|Allen, H. lee|
Submitted to: Global Change Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/9/2008
Publication Date: 1/5/2009
Publication URL: http://hdl.handle.net/10113/36124
Citation: Lui, L., King, J.S., Booker, F.L., Giardina, C., Allen, H., Hu, S. 2009. Enhanced litter input rather than changes in litter chemistry drive soil carbon and nitrogen cycles under elevated CO2: a microcosm study. Global Change Biology. 15:441-453. Interpretive Summary: It is uncertain whether increased primary productivity under elevated atmospheric CO2 will lead to increased long-term carbon storage in soils and offset rising atmospheric CO2 concentrations in the future. To examine how changes in litter biochemistry and productivity under elevated CO2 influence microbial activity and soil carbon and nitrogen dynamics, we conducted a 230-day microcosm incubation study with five levels of litter mass additions. Litter and soil were collected from aspen stands growing under control and elevated CO2 treatments at the AspenFACE experiment in Rhinelander, WI. Throughout the incubation, we measured soil microbial respiration and biomass, dissolved carbon and nitrogen, delta13C, %C and %N in the microcosm soil. We found that small decreases in litter nitrogen under elevated CO2 had minor impacts on microbial biomass and dissolved nitrogen concentrations. Increasing mass addition rates resulted in higher carbon accumulation in the microcosm soils, despite higher cumulative carbon loss by respiration. Total nitrogen retained in the soil also increased with litter mass addition. We concluded that enhanced carbon input is the dominant force driving soil carbon formation and turnover under elevated CO2. Increased litter inputs resulted in higher recalcitrant and labile carbon fluxes to the soil, and both made significant contributions to soil C formation. However, our analysis suggests the effects of changes in litter biochemistry caused by elevated CO2 could become significant at a higher level of litter production rate.
Technical Abstract: Elevated CO2 has been shown to stimulate plant productivity and change litter chemistry. These changes in substrate availability may then alter soil microbial processes and possibly lead to feedback effects on N availability. However, the strength of this feed back, and even its direction, remain unknown. Further, uncertainty remains whether sustained increases in net primary productivity will lead to increased long-term C storage in soils. To examine how changes in litter chemistry and productivity under elevated CO2 influence microbial activity and soil C formation, we conducted a 230-day microcosm incubation with five levels of litter addition rate that represented 0, 0.5, 1.0, 1.4 and 1.8 × litterfall rates observed in the field for aspen stand growing under control treatments at the Aspen FACE experiment in Rhinelander, WI. Litter and soil samples were collected from the aspen community under control and elevated CO2 treatments. We found that small decreases in litter [N] under elevated CO2 had minor effects on microbial biomass carbon (MBC), microbial biomass nitrogen (MBN) and dissolved inorganic nitrogen (DIN). Increasing litter addition rates resulted in linear increases in total C and new C (C from added litter) that accumulated in whole soil as well as in the high density soil fraction (HDF), despite higher cumulative C loss by respiration. Total N retained in whole soil and in HDF also increased with litter addition rate as did accumulation of new C per unit of accumulated N. Based on our microcosm comparisons and regression models, we expected that enhanced C inputs rather than changes in litter chemistry would be the dominant factor controlling soil C levels and turnover at the current level of litter production rate (230 g C•m-2• yr-1 under ambient CO2). However, our analysis also suggests that the effects of changes in biochemistry caused by elevated CO2 could become significant at a higher level of litter production rate, with a trend of decreasing total C in HDF, new C in whole soil, as well as total N in whole soil and HDF.