A 3-year continuous record of nitrogen trace gas fluxes from untreated and limed soil of a N-saturated spruce and beech forest ecosystem in Germany: 2. NO and NO₂ fluxes

For 3 years we followed the complete annual cycles of NO and NO$_{2}$ flux rates from soil of a spruce control site, a limed spruce site, and a beech site at the Höglwald Forest, Bavaria, Germany, with high temporal resolution in order to gain detailed information about (1) the impacts of forest type, liming, and atmospheric N input by wet deposition on the magnitude of NO and NO$_{2}$ flux rates and (2) the microbial processes involved in NO production and emission. In addition to identification of seasonal variations of flux rates the huge database allowed calculation of annual mean NO and NO$_{2}$ fluxes with high accuracy and identification of interannual variations of fluxes. The long-term annual mean NO$_{X}$ emission was 61.7 $\mu$g NO$_{X}$ N m$^{-2}$ h$^{-1}$ for the spruce control site, 17.3 $\mu$g NO$_{X}$ N m$^{-^2}$ h${^-1}$ for the limed spruce site, and 4.0 $\mu$g NO$_{X}$ N m$^{-2}$ h$^{-1}$ for the beech site. These extremely high soil NO$_{X}$ emissions from a temperate forest most likely reflect the status of N saturation of the Höglwald Forest as a consequence of year-long heavy atmospheric N input. Multiple regression analyses revealed the following sequence of importance of environmental factors on NO flux: soil temperature to waterfilled pore space to soil NO$^{-}_{3}$ concentrations to soil NH$^{+}_{4}$ concentrations. Nitrification was the dominating biotic modulator of NO emission at all sites: $>$60% of the variation of NO emission rates were associated with variations of net nitrification rates. There was a strong positive correlation between amount of in situ N input by wet deposition and magnitude of in situ NO flux rates. Approximately 15% and 7% of the actual N input was lost as NO from the soil stocked with spruce and beech, respectively. Liming resulted in 49% reduction of NO emissions as compared to an unlimed spruce control site. The results indicate that the reduction in NO emission was due to an increase in NO consumption within the limed soil. In contrast to NO flux, NO$_{2}$ flux was modulated by physico-chemical rather than biological factors. Using the data of this study, we estimate that the contribution of N-affected temperate coniferous and deciduous forests to the global NO$_{X}$ release is 0.3 Tg NO$_{X}$ N yr$^{-1}$

Impacts of slurry acidification and injection on fertilizer nitrogen fates in grassland

Low nitrogen (N) use efficiency of broadcast slurry application leads to nutrient losses, air and water pollution, greenhouse gas emissions and—in particular in a warming climate—to soil N mining. Here we test the alternative slurry acidification and injection techniques for their mitigation potential compared to broadcast spreading in montane grassland. We determined (1) the fate of $^{15}$N labelled slurry in the plant-soil-microbe system and soil-atmosphere exchange of greenhouse gases over one fertilization/harvest cycle and (2) assessed the longer-term contribution of fertilizer $^{15}$N to soil organic N formation by the end of the growing season. The isotope tracing approach was combined with a space for time climate change experiment. Simulated climate change increased productivity, ecosystem respiration, and net methane uptake irrespective of management, but the generally low N$_2$O fluxes remained unchanged. Compared to the broadcast spreading, slurry acidification showed lowest N losses, thus increased productivity and fertilizer N use efficiency (38% $^{15}$N recovery in plant aboveground plant biomass). In contrast, slurry injection showed highest total fertilizer N losses, but increased fertilization-induced soil organic N formation by 9–12 kg N ha$^{−1}$ season$^{−1}$. Slurry management effects on N$_2$O and CH$_4$ fluxes remained negligible. In sum, our study shows that the tested alternative slurry application techniques can increase N use efficiency and/or promote soil organic N formation from applied fertilizer to a remarkable extent. However, this is still not sufficient to prevent soil N mining mostly resulting from large plant N exports that even exceed total fertilizer N inputs

The impact of management and climate on soil nitric oxide fluxes from arable land in the southern Ukraine

