Mineralizable C and Nitrate Limitation On Nitrous Oxide from an Intensive Maize System: Interaction Between Cover Crop and Fertilizer Rate.

Use of cover crops has been proposed to reduce loss of nitrogen (N) from cereal production systems, including as nitrous oxide (N₂O). This effect on N₂O has been attributed to uptake of soil nitrate (NO₃) by the cover crops. However, previous studies have not found consistent effects of cover crops on N₂O emissions. It is necessary to examine the biochemical mechanisms by which cover crops affect N₂O emissions. In a field study in central Iowa, we observed a winter rye (Secale cereale L.) cover crop in maize (Zea mays L.) to decrease N₂O emissions only in unfertilized plots and to increase N₂O emissions at 135 kg N fertilizer ha^-1. The rye decreased soil NO₃ concentrations at all N fertilizer rates but did not affect soil moisture. We incubated soils from this site at constant temperature and moisture in the laboratory and measured N₂O production in the soils with and without addition of NO₃ and glucose. Nitrate addition increased N₂O production in unfertilized soils but had no effect in fertilized soils. Glucose addition increased N₂O production at all fertilizer rates. These results show that soil NO₃ concentration did not limit N₂O production except in unfertilized soils. In fertilized soils, N₂O production was limited by availability of mineralizable carbon (C). These results indicate that input of mineralizable C by cover crops may have increased N₂O emissions in fertilized soils at this site. The effect of cover crops on mineralizable C availability should be considered in evaluations of their effect on N₂O.