Dinitrogen emissions: an overlooked key component of the N balance of montane grasslands

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

Biotic and abiotic controls on carbon storage in aggregates in calcareous alpine and prealpine grassland soils

Alpine and prealpine grasslands provide various ecosystem services and are hotspots for the storage of soil organic C (SOC) in Central Europe. Yet, information about aggregate-related SOC storage and its controlling factors in alpine and prealpine grassland soils is limited. In this study, the SOC distribution according to the aggregate size classes large macroaggregates (> 2000 μm), small macroaggregates (250–2000 μm), microaggregates (63–250 μm), and silt-/clay-sized particles (< 63 μm) was studied in grassland soils along an elevation gradient in the Northern Limestone Alps of Germany. This was accompanied by an analysis of earthworm abundance and biomass according to different ecological niches. The SOC and N stocks increased with elevation and were associated with relatively high proportions of water-stable macroaggregates due to high contents of exchangeable Ca$^{2+}$ and Mg$^{2+}$. At lower elevations, earthworms appeared to act as catalyzers for a higher microaggregate formation. Thus, SOC stabilization by aggregate formation in the studied soils is a result of a joined interaction of organic matter and Ca$^{2+}$ as binding agents for soil aggregates (higher elevations), and the earthworms that act as promoters of aggregate formation through the secretion of biogenic carbonates (low elevation). Our study highlights the importance of aggregate-related factors as potential indices to evaluate the SOC storage potential in other mountainous grassland soils

Short term effects of climate change and intensification of management on the abundance of microbes driving nitrogen turnover in montane grassland soils.

Montane grasslands in Europe are exposed to increasing temperatures twice as fast as the global average. Changes in climatic conditions are possibly accompanied by an increase in land use intensity, caused by a prolongation of the vegetation period and the need to improve productivity. Therefore, the investigation of combined effects of climate change and land use intensity is needed to further implement agricultural management strategies. Here we present results from a study performed in the pre-alpine region of southern Germany, where intact plant-soil mesocosms from grasslands, were translocated along an altitudinal gradient, resulting in an increase in soil temperature (moderate treatment: +0.5 K; strong treatment: +1.9 K warming) during the experimental period. Additionally, we applied an extensive or intensive agricultural management (two vs. five times of mowing and slurry application) on the transplanted mesocosms. After an exposure of one year, we measured plant growth and soil properties and quantified abundances of soil microorganisms catalyzing key steps in the nitrogen (N) cycle. Our data indicate, significant interactions between climate change and management. For example, microbial biomass was significantly reduced (−47.7% and −49.8% for C and N respectively), which was further accompanied by lower abundances of N -fixing bacteria (up to −89,3%), as well as ammonia oxidizing bacteria (−81.4%) under intensive management, whereas N-mineralizing bacteria increased in abundance (up to +139.8%) under extensive management. Surprisingly, the abundances of denitrifying bacteria as well as mean N O emissions were not affected by the treatments. Overall, our data suggest pronounced shifts in the abundance of microbes driving the N cycle in soil as a result of combined climate change and land use intensification already after a short simulation period of one year. mic mic 2