NO fluxes from soils are a significant source for tropospheric NOx, though global and regional estimates of the soil source strength are constrained by the paucity of measurements. In a continuous 18 month effort (2012–2014) soil NO fluxes from an intensively managed arable site in the black soil region of the Southern Ukraine (Odessa region) were measured using an automated dynamic chamber system. Measurements revealed three periods of peak NO emissions (fertigation, re-wetting of soils, and to a lower extend during winter), with a pulse emission peak during soil re-wetting in summer of 88.4 μg N m−2 h−1. The mean annual NO flux was 5.1 ± 8.9 μg N m−2 h−1 and total annual NO emissions were 0.44 ± 0.78 kg N ha−1 yr−1. The fertilizer induced emission factor for NO was 0.63% under beetroot. The combined effect of soil temperature, soil moisture and soil DIN (NH4+ and NO3−) concentrations were identified as drivers of the temporal and spatial variability of soil NO fluxes. This work shows that long-term measurements are needed for estimating annual fluxes and the importance of soils as a source for tropospheric NOx as the contribution of different seasons and crop growing periods to the annual budget differed markedly

Climate change and management intensity alter spatial distribution and abundance of P mineralizing bacteria and arbuscular mycorrhizal fungi in mountainous grassland soils

In mountainous grasslands management adaptations are required to maintain soil functions. We investigated climate change (CC) and management effects on the abundance and potential activity of microbiota catalyzing the major steps of P transformation which are still unknown in these grasslands. Soil samples were taken from intact plant-soil mesocosms managed extensively or intensively (two vs. five mowing and slurry applications, respectively). These mesocosms were previously translocated from high to lower altitudes to simulate two CC scenarios (CC1: +1 °C warming and mean annual precipitation (MAP) of 1347 mm and CC2: +3 °C warming and MAP of 956 mm), while control mesocosms (CC0) were relocated at their original site (6 °C and MAP of 1400 mm). Specific marker genes for P-solubilization (gcd), P-mineralization (phoN, phoD, phnX and appA), P-uptake (pitA and pstS), total bacteria and arbuscular mycorrhizal fungi (AMF) were quantified by quantitative real-time PCR. Spatial distributions of phosphatase activities were analyzed in situ by zymography analysis and total organic C, N and P contents were measured. Gene abundances and enzymatic activities were comparable for both managements under CC0, except for phytase-harboring (appA) microbiota which decreased under intensive management. The abundance of microbiota which catalyzes organic P (Po) mineralization (phoN and appA) and those harboring quinoprotein glucose dehydrogenase (gcd) for P solubilization significantly dropped by interacting effects of CC2 and extensive management. The same effect was found for microbes harboring specific P transporters (pitA and pstS). Under intensive management, microbiota catalyzing Po mineralization (phoN and appA), and alkaline phosphatase activities tended to increase in CC2. Noteworthy, the AMF abundance was reduced at 0–5 cm soil depth under CC. Our results suggest that CC and extensive management reduced microbial P solubilization, mineralization and uptake, while management intensification may increase P availability, which leads to shifts in nutrient stoichiometry and decreased AMF abundance

Lysimeter-based full fertilizer 15N balances corroborate direct dinitrogen emission measurements using the 15N gas flow method

The $^{15}$N gas flux ($^{15}$NGF) method allows for direct in situ quantification of dinitrogen (N$_2$) emissions from soils, but a successful cross-comparison with another method is missing. The objectives of this study were to quantify N$_2$ emissions of a wheat rotation using the $^{15}$NGF method, to compare these N$_2$ emissions with those obtained from a lysimeter-based $^{15}$N fertilizer mass balance approach, and to contextualize N$_2$ emissions with $^{15}$N enrichment of N$_2$ in soil air. For four sampling periods, fertilizer-derived N$_2$ losses ($^{15}$NGF method) were similar to unaccounted fertilizer N fates as obtained from the $^{15}$N mass balance approach. Total N$_2$ emissions ($^{15}$NGF method) amounted to 21 ± 3 kg N ha− 1, with 13 ± 2 kg N ha− 1 (7.5% of applied fertilizer N) originating from fertilizer. In comparison, the $^{15}$N mass balance approach overall indicated fertilizer-derived N$_2$ emissions of 11%, equivalent to 18 ± 13 kg N ha− 1. Nitrous oxide (N$_2$O) emissions were small (0.15 ± 0.01 kg N ha− 1 or 0.1% of fertilizer N), resulting in a large mean N$_2$:(N$_2$O + N$_2$) ratio of 0.94 ± 0.06. Due to the applied drip fertigation, ammonia emissions accounted for < 1% of fertilizer-N, while N leaching was negligible. The temporal variability of N$_2$ emissions was well explained by the δ$^{15}$N$_2$ in soil air down to 50 cm depth. We conclude the $^{15}$NGF method provides realistic estimates of field N$_2$ emissions and should be more widely used to better understand soil N$_2$ losses. Moreover, combining soil air δ$^{15}$N$_2$ measurements with diffusion modeling might be an alternative approach for constraining soil N$_2$ emissions

