Peatlands represent large terrestrial carbon banks. Given that most peat accumulates in boreal regions, where low temperatures and water saturation preserve organic matter, the existence of peat in (sub)tropical regions remains enigmatic. Here we examined peat and plant chemistry across a latitudinal transect from the Arctic to the tropics. Near-surface low-latitude peat has lower carbohydrate and greater aromatic content than near-surface high-latitude peat, creating a reduced oxidation state and resulting recalcitrance. This recalcitrance allows peat to persist in the (sub)tropics despite warm temperatures. Because we observed similar declines in carbohydrate content with depth in high-latitude peat, our data explain recent field-scale deep peat warming experiments in which catotelm (deeper) peat remained stable despite temperature increases up to 9 °C. We suggest that high-latitude deep peat reservoirs may be stabilized in the face of climate change by their ultimately lower carbohydrate and higher aromatic composition, similar to tropical peats.

Peatlands store one-third of global soil carbon 1 . Drought/drainage coupled with climate warming present the main threat to these stores 1-4 . Hence, understanding drought e ects and inherent feedbacks related to peat decomposition has been a primary global challenge 5,6 . However, widely divergent results concerning drought in recent studies 3,7-11 challenge the accepted paradigm that waterlogging and associated anoxia are the overarching controls locking up carbon stored in peat. Here, by linking field and microcosm experiments, we show how previously unrecognized mechanisms regulate the build-up of phenolics, which protects stored carbon directly by reducing phenol oxidase activity during short-term drought and, indirectly, through a shift from low-phenolic Sphagnum/herbs to high-phenolic shrubs after long-term moderate drought. We demonstrate that shrub expansion induced by drought/warming 2,6,10,12,13 in boreal peatlands might be a long-term self-adaptive mechanism not only increasing carbon sequestration but also potentially protecting historic soil carbon. We therefore propose that the projected 'positive feedback loop' between carbon emission and drought in peatlands 2,3,14,15 may not occur in the long term.Peatlands, covering only 3% of Earth's land area, store about 445 Pg of carbon 1 . These stores result from a small imbalance between production and decomposition over millennia under predominantly waterlogged conditions. However, drought and drainage coupled with warming have been substantially lowering water levels for decades and have resulted in a degradation of more than 11% of global peatlands 1 . These hydrologic shifts often threaten carbon stores, changing peatlands from a carbon sink to a carbon source by increasing decomposition 2,3,[14][15][16] ; concomitantly, the crucial peat-forming Sphagnum mosses are replaced by shrubs/trees 2,10,12 with possibly substantial feedbacks on climate change 6,10 . However, recent evidence showed that in some peatlands drought had little impact or even decreased CO 2 emission and increased carbon accumulation [7][8][9][10][11][17][18][19] (Supplementary Table 1). These contrasting results raise uncertainty on the future fate of peat carbon and question the conventional theory that anoxia is the key to sustaining peat carbon.Detailed comparisons of these drought studies (for example, refs 2,3,7-11,14,15,17,18) show that the initial water level and dominant species varied before drought manipulation (Supplementary Table 1), indicating that these peatlands were in different successional stages 6 . In Sphagnum peatlands, where water levels were mostly above or near the ground surface, drought increased CO 2 emission. However, drought seemed to have less impact in unsaturated and shrub/tree-dominated peatlands. Phenolic inhibitory e ect is defined as a negative value of Pearson's r between soil respiration and soluble phenolics in the surface soil. The grey square is one masked value (removed from the regression) in a drained site where samples were collected a...

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Despite the rapid rise in diversity and quantities of engineered nanomaterials produced, the impacts of these emerging contaminants on the structure and function of ecosystems have received little attention from ecologists. Moreover, little is known about how manufactured nanomaterials may interact with nutrient pollution in altering ecosystem productivity, despite the recognition that eutrophication is the primary water quality issue in freshwater ecosystems worldwide. In this study, we asked two main questions: (1) To what extent do manufactured nanoparticles affect the biomass and productivity of primary producers in wetland ecosystems? (2) How are these impacts mediated by nutrient pollution? To address these questions, we examined the impacts of a citrate-coated gold nanoparticle (AuNPs) and of a commercial pesticide containing Cu(OH) nanoparticles (CuNPs) on aquatic primary producers under both ambient and enriched nutrient conditions. Wetland mesocosms were exposed repeatedly with low concentrations of nanoparticles and nutrients over the course of a 9-month experiment in an effort to replicate realistic field exposure scenarios. In the absence of nutrient enrichment, there were no persistent effects of AuNPs or CuNPs on primary producers or ecosystem productivity. However, when combined with nutrient enrichment, both NPs intensified eutrophication. When either of these NPs were added in combination with nutrients, algal blooms persisted for >50 d longer than in the nutrient-only treatment. In the AuNP treatment, this shift from clear waters to turbid waters led to large declines in both macrophyte growth and rates of ecosystem gross primary productivity (average reduction of 52% ± 6% and 92% ± 5%, respectively) during the summer. Our results suggest that nutrient status greatly influences the ecosystem-scale impact of two emerging contaminants and that synthetic chemicals may be playing an under-appreciated role in the global trends of increasing eutrophication. We provide evidence here that chronic exposure to Au and Cu(OH) nanoparticles at low concentrations can intensify eutrophication of wetlands and promote the occurrence of algal blooms.