Soda Lime Method

From 1991 through 1994, soil respiration at the Harvard Forest DIRT experiment was measured with the soda lime technique (Edwards et al. 1982, Raich et al. 1990). Carbon dioxide emitted from soil was absorbed in 60 g of 105 °C-oven-dry soda lime contained in an open tin on the soil surface, underneath a white plastic bucket approximately 20 cm tall and 28 cm in diameter. The bucket's upper surface was covered with aluminum foil to minimize solar heating. Rocks (5 kg) placed on top of each bucket held it firmly against the forest floor. Buckets were seated no more than 1 cm deep to avoid severing live roots. After 24 hours, the soda lime tins were removed, oven-dried (105°C), and reweighed. CO₂ efflux was measured simultaneously in all 21 plots on each collection date. The same location within each plot was sampled on each collection date. The weight gain of the soda lime during the 24 hours of CO₂ absorption was used to calculate the CO₂ efflux from the soil surface.

IRGA Method

In 1994, soil respiration data was collected using the soda lime technique and with an IRGA (infrared gas analyzer). Comparison of 1994 soda lime and IRGA results showed that the soda lime technique underestimated high soil CO₂ efflux and overestimated low soil CO₂ efflux compared to IRGA data. All soil respiration measurements after 1994 have been made with IRGA only.

The IRGA system consists of a LI-CORâ model 6262 IRGA connected to a closed chamber, approximately 20 cm tall and 30 cm in diameter, placed over the soil surface. A pump circulates air between the soil surface and the IRGA. CO₂ concentrations in the chamber air are recorded for four minutes at each plot at dawn and midday to capture the times of highest and lowest soil CO₂ efflux. The morning and afternoon fluxes are averaged to obtain the daily flux. On several dates in 1995, 1998, and 2000 and 2001, only the a.m. or p.m. data were obtained; in these cases, the average daily flux given is equal to the data obtained.

Description of data:

Soil CO₂ efflux was measured by soda lime in 1991, 1992, and 1993. Soda lime data are corrected for the IRGA/soda lime relationship. Soil CO₂ efflux data given for 1994 and on were measured by IRGA.
YEAR = year flux data collected
DOY = Day of year
CH = chamber number; this replaces plot number in 1991 data
PLOT = plot number
TRTMT = treatment
Avg = Average daily soil CO₂ efflux by treatment, expressed as mg CO₂-C/m²/hr.