology determinants of biomass in the model.

How to cite: Thentu, T., Forster, D., Virkajärvi, P., Harrison, M. T., and Shurpali, N.: Accurate simulation of greenhouse gas emissions across fertilizer scenarios with DNDC, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7642, https://doi.org/10.5194/egusphere-egu24-7642, 2024.