Greater ecosystem carbon in the Mojave Desert after ten years exposure to elevated CO2

Carbon dioxide is the main greenhouse gas inducing climate change. Increased global CO₂ emissions, estimated at 8.4 Pg C yr⁻¹ at present, have accelerated from 1% yr⁻¹during 1990–1999 to 2.5% yr⁻¹ during 2000–2009 (ref. 1). The carbon balance of terrestrial ecosystems is the greatest unknown in the global C budget because the actual magnitude, location and causes of terrestrial sinks are uncertain²; estimates of terrestrial C uptake, therefore, are often based on the residuals between direct measurements of the atmospheric sink and well-constrained models of ocean uptake of CO₂ (ref. 3). Here we report significant terrestrial C accumulation caused by CO₂ enhancement to net ecosystem productivity in an intact, undisturbed arid ecosystem^{4, 5, 6, 7, 8} following ten years of exposure to elevated atmospheric CO₂. Results provide direct evidence that CO₂fertilization substantially increases ecosystem C storage and that arid ecosystems are significant, previously unrecognized, sinks for atmospheric CO₂ that must be accounted for in efforts to constrain terrestrial and global C cycles.

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Authors:Evans RD Koyama A Sonderegger DL Charlet TN Newingham BA Fenstermaker LF Harlow B Jin VL Ogle K Smith SD Nowak RS

Forest ecosystems sequester approximately 12% of anthropogenic carbon emissions, and efforts to increase forest carbon uptake are central to climate change mitigation policy. Managing forests to store carbon has focused on increasing forested area, decreasing area lost to logging and clearing, and increasing forest carbon density. Warming, drought, and wildfires challenge the stability of carbon stored in forests. By contrast, natural cycles of low-intensity fires in dry forests can, over the long term, promote forest carbon storage by protecting carbon in soil and in large, old trees. The conundrum is how to balance immediate, disturbance-driven carbon loss with long-term, stable carbon storage and account for these risks in policies for forest carbon management.

Temperature is a primary driver of microbial community composition and taxonomic diversity; however, it is unclear to what extent temperature affects characteristics of central carbon metabolic pathways (CCMPs). In this study, 16S rRNA gene amplicon and metagenome sequencing were combined with 13C-labeled metabolite probing of the CCMPs to assess community carbon metabolism along a temperature gradient (60-95 °C) in Great Boiling Spring, NV. 16S rRNA gene amplicon diversity was inversely proportional to temperature, and Archaea were dominant at higher temperatures. Metagenomes spanning the temperature gradient hosted abundant CCMPs genes and many individual metagenome-assembled genomes had complete pathways. In contrast, genes encoding cellulosomes and some others involved in plant matter degradation and most for photosynthesis and were absent at higher temperatures. In situ 13C-CO2 production from labeled isotopomer pairs of glucose, pyruvate, and acetate suggested lower relative oxidative pentose phosphate pathway activity and/or fermentation at 60°C, and a stable or decreased maintenance energy demand at higher temperatures. Catabolism of 13C-labeled citrate, succinate, L-alanine, L-serine, and L-cysteine was observed at 85°C, demonstrating broad heterotrophic activity. Together, these results suggest that temperature-driven losses in biodiversity in geothermal systems may not alter CCMP function or maintenance energy demands at a community level.