Carbon sink reduction by fruit removal triggers respiration but not nitrous oxide emissions from the root zone of cucumber

Abstract

AbstractRoot exudation of organic carbon (C) is generally believed to be the cause of positive effects of root activity on nitrous oxide (N2O) emissions. We tested the effects of root exudation in an actual soil–plant system on N2O emissions while excluding most other potential factors. The C source/sink ratio in cucumber was changed by removing fruits to increase root exudation. Root‐zone emissions of carbon dioxide (CO2) and N2O were monitored in complete stands of adult plants in a greenhouse. Whereas CO2 emissions rapidly increased as a result of fruit removal the N2O emissions were completely unaffected. After cutting the shoots CO2 emissions decreased within 2 weeks in both the fruit removal treatment and the control to a value significantly lower than that before the start of the treatments. However, N2O emissions immediately exhibited a short peak, which was significantly higher in the fruit removal treatment compared to the control. Thereafter N2O emissions in both treatments remained on the same level but considerably higher than before shoot cutting. We concluded that in a well‐aerated root zone, a root exudation pulse does not necessarily increase N2O emissions, because C substrates are quickly respired by microorganisms before they can support heterotrophic denitrification. The results further indicate the significance of dying/dead roots for the creation of denitrificaton hot‐spots, which likely result from providing C substrates as well as poorly aerated habitats.