Frost-free zone on leaves revisited.

Particular frost patterns on natural leaves had prompted Yao et al. [Y. Yao et al., Proc. Natl. Acad. Sci. U.S.A. 117, 6323-6329 (2020)] to investigate the underlying physics. Their work revealed why on corrugated surfaces ice forms on crests and dries out adjacent grooves. In the absence of frost, in contrast, grooves tend to constitute niches on a leaf where microorganisms are less limited by moisture than in other locations. Here, we show that microorganisms able to nucleate ice before it forms on crests can modify the frosting pattern to their advantage. This ability might drive in cold arid environments the association between certain microorganisms and plants.

Bioprecipitation: a feedback cycle linking earth history, ecosystem dynamics and land use through biological ice nucleators in the atmosphere.

Landscapes influence precipitation via the water vapor and energy fluxes they generate. Biologically active landscapes also generate aerosols containing microorganisms, some being capable of catalyzing ice formation and crystal growth in clouds at temperatures near 0 °C. The resulting precipitation is beneficial for the growth of plants and microorganisms. Mounting evidence from observations and numerical simulations support the plausibility of a bioprecipitation feedback cycle involving vegetated landscapes and the microorganisms they host. Furthermore, the evolutionary history of ice nucleation-active bacteria such as Pseudomonas syringae supports that they have been part of this process on geological time scales since the emergence of land plants. Elucidation of bioprecipitation feedbacks involving landscapes and their microflora could contribute to appraising the impact that modified landscapes have on regional weather and biodiversity, and to avoiding inadvertent, negative consequences of landscape management.

δ15N natural abundance may directly disclose perturbed soil when related to C:N ratio.

RATIONALE: Natural abundance δ(15) N values in soil samples analysed by isotope ratio mass spectrometry (IRMS) are often used to confirm a perturbation after it has been indicated by other parameters. We propose a concept of how δ(15)N values may a priori indicate a perturbation. METHODS: We analysed the δ(15)N values and C:N ratios of 102 soil samples from five regions in northern Eurasia by using an elemental analyser coupled to an isotope ratio mass spectrometer. RESULTS: Unperturbed samples ranged in δ(15)N values from -4.8 to 9.7 ‰ and in C:N ratio from 6.4 to 48.1. The δ(15)N values were linearly proportional to the inverse of the square root of the C:N ratio (R(2) = 0.79). At any particular C:N ratio, 94 % of the δ(15)N values of the unperturbed samples were within ±2.4 ‰, but 72 % of perturbed samples had values outside this range. CONCLUSIONS: The δ(15)N natural abundance values, when related to the C:N ratios, may readily indicate perturbation of soil N cycling prior to other, more demanding investigations into related processes and extend the current use of IRMS in ecosystem research.

Export of ice-nucleating particles from watersheds: results from the Amazon and Tocantins river plumes.

We examined ice-nucleating particles (INPs) in the plumes of the Tocantins and Amazon rivers, which drain watersheds with different proportions of degraded land. The concentration of INPs active at -15°C (INP-15) was an order of magnitude lower in the Tocantins (mean = 13.2 ml-1; s.d. = 7.8 ml-1), draining the more degraded watershed, compared with the Amazon (mean = 175.8 ml-1; s.d. = 11.2 ml-1), where the concentration was also significantly higher than in Atlantic surface waters (mean = 3.2 ml-1; s.d. = 2.3 ml-1). Differences in heat tolerance suggest that INPs emitted by the Amazon rainforest to the atmosphere or washed into the river might originate from contrasting sources on top of and below the rainforest canopy, respectively. For the Amazon River, we estimate a daily discharge of 1018 INP-15 to Atlantic waters. Rivers in cooler climate zones tend to have much higher concentrations of INPs and could, despite a smaller water volume discharged, transfer even larger absolute numbers of INP-15 to shelf waters than does the Amazon. To what extent these terrestrial INPs become aerosolized by breaking waves and bubble-bursting remains an open question.

Regionally sourced bioaerosols drive high-temperature ice nucleating particles in the Arctic.

