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Forest Litter Production and Soil Carbon Sequestration
Liu, L., King, J.S., Booker, F.L., Giardina, C.P., Allen, H.L. and 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.

The authors write that "elevated CO2 has been shown to stimulate plant productivity and change litter chemistry," and that "these changes in substrate availability may then alter soil microbial processes and possibly lead to feedback effects on nitrogen availability." However, they state that "the strength of this feedback, and even its direction, remains unknown," and that "uncertainty remains whether sustained increases in net primary productivity [such as typically occur in response to increases in the air's CO2 content] will lead to increased long-term carbon storage in soil."

What was done
To explore this situation and "examine how changes in litter chemistry and productivity under elevated CO2 influence microbial activity and soil carbon formation," Liu et al. conducted a 230-day microcosm incubation experiment with five levels of litter addition rate that represented 0, 0.5, 1.0, 1.4 and 1.8 times the litter-fall rates observed in aspen stands that had been growing for seven years under ambient and elevated (560 ppm) CO2 concentrations at the Aspen FACE facility in Rhinelander, Wisconsin, USA.

What was learned
The six scientists found that small CO2-induced decreases in litter nitrogen concentration had only "minor effects on microbial biomass carbon, microbial biomass nitrogen and dissolved inorganic nitrogen," but they found that "increasing litter addition rates resulted in linear increases in total carbon and new carbon (carbon from added litter) that accumulated in whole soil as well as in the high density soil fraction (HDF)." In addition, they found that "total nitrogen retained in whole soil and in HDF also increased with litter addition rate as did accumulation of new carbon per unit of accumulated nitrogen."

What it means
Liu et al. say their results suggest that "changes in litter inputs under elevated CO2 [slight decreases in nitrogen concentration but sizable increases in biomass] should lead to higher long-term carbon storage in soil," even "despite higher rates of soil respiration."

Reviewed 29 April 2009