Quantifying Nitrogen Mineralization From Weed Residue.

Understanding nutrient cycling in agro-ecosystems is essential for maximizing corn grain yield with minimal environmental impact.  Although weeds assimilate large quantities of nitrogen (N), little is understood about the fate of weed residues subsequent to postemergence weed control.  A laboratory incubation study was initiated to compare N mineralization of three weed species (common lambsquarters, common ragweed, and giant foxtail) at two plant heights (10 and 15 cm) grown under four N application rates (0, 67, 134, and 202 kg N ha^-1).  Total carbon (C) and N content of the weed residues was determined by the Dumas method.  Weed residues were placed in specimen cups containing 20 g dry weight equivalent field soil at a rate of 60 mg N kg^-1 soil.  Specimen cups were incubated at room temperature and soil nitrate-N (NO₃-N) and ammonium-N (NH₄-N) were destructively measured at 0, 1, 2, 4, 8, and 12 weeks after incubation.  Control (soil only) NO₃-N and NH₄-N were measured at each incubation time to correct for N mineralization from soil organic matter.  Nitrogen mineralization was considered to be the total inorganic N content of the soil after subtracting N mineralization from the control.  Nitrogen mineralization was analyzed using ANOVA in Proc Mixed and modeled over the twelve week period.  The C:N ratio of weed residues was 6 to 9 among all treatments.  Preliminary results indicate over half of weed residue N was released as inorganic N during the first 4 weeks of incubation.  From 4 to 12 weeks, there was little additional N released from weed residue.  When weeds were grown under the 0 kg N ha^-1 application rate, N release from giant foxtail was less than common lambsquarters and common ragweed.  There were no differences in N release or rate of release among any of the other treatments.  These results indicate that weed species and plant height did not influence N mineralization when weeds were grown under 67 to 202 kg N ha^-1.