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Nitrous oxide (N 2 O) is an important greenhouse gas contributing to global climate change. Emissions of N 2 O from acidic forests are increasing rapidly; however, little is known about the mechanisms driving these emissions. We analyzed soil samples from a high N 2 O emission area (HEA, 224-601 μg N m -2  h -1 ) and an adjacent low emission area (LEA, 20-30 μg N m -2  h -1 ) of a highly acidified forest. HEA showed similar carbon and nitrogen (N) pools and microbial biomass to LEA, but significantly higher moisture and extractable nutrients than LEA did. GeoChip 4 detected 298 gene families (unadjusted P < 0.05; 94, adjusted P < 0.05) showing significantly different structures between HEA and LEA. Both areas had highly diverse N cycling functional genes. However, HEA had higher relative abundances of nor, P450nor, and archaeal nitrifier nirK, which provided evidence for the importance of denitrifiers in N 2 O emission. HEA also showed significantly higher relative abundances of lignin- and cellulose-degrading genes, oxygen-limitation-response genes and denitrifier ppk, but lower abundances of N- and phosphorus (P) -limitation-response genes especially denitrifier pstS, corresponding to the higher moisture and extractable nutrients conducive to denitrification. The moisture, extractable nutrients and pH explained over 50% variation in microbial communities, and extractable P appeared as the key factor driving community variation and consequently regulated N 2 O production. CAPSULE ABSTRACT: N 2 O emission in highly acidified forest soils was related to the diverse N functional genes, especially denitrification genes, and was affected by soil properties., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

The continuously increasing concentration of atmospheric CO 2 has considerably altered ecosystem functioning. However, few studies have examined the long-term (i.e. over a decade) effect of elevated CO 2 on soil microbial communities. Using 16S rRNA gene amplicons and a GeoChip microarray, we investigated soil microbial communities from a Californian annual grassland after 14 years of experimentally elevated CO 2 (275 ppm higher than ambient). Both taxonomic and functional gene compositions of the soil microbial community were modified by elevated CO 2 . There was decrease in relative abundance for taxa with higher ribosomal RNA operon (rrn) copy number under elevated CO 2 , which is a functional trait that responds positively to resource availability in culture. In contrast, taxa with lower rrn copy number were increased by elevated CO 2 . As a consequence, the abundance-weighted average rrn copy number of significantly changed OTUs declined from 2.27 at ambient CO 2 to 2.01 at elevated CO 2 . The nitrogen (N) fixation gene nifH and the ammonium-oxidizing gene amoA significantly decreased under elevated CO 2 by 12.6% and 6.1%, respectively. Concomitantly, nitrifying enzyme activity decreased by 48.3% under elevated CO 2 , albeit this change was not significant. There was also a substantial but insignificant decrease in available soil N, with both nitrate (NO 3 - ) (-27.4%) and ammonium (NH 4 + ) (-15.4%) declining. Further, a large number of microbial genes related to carbon (C) degradation were also affected by elevated CO 2 , whereas those related to C fixation remained largely unchanged. The overall changes in microbial communities and soil N pools induced by long-term elevated CO 2 suggest constrained microbial N decomposition, thereby slowing the potential maximum growth rate of the microbial community., (Copyright © 2018 Elsevier B.V. All rights reserved.)