QUANTIFYING RHIZOSPHERE RESPIRATION IN A CORN CROP UNDER FIELD CONDITIONS

In order to quantify rhizosphere respiration (R-rh) during the growing season, we monitored the isotope ratio (delta(13)C) of soil CO2 in a corn (Zea mays L.) crop that was grown on a soil developed from C-3 pl ant material. The R-rh was defined as the sum of CO2 respired by plant roots and CO2 respired by microbes that feed on organic material prod uced by the roots. The delta(13)C of soil CO2 in the corn plot changed from an early season low of approximately -20 parts per thousand, to mid-season values of approximately -14 parts per thousand, before decl ining again at the end of the growing season. In the control plot (no C-4 plants present) delta(13)C values of soil CO2 were significantly l ower than values in the corn plot, averaging approximately -22 parts p er thousand throughout most of the season. We observed no significant change in the delta(13)C value of soil CO2 in either the corn or contr ol plots during a diurnal sampling period. The value of R-rh was 1.71 g CO2 m(-2) d(-1) 27 d after planting (DAP), reached 10 g CO2 m(-2) d( -1) at 44 DAP, remained at about that value until 76 DAP, and graduall y decreased to 2.6 g CO2 m(-2) d(-1) at 141 DAP. The CO2 respired by t he rhizosphere was equivalent to 18 to 25% of crop net photosynthesis and 24 to 35% of crop net CO2 assimilation during most of the growing season.