Primary biological aerosol particles (PBAP) play an important role in the climate system, facilitating the formation of ice within clouds, consequently PBAP may be important in understanding the rapidly changing Arctic. Within this work, we use single-particle fluorescence spectroscopy to identify and quantify PBAP at an Arctic mountain site, with transmission electronic microscopy analysis supporting the presence of PBAP. We find that PBAP concentrations range between 10-3-10-1 L-1 and peak in summer. Evidences suggest that the terrestrial Arctic biosphere is an important regional source of PBAP, given the high correlation to air temperature, surface albedo, surface vegetation and PBAP tracers. PBAP clearly correlate with high-temperature ice nucleating particles (INP) (>-15 °C), of which a high a fraction (>90%) are proteinaceous in summer, implying biological origin. These findings will contribute to an improved understanding of sources and characteristics of Arctic PBAP and their links to INP.

δ15N natural abundance in permafrost soil indicates impact of fire on nitrogen cycle.

The impact of fire on the nitrogen (N) cycle of natural ecosystems is arguable. Here we report and interpret an observation from boreal ecosystems in the Lena River basin, Sakha Republic (Yakutia), Russian Federation. Different types of permafrost soil (0-30 cm depth) were sampled along transects (60-150 m length) from the forest edge towards the centre of four separate thermokarst depressions under grassland. The average values of δ(15)N were remarkably similar within three transects, but differed systematically between them. Three findings point towards fire being the cause of the observed pattern. First, the spatial extent of systematic differences in soil δ(15)N coincides with the extent of typical fire scars in the region. Second, soil enrichment in (15)N is larger in the proximity of settlements, where fire is generally more frequent than in more remote places. Third, there is a significant positive correlation between δ(15)N values and the ratio of black C to total N. These findings point towards fire having a marked impact on soil δ(15)N and, accordingly, on the N cycle of this cold and dry ecosystem.

Export of ice nucleating particles from a watershed.

Ice nucleating particles (INP) active at a few degrees below 0°C are produced by a range of organisms and released into the environment. They may affect cloud properties and precipitation when becoming airborne. So far, our knowledge about sources of biological INP is based on grab samples of vegetation, soil or water studied in the laboratory. By combining measurements of INP concentrations in river water with river water discharge rates over the course of 16 months, we obtained a lower limit for the production rate of INP in a watershed covering most of Switzerland (4 × 105 INP-8 m-2 d-1). Coincidentally, we found that INP-8 are likely to retain their potential for catalysing ice formation in the natural environment for at least several months before they are mobilized by an intensive rainfall, washed into the river and exported from the watershed.

Ice nucleation active particles are efficiently removed by precipitating clouds.

Ice nucleation in cold clouds is a decisive step in the formation of rain and snow. Observations and modelling suggest that variations in the concentrations of ice nucleating particles (INPs) affect timing, location and amount of precipitation. A quantitative description of the abundance and variability of INPs is crucial to assess and predict their influence on precipitation. Here we used the hydrological indicator δ(18)O to derive the fraction of water vapour lost from precipitating clouds and correlated it with the abundance of INPs in freshly fallen snow. Results show that the number of INPs active at temperatures ≥ -10 °C (INPs-10) halves for every 10% of vapour lost through precipitation. Particles of similar size (>0.5 µm) halve in number for only every 20% of vapour lost, suggesting effective microphysical processing of INPs during precipitation. We show that INPs active at moderate supercooling are rapidly depleted by precipitating clouds, limiting their impact on subsequent rainfall development in time and space.

Fractionation factors for stable isotopes of N and O during N2O reduction in soil depend on reaction rate constant.

Nitrous oxide (N(2)O) is a major greenhouse gas that is mainly produced but also reduced by microorganisms in soils. We determined factors for N and O isotope fractionation during the reduction of N(2)O to N(2) in soil in a flow-through incubation experiment. The absolute value of the fractionation factors decreased with increasing reaction rate constant. Reaction rates constants ranged from 1.7 10(-4) s(-1) to 4.5 10(-3) s(-1). The minimum, maximum and median of the observed fractionation factors were for N -36.0 per thousand, -1.0 per thousand and -9.3 per thousand and for O -74.0 per thousand, -6.9 per thousand and -26.3 per thousand, respectively. The ratio of O isotope fractionation to N isotope fractionation was 2.4 +/- 0.3 and it was independent from the reaction rate constants. This leads us to conclude that fractionation factors are variables while their ratio in this particular reaction might be a constant.