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a D. Martens (deceased), USDA-ARS, Southwest Watershed Research Ctr., 2000 E. Allen Rd., Tucson, AZ 85719
b USDA-ARS, Natl. Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011
* Corresponding author (jaynes{at}NSTL.gov)
Understanding soil organic N (ON) pool enrichment may help explain why rates of N fertilization required to attain maximum corn (Zea mays L.) yields are usually lower for corn following soybean [Glycine max