Nitrogen Mineralization from Weed Residues
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.
Our objectives were to measure the total N and C content and rate of N mineralization of weeds grown under field conditions and varying N supplies.
Preliminary results are given in the summary paragraph. The first run of the experiment is completed and analyzed. The experiment was run again in 2011 and results are in the process of being analyzed.
Impacts and Contributions/Outcomes
These preliminary results have contributed to the understanding of nitrogen cycling in agro-ecosystems and will serve as a basis for future studies.
In 2011, preliminary results were discussed at the Michigan State University Extension Diagnostic Day at the Saginaw Valley Research Center and presented at the ASA-CSSA-SSSA annual meetings in San Antonio, TX.
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