Root-derived respiration and nitrous oxide production as affected by crop phenology and nitrogen fertilization

In annual crops, the partitioning of photosynthates to support root growth, respiration and rhizodeposition should be greater during early development than in later reproductive stages due to source/sink relationships in the plant. Therefore, seasonal fluctuations in carbon dioxide (CO2) and nitrous oxide (N2O) production from roots and root-associated soil may be related to resource partitioning by the crop. Greenhouse studies used C-13 and N-15 stable isotopes to evaluate the carbon (C) partitioning and nitrogen (N) uptake by corn and soybean. We also measured the CO2 and N2O production from planted pots as affected by crop phenology and N fertilization. Specific root-derived respiration was related to the C-13 allocated to roots and was greatest during early vegetative growth. Root-derived respiration and rhizodeposition were greater for corn than soybean. The N-15 uptake by corn increased between vegetative growth, tasseling and milk stages, but the N-15 content in soybean was not affected by phenology. A peak in N2O production was observed with corn at the milk stage, suggesting that the corn rhizosphere supported microbial communities that produced N2O. Most of the N-15-NO3 applied to soybean was not taken up by the plant and negative N2O production during vegetative growth and floral initiation stages suggests that soybean roots supported the reduction of N2O to dinitrogen (N-2). We conclude that crop phenology and soil N availability exert important controls on rhizosphere processes, leading to temporal variation in CO2 and N2O production.