This document on the CMB-S4 Science Case, Reference Design, and Project Plan is the product of a global community of scientists who are united in support of advancing CMB-S4 to cross key thresholds in our understanding of the fundamental nature of space and time and the evolution of the Universe. CMB-S4 is planned to be a joint National Science Foundation (NSF) and Department of Energy (DOE) project, with the construction phase to be funded as an NSF Major Research Equipment and Facilities Construction (MREFC) project and a DOE High Energy Physics (HEP) Major Item of Equipment (MIE) project. At the time of this writing, an interim project office has been constituted and tasked with advancing the CMB-S4 project in the NSF MREFC Preliminary Design Phase and toward DOE Critical Decision CD-1. DOE CD-0 is expected imminently.CMB-S4 has been in development for six years. Through the Snowmass Cosmic Frontier planning process, experimental groups in the cosmic microwave background (CMB) and broader cosmology communities came together to produce two influential CMB planning papers, endorsed by over 90 scientists, that outlined the science case as well as the CMB-S4 instrumental concept [1, 2]. It immediately became clear that an enormous increase in the scale of ground-based CMB experiments would be needed to achieve the exciting thresholdcrossing scientific goals, necessitating a phase change in the ground-based CMB experimental program. To realize CMB-S4, a partnership of the university-based CMB groups, the broader cosmology community, and the national laboratories would be needed.The community proposed CMB-S4 to the 2014 Particle Physics Project Prioritization Process (P5) as a single, community-wide experiment, jointly supported by DOE and NSF. Following P5's recommendation of CMB-S4 under all budget scenarios, the CMB community started in early 2015 to hold biannual workshops -open to CMB scientists from around the world -to develop and refine the concept. Nine workshops have been held to date, typically with 150 to 200 participants. The workshops have focused on developing the unique and vital role of the future ground-based CMB program. This growing CMB-S4 community produced a detailed and influential CMB-S4 Science Book [3] and a CMB-S4 Technology Book [4]. Over 200 scientists contributed to these documents. These and numerous other reports, workshop and working group wiki pages, email lists, and much more may be found at the website http://CMB-S4.org.Soon after the CMB-S4 Science Book was completed in August 2016, DOE and NSF requested the Astronomy and Astrophysics Advisory Committee (AAAC) to convene a Concept Definition Taskforce (CDT) to conduct a CMB-S4 concept study. The resulting report was unanimously accepted in late 2017. 1 One recommendation of the CDT report was that the community should organize itself into a formal collaboration. An Interim Collaboration Coordination Committee was elected by the community to coordinate this process. The resulting draft bylaws were refined at the Spring 2018 CMB-S4...

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Abstract. Trace gas exchange of N 2 O, NO/NO 2 and CH 4 between soil and the atmosphere was measured in a typical Mediterranean pine (Pinus pinaster) forest during two intensive field campaigns in spring and autumn 2003. Furthermore, gross and net turnover rates of N mineralization and nitrification as well as soil profiles of N 2 O and CH 4 concentrations were determined. For both seasons a weak but significant N 2 O uptake from the atmosphere into the soil was observed. During the unusually dry and hot spring mean N 2 O uptake was −4.32 µg N m −2 h −1 , whereas during the wet and mild autumn mean N 2 O uptake was −7.85 µg N m −2 h −1 . The observed N 2 O uptake into the soil was linked to the very low availability of inorganic nitrogen at the study site. Organic layer gross N mineralization decreased from 5.06 mg N kg −1 SDW d −1 in springtime to 2.68 mg N kg −1 SDW d −1 in autumn. Mean NO emission rates were significantly higher in springtime (9.94 µg N m −2 h −1 ) than in autumn (1.43 µg N m −2 h −1 ). A significant positive correlation between NO emission rates and gross N mineralization as well as nitrification rates was found. The negative correlation between NO emissions and soil moisture was explained with a stimulation of aerobic NO uptake under N limiting conditions. Since NO 2 deposition was continuously higher than NO emission rates the examined forest soil functioned as a net NO x sink. Observed mean net CH 4 uptake rates were in spring significantly higher (−73.34 µg C m −2 h −1 ) than in autumn (−59.67 µg C m −2 h −1 ). Changes in CH 4 uptake rates were strongly negatively correlated with changes in soil moisture. The N 2 O and CH 4 concentrations in different soil depths revealed the organic layer and the upper 0.1 m of mineral soil as the most important soil horizons for N 2 O and CH 4 consumption.

Using the free-air CO 2 enrichment (FACE) techniques, we carried out a 3-year monofactorial experiment in temperate paddy rice fields of Japan (1998)(1999)(2000) and a 3-year multifactorial experiment in subtropical paddy rice fields in the Yangtze River delta in China (2001China ( -2003, to investigate the methane (CH 4 ) emissions in response to an elevated atmospheric CO 2 concentration (200 AE 40 mmol mol À1 higher than that in the ambient atmosphere). No significant effect of the elevated CO 2 upon seasonal accumulative CH 4 emissions was observed in the first rice season, but significant stimulatory effects (CH 4 increase ranging from 38% to 188%, with a mean of 88%) were observed in the second and third rice seasons in the fields with or without organic matter addition. The stimulatory effects of the elevated CO 2 upon seasonal accumulative CH 4 emissions were negatively correlated with the addition rates of decomposable organic carbon (Po0.05), but positively with the rates of nitrogen fertilizers applied in either the current rice season (Po0.05) or the whole year (Po0.01). Six mechanisms were proposed to explain collectively the observations. Soil nitrogen availability was identified as an important regulator. The effect of soil nitrogen availability on the observed relation between elevated CO 2 and CH 4 emission can be explained by (a) modifying the C/N ratio of the plant residues formed in the previous growing season(s); (b) changing the inhibitory effect of high C/N ratio on plant residue decomposition in the current growing season; and (c) altering the stimulatory effects of CO 2 enrichment upon plant growth, as well as nitrogen uptake in the current growing season. This study implies that the concurrent enrichment of reactive nitrogen in the global ecosystems may accelerate the increase of atmospheric methane by initiating a stimulatory effect of the ongoing dramatic atmospheric CO 2 enrichment upon methane emissions from nitrogen-poor paddy rice ecosystems and further amplifying the existing stimulatory effect in nitrogen-rich paddy rice ecosystems.