Climate Change Impairs Nitrogen Cycling in European Beech Forests

European beech forests growing on marginal calcareous soils have been proposed to be vulnerable to decreased soil water availability. This could result in a large-scale loss of ecological services and economical value in a changing climate. In order to evaluate the potential consequences of this drought-sensitivity, we investigated potential species range shifts for European beech forests on calcareous soil in the 21st century by statistical species range distribution modelling for present day and projected future climate conditions. We found a dramatic decline by 78% until 2080. Still the physiological or biogeochemical mechanisms underlying the drought sensitivity of European beech are largely unknown. Drought sensitivity of beech is commonly attributed to plant physiological constraints. Furthermore, it has also been proposed that reduced soil water availability could promote nitrogen (N) limitation of European beech due to impaired microbial N cycling in soil, but this hypothesis has not yet been tested. Hence we investigated the influence of simulated climate change (increased temperatures, reduced soil water availability) on soil gross microbial N turnover and plant N uptake in the beech-soil interface of a typical mountainous beech forest stocking on calcareous soil in SW Germany. For this purpose, triple 15N isotope labelling of intact beech seedling-soil-microbe systems was combined with a space-for-time climate change experiment. We found that nitrate was the dominant N source for beech natural regeneration. Reduced soil water content caused a persistent decline of ammonia oxidizing bacteria and therefore, a massive attenuation of gross nitrification rates and nitrate availability in the soil. Consequently, nitrate and total N uptake of beech seedlings were strongly reduced so that impaired growth of beech seedlings was observed already after one year of exposure to simulated climatic change. We conclude that the N cycle in this ecosystem and here specifically nitrification is vulnerable to reduced water availability, which can directly lead to nutritional limitations of beech seedlings. This tight link between reduced water availability, drought stress for nitrifiers, decreased gross nitrification rates and nitrate availability and finally nitrate uptake by beech seedlings could represent the Achilles’ heel for beech under climate change stresses

Attribution of N₂O sources in a grassland soil with laser spectroscopy based isotopocule analysis

Nitrous oxide (N2O) is the primary atmospheric constituent involved in stratospheric ozone depletion and contributes strongly to changes in the climate system through a positive radiative forcing mechanism. The atmospheric abundance of N2O has increased from 270 ppb (parts per billion, 10−9 mole mole−1) during the pre-industrial era to approx. 330 ppb in 2018. Even though it is well known that microbial processes in agricultural and natural soils are the major N2O source, the contribution of specific soil processes is still uncertain. The relative abundance of N2O isotopocules (14N14N16N, 14N15N16O, 15N14N16O, and 14N14N18O) carries process-specific information and thus can be used to trace production and consumption pathways. While isotope ratio mass spectroscopy (IRMS) was traditionally used for high-precision measurement of the isotopic composition of N2O, quantum cascade laser absorption spectroscopy (QCLAS) has been put forward as a complementary technique with the potential for on-site analysis. In recent years, pre-concentration combined with QCLAS has been presented as a technique to resolve subtle changes in ambient N2O isotopic composition. From the end of May until the beginning of August 2016, we investigated N2O emissions from an intensively managed grassland at the study site Fendt in southern Germany. In total, 612 measurements of ambient N2O were taken by combining pre-concentration with QCLAS analyses, yielding δ15Nα, δ15Nβ, δ18O, and N2O concentration with a temporal resolution of approximately 1 h and precisions of 0.46 ‰, 0.36 ‰, 0.59 ‰, and 1.24 ppb, respectively. Soil δ15N-NO−3 values and concentrations of NO−3 and NH+4 were measured to further constrain possible N2O-emitting source processes. Furthermore, the concentration footprint area of measured N2O was determined with a Lagrangian particle dispersion model (FLEXPART-COSMO) using local wind and turbulence observations. These simulations indicated that night-time concentration observations were largely sensitive to local fluxes. While bacterial denitrification and nitrifier denitrification were identified as the primary N2O-emitting processes, N2O reduction to N2 largely dictated the isotopic composition of measured N2O. Fungal denitrification and nitrification-derived N2O accounted for 34 %–42 % of total N2O emissions and had a clear effect on the measured isotopic source signatures. This study presents the suitability of on-site N2O isotopocule analysis for disentangling source and sink processes in situ and found that at the Fendt site bacterial denitrification or nitrifier denitrification is the major source for N2O, while N2O reduction acted as a major sink for soil-produced N2O.ISSN:1726-4170ISSN:1726-417

