Research ArticleECOLOGY

Large losses of ammonium-nitrogen from a rice ecosystem under elevated CO2

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Science Advances  16 Oct 2020:
Vol. 6, no. 42, eabb7433
DOI: 10.1126/sciadv.abb7433
  • Fig. 1 Losses of ammonium (NH4+)–nitrogen from a rice ecosystem exposed to long-term FACE.

    (A to C) Experimental scheme: experiment 1 (A), two parallel microcosms in which soils, taken from ambient (aCO2) and elevated (eCO2) CO2 plots at the heading stage of rice plants in 2014 and 2015, were incubated with the addition of 15NH4+ under strictly anaerobic conditions; experiment 2 (B), a field work with an anaerobic bioreactor conducted in the 2016 rice growing season; experiment 3 (C), two parallel SIP microcosms in which soils, taken from aCO2 and eCO2 plots in the 2017 rice growing season, were anaerobically incubated with the addition of 13C-labeled either 13CH313COO + NH4+ (SIP 1) or 13CO2 + NH4+ (SIP 2). (D) Influence of eCO2 on 15N-N2 production (total amount of 29N2 and 30N2; light blue bars) in experiments 1 (P < 0.05) and 2 (P < 0.01), and NH4+ consumption (dark blue bar) in experiment 3 (P < 0.05). The net CO2 effect was calculated: (value at eCO2 − value at aCO2)/(value at aCO2). Values are means (n = 3) ± SEM.

  • Fig. 2 AOA coupled to IR under elevated CO2.

    Elevated CO2 increased the production of 15N-N2 (total amount of 29N2 and 30N2) (A and D) and Fe2+ (B and E) following the addition of 15NH4+ + Fe(III) (A and B) or 15NH4+ + Fe(III) + C2H2 together (D and E) in experiment 1 (see fig. S4A for the experimental scheme). (A and D) AOA (NH4+ → N2); CO2 effect: P < 0.01; C2H2 effect: P < 0.05. (B and E) IR [Fe(III) → Fe2+]; CO2 effect: P < 0.05; C2H2 effect: P = 0.61. Open bars, ambient CO2 (aCO2); filled bars, elevated CO2 (eCO2). Data represent means (n = 3) ± SEM. (C and F) The stoichiometry of AOA-IR reactions and the overall molar ratio (R) of Fe2+ to 15N-N2, following the addition of either +15NH4+ + Fe(III) (C) or +15NH4+ + Fe(III) + C2H2 (F), were estimated using a general linear model.

  • Fig. 3 Microbial mediation of the coupling of AOA and IR.

    (A) Fe2+ production following the addition of Fe(III) or NH4+ + Fe(III) after 2-week incubation using sterilized or nonsterilized soils. (B) 13CO2 consumption following the addition of NH4+, Fe(III), or NH4++Fe(III) (experiment 3). Different letters above bars denote statistically significant differences (P < 0.05). (C) Nonmetric multidimensional analysis (NMDS) of community dissimilarities based on 16S rRNA gene profiling in experiment 3 (see fig. S4C for the experimental scheme). Squares (I and II), 12C-DNA; triangles (III and IV), 13C-DNA; green symbols (I and III), +NH4+; red symbols (II and IV), +NH4+ + Fe(III). (D) Images of live members of β-Proteobacteria using FISH during incubation (experiment 3). DAPI, 4′,6-diamidino-2-phenylindole (blue). The BET 42a probe (red) was used to target β-Proteobacteria. (E) Unclassified OTU dominated in both 12C- and 13C-labeled microbiota (experiment 3). Data are the relative abundance of taxa.

  • Fig. 4 A conceptual framework of microbial mediation of AOA coupled to IR in a rice ecosystem in response to long-term FACE.

    CO2 stimulation of root and microbial activities increases soil CO2 production, generating a novel niche for certain autotrophic anaerobes that take advantage of CO2 as their C source; meanwhile, they conserve energy by catalyzing AOA-IR. Left inset: Reactions 1 to 3 of AOA. Right inset: IR. Solid and dashed arrows represent positive and negative CO2 effects, respectively.

Supplementary Materials

  • Supplementary Materials

    Large losses of ammonium-nitrogen from a rice ecosystem under elevated CO2

    Chenchao Xu, Kaihang Zhang, Wanying Zhu, Jing Xiao, Chen Zhu, Naifang Zhang, Fangjian Yu, Shuyao Li, Chunwu Zhu, Qichao Tu, Xin Chen, Jianguo Zhu, Shuijin Hu, Roger T. Koide, Mary K. Firestone, Lei Cheng

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    • Supplementary Discussion
    • I. Ecosystem N cycling
    • II. AOA coupled to other soil oxidants
    • III. Direct CO2 effects on AOA-IR
    • IV. Ecosystem N balance under eCO2
    • Figs. S1 to S10
    • References

    Other Supplementary Material for this manuscript includes the following:

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