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DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Soil Carbon Changes

Comparing Flux Monitoring and Mass Balance in a Box Lysimeter Experiment

USDA Forest Service, PNW Research Station., Corvallis, OR 97331 USA

mark.nay{at}orst.edu

Direct measures of soil-surface respiration are needed to evaluate belowground biological processes, forest productivity, and ecosystem responses to global change. Although infra-red gas analyzer (IRGA) methods track reference CO₂ flows in lab studies, questions remain for extrapolating IRGA methods to field conditions. We constructed 10 box lysimeters with homogenized mixtures of sandy loam and cattle manure and kept them free of plants to create a range of CO₂ fluxes. Infra-red gas analyzer measurements, applied biweekly, were then compared to mass balance–based measures of changes in soil C over 8 mo. The CO₂ fluxes measured with IRGA were not significantly different (P < 0.05) from the mass balance measure in 9 of the 10 boxes. The only statistically significant difference was in the lysimeter with the highest initial C content; this box had elevated soil temperatures early in the trial, suggesting a composting effect that may have interfered with IRGA measures. Variations in the mass balance estimates were higher than expected, demonstrating how difficult establishing a true reference in field studies is. We conclude that fluxes of CO₂ from soils can be monitored with an IRGA-based chamber system in the field to produce reliable estimates of cumulative C loss. Such field measures will likely be much more variable than laboratory measures, however, and thus will require extensive sampling.

Abbreviations: IRGA, infra-red gas analyzer • LOI, loss on ignition • TOC, total organic carbon