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 (δ 13 C) of soil CO 2 in a corn ( Zea mays L.) crop that was grown on a soil developed from C 3 plant material. The R rh was defined as the sum of CO 2 respired by plant roots and CO 2 respired by microbes that feed on organic material produced by the roots. The δ 13 C of soil CO 2 in the corn plot changed from an early season low of approximately −20‰, to mid‐season values of approximately −14‰, before declining again at the end of the growing season. In the control plot (no C 4 plants present) δ 13 C values of soil CO 2 were significantly lower than values in the corn plot, averaging approximately −22‰ throughout most of the season. We observed no significant change in the δ 13 C value of soil CO 2 in either the corn or control plots during a diurnal sampling period. The value of R rh was 1.71 g CO 2 m −2 d −1 27 d after planting (DAP), reached 10 g CO 2 m −2 d −1 at 44 DAP, remained at about that value until 76 DAP, and gradually decreased to 2.6 g CO 2 m −2 d −1 at 141 DAP. The CO 2 respired by the rhizosphere was equivalent to 18 to 25% of crop net photosynthesis and 24 to 35% of crop net CO 2 assimilation during most of the growing season.