NITROGEN EFFECTS ON CONSERVATION OF CARBON DURING CORN RESIDUE DECOMPOSITION IN SOIL

Nitrogen fertilization is widely recognized for its potential to help maintain soil organic matter (SOM) concentrations by increasing plant growth and amounts of plant material decomposed in soils. We studied possible mechanisms by which annual additions of more N than needed to maximize yields of corn (Zea mays L.) could cause losses of SOM. Various amounts of stover and NO3- were added to a Galva silty clay loam (fine-silty, mixed, mesic Typic Hapludoll), and NO3- concentrations and CO2 evolution were monitored for periods up to 90 d at 24-degrees-C in the laboratory. The added NO3- suppressed mineralization of C from SOM and stimulated mineralization of C from stover. Adjustment of rates of stover decomposition to temperature regimes normally encountered in Iowa fields after harvest showed that stover decomposition would not be complete within 1 yr and that increases in NO3- availability decreased the amount of stover C remaining in the soil. This effect of NO3- could explain how additions of unneeded N could decrease concentrations of SOM in long-term studies in which residues and NO3- are added in annual cycles. These observations suggest that practices that reduce unnecessary fertilization could help conserve SOM and reduce net amounts of CO2 released to the atmosphere.