Soil Carbon Changes: Comparing Flux Monitoring and Mass Balance in a Box Lysimeter Experiment

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 COz flows in lab studies, questions remain for extrapolating I R G A 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 COs fluxes. Infra-red gas analyzer measurements, applied biweekly, were then compared to mass baiance--based measures of changes in soil C over 8 mo. The COz fluxes measured with I R G A 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 COs 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. I~ SPIRATION OF CO2 f rom soil surfaces is one of the most important ecosystem processes. Soil respiration nearly balances net photosynthetic uptake of atmospheric CO2 by plants, when stores of soil C are constant. With changing global climates, however, global stores of soil C a b o u t two to three times greater than C in atmospheric CO2--may not remain constant. For example, warmer temperatures may promote faster decomposition, and higher atmospheric CO2 may increase photosynthesis, in turn increasing detritus production. Thus, measures of soil respiration may play an important part in unraveling how the global ecosystem will respond to changing atmospheric CO2. Measures of soil respiration are important in many other ways as well, such as in assessing belowground biological activity; calculating C budgets and net primary production; and increasing understanding of the effects of soil disturbance, fertilization amendments , and contamination by pollutants. USDA Forest Service, PNW Research Station.. Corvallis, OR 97331. Received 17 May 1999. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 64:943-948 (2000). Studies use different methods to measure soil respiration. Reviews of the different methodologies can be found in Schlesinger (1977), Anderson (1982), Rolston (1986), Raich and Nadelhoffer (1989), and Nakayama (1990). The accuracy of methods has long been debated in the literature. Numerous field studies have compared methodologies (de Jong et al., 1979; Edwards, 1982; Cropper et al., 1985; Freijer and Bouten, 1991; Norman et al., 1992; Rochet te et al., 1992; Nakadai et al., 1993; Jensen et al., 1996; Norman et al., 1997; Rochet te et al., 1997); however, these types of studies are difficult to evaluate because a reference to compare responses is lacking (Nakayama, 1990, and Nay et al., 1994). The accuracy and precision of instantaneous flux methods relative to a known reference, under field conditions, remains untested. Establishing a reference in the field is difficult or impossible because of the difficulty in detecting soil C changes that can be attributed to respira t ion--because of inherent soil variability. The presence of plants further complicates C accounting. To test an I R G A b a s e d flux method under quasifield conditions, we established field lysimeters without plants and with homogenized soils amended with manure that permit ted us to calculate a reference C loss by mass balance. Instantaneous methods were used to monitor respiration rates and calculate long-term fluxes for comparison to mass balances. M A T E R I A L S A N D M E T H O D S B o x L y s i m e t e r s To compare IRGA-based flux methods against a mass balance reference, we constructed 10 box lysimeters with homogenized soils of varying C contents (Fig. 1). The study site was located in Corvallis, Oregon. The 152by 152by 70-cm box lysimeters were made of plywood, lined with 0.15-mm-thick polyethylene and constrained between concrete barriers. Each box included a 10-cm-diam. drain plumbed to a catchment area. All boxes had a 10-cm layer of coarse river rock overlaid with a 10-cm layer of coarse sand. Above the coarse sand was 50 cm of soil amended with manure. Box lysimeter soils were created by combining a sandy loam-loamy sand (USDA textural classification) and dewaAbbreviations: IRGA, infra-red gas analyzer: LOI, loss on ignition; TOC, total organic carbon. 944 SOIL SCI. SOC. AM. J., VOL. 64, MAY-JUNE 2000