Effects of soil temperature and moisture on soil respiration in barley andfallow plots

Agricultural systems are sources and sinks for carbon and to quantify the n et effect of these systems on atmospheric CO2 concentration, the amounts of carbon fixed in primary production and that respired by the soil must be k nown. The objectives of our study were (1) to quantify the amount of soil r espiration from fallow and barley plots during the growing season; and (2) to determine the relationship between these fluxes and soil temperature and moisture. This study was conducted on field plots measuring 200 by 200 m w ith one plot planted to barley (Hordeum vulgare L.) while the other plot wa s in fallow. Two automated chambers were permanently installed in the fallo w plot and three in the barley plot at the start of the growing season. Whe n CO2 fluxes were integrated over a 24-h period, the daily soil respiration under fallow ranged from a low of 1.6 g CO2 m(-2) d(-1) on a dry day to a high of 8.3 g CO2 m(-2) d(-1) on a wet day. The corresponding values for ba rley were 3.3 and 18.5 g CO2 m(-2) d(-1) in 1994. Similar values were obtai ned in 1996 and, on average, daily soil respiration under barley was twice of that under fallow. The integrated daily CO2 flux under fallow was strong ly related to daily soil moisture and mean soil temperature with moisture a lone accounting for 76 to 80% of the variation in CO2 flux. While good rela tionships were obtained between soil moisture and CO2 flux under fallow, th e relationship under barley was not as good. The CO2 fluxes, measured eight times per day, displayed a diurnal pattern similar to that of soil tempera ture; however, there was no consistent quantitative relationship between th ese 3-hourly fluxes and temperature. A poor relationship was obtained when the fluxes during several days were related to soil temperature as soil moi sture confounded flux-temperature relationship. Under the semi-arid conditi ons of southern Alberta, moisture is the main parameter controlling soil re spiration during the growing season.