Isotopic analysis (δ13C and δ2H) of lignin methoxy groups in forest soils to identify and quantify lignin sources.

The relative apportion of above and below ground carbon sources is known to be an important factor in soil organic matter formation. Although lignin is the most abundant aromatic plant material in the terrestrial biosphere, our understanding of lignin source contributions to soil organic matter (SOM) is limited due to the complex molecular structure and analysis of lignin. In this study, we novelly apply the dual isotopic analysis (δ13C and δ2H values) of lignin methoxy groups (LMeO) with the Bayesian mixing model, MixSIAR, to apportion lignin sources in two contrasting soil types, a podzol and a stagnosol. Results of the isotopic analysis of LMeO demonstrate the ability of δ2H LMeO values to discriminate between above and below ground lignin sources, while δ13C LMeO values discriminated between photosynthesising and non-photosynthesising tissues. In the stagnosol subsurface horizons, a decreasing proportion of the leaf litter lignin was observed with increasing organic matter degradation, cumulating in the Ah horizon being dominated by lignin from roots. The podzol sites indicated a similar reduction in leaf litter lignin with an increase in organic matter degradation and depth. However, the Ah horizon was shown to accumulate lignin from the above ground woody material. Furthermore, given the significant abundance of LMeO groups in the terrestrial biosphere and the extremely depleted δ13C LMeO values in leaf litter, we employed a mass balance approach to determine the extent in which the 13C bulk enrichment generally associated with isotopic fractionation during organic matter decomposition can be attributed to the shift in lignin sources. Analysis reveals that 14 % and 11 % of bulk 13C enrichment can be attributed to the transition in LMeO sources from leaf litter to roots in the stagnosol and podzol, respectively. Thus, models relying on 13C enrichment with depth as an indicator of carbon turnover may be partially overestimating rates.

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.

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.