Trade-offs in soil carbon protection mechanisms under aerobic and anaerobic conditions.

Oxygen (O2 ) limitation is generally understood to suppress oil carbon (C) decomposition and is a key mechanism impacting terrestrial C stocks under global change. Yet, O2 limitation may differentially impact kinetic or thermodynamic versus physicochemical C protection mechanisms, challenging our understanding of how soil C may respond to climate-mediated changes in O2 dynamics. Although O2 limitation may suppress decomposition of new litter C inputs, release of physicochemically protected C due to iron (Fe) reduction could potentially sustain soil C losses. To test this trade-off, we incubated two disparate upland soils that experience periodic O2 limitation-a tropical rainforest Oxisol and a temperate cropland Mollisol-with added litter under either aerobic (control) or anaerobic conditions for 1 year. Anoxia suppressed total C loss by 27% in the Oxisol and by 41% in the Mollisol relative to the control, mainly due to the decrease in litter-C decomposition. However, anoxia sustained or even increased decomposition of native soil-C (11.0% vs. 12.4% in the control for the Oxisol and 12.5% vs. 5.3% in the control for the Mollisol, in terms of initial soil C mass), and it stimulated losses of metal- or mineral-associated C. Solid-state 13 C nuclear magnetic resonance spectroscopy demonstrated that anaerobic conditions decreased protein-derived C but increased lignin- and carbohydrate-C relative to the control. Our results indicate a trade-off between physicochemical and kinetic/thermodynamic C protection mechanisms under anaerobic conditions, whereby decreased decomposition of litter C was compensated by more extensive loss of mineral-associated soil C in both soils. This challenges the common assumption that anoxia inherently protects soil C and illustrates the vulnerability of mineral-associated C under anaerobic events characteristic of a warmer and wetter future climate.

How do Elevated CO2 and Nitrogen Addition Affect Functional Microbial Community Involved in Greenhouse Gas Flux in Salt Marsh System.

Salt marshes are unique ecosystem of which a microbial community is expected to be affected by global climate change. In this study, by using T-RFLP analysis, quantitative PCR, and pyrosequencing, we comprehensively analyzed the microbial community structure responding to elevated CO2 (eCO2) and N addition in a salt marsh ecosystem subjected to CO2 manipulation and N addition for about 3 years. We focused on the genes of microbes relevant to N-cycling (denitrification and nitrification), CH4-flux (methanogens and methanotrophs), and S-cycling (sulfate reduction) considering that they are key functional groups involved in the nutrient cycle of salt marsh system. Overall, this study suggests that (1) eCO2 and N addition affect functional microbial community involved in greenhouse gas flux in salt marsh system. Specifically, the denitrification process may be facilitated, while the methanogenesis may be impeded due to the outcompeting of sulfate reduction by eCO2 and N. This implies that future global change may cause a probable change in GHGs flux and positive feedback to global climate change in salt marsh; (2) the effect of eCO2 and N on functional group seems specific and to contrast with each other, but the effect of single factor would not be compromised but complemented by combination of two factors. (3) The response of functional groups to eCO2 and/or N may be directly or indirectly related to the plant community and its response to eCO2 and/or N. This study provides new insights into our understanding of functional microbial community responses to eCO2 and/or N addition in a C3/C4 plant mixed salt marsh system.