ECLAIRE: Effects of Climate Change on Air Pollution Impacts and Response Strategies for European Ecosystems. Project final report

The central goal of ECLAIRE is to assess how climate change will alter the extent to which air pollutants threaten terrestrial ecosystems. Particular attention has been given to nitrogen compounds, especially nitrogen oxides (NOx) and ammonia (NH3), as well as Biogenic Volatile Organic Compounds (BVOCs) in relation to tropospheric ozone (O3) formation, including their interactions with aerosol components. ECLAIRE has combined a broad program of field and laboratory experimentation and modelling of pollution fluxes and ecosystem impacts, advancing both mechanistic understanding and providing support to European policy makers. The central finding of ECLAIRE is that future climate change is expected to worsen the threat of air pollutants on Europe’s ecosystems. Firstly, climate warming is expected to increase the emissions of many trace gases, such as agricultural NH3, the soil component of NOx emissions and key BVOCs. Experimental data and numerical models show how these effects will tend to increase atmospheric N deposition in future. By contrast, the net effect on tropospheric O3 is less clear. This is because parallel increases in atmospheric CO2 concentrations will offset the temperature-driven increase for some BVOCs, such as isoprene. By contrast, there is currently insufficient evidence to be confident that CO2 will offset anticipated climate increases in monoterpene emissions. Secondly, climate warming is found to be likely to increase the vulnerability of ecosystems towards air pollutant exposure or atmospheric deposition. Such effects may occur as a consequence of combined perturbation, as well as through specific interactions, such as between drought, O3, N and aerosol exposure. These combined effects of climate change are expected to offset part of the benefit of current emissions control policies. Unless decisive mitigation actions are taken, it is anticipated that ongoing climate warming will increase agricultural and other biogenic emissions, posing a challenge for national emissions ceilings and air quality objectives related to nitrogen and ozone pollution. The O3 effects will be further worsened if progress is not made to curb increases in methane (CH4) emissions in the northern hemisphere. Other key findings of ECLAIRE are that: 1) N deposition and O3 have adverse synergistic effects. Exposure to ambient O3 concentrations was shown to reduce the Nitrogen Use Efficiency of plants, both decreasing agricultural production and posing an increased risk of other forms of nitrogen pollution, such as nitrate leaching (NO3-) and the greenhouse gas nitrous oxide (N2O); 2) within-canopy dynamics for volatile aerosol can increase dry deposition and shorten atmospheric lifetimes; 3) ambient aerosol levels reduce the ability of plants to conserve water under drought conditions; 4) low-resolution mapping studies tend to underestimate the extent of local critical loads exceedance; 5) new dose-response functions can be used to improve the assessment of costs, including estimation of the value of damage due to air pollution effects on ecosystems, 6) scenarios can be constructed that combine technical mitigation measures with dietary change options (reducing livestock products in food down to recommended levels for health criteria), with the balance between the two strategies being a matter for future societal discussion. ECLAIRE has supported the revision process for the National Emissions Ceilings Directive and will continue to deliver scientific underpinning into the future for the UNECE Convention on Long-range Transboundary Air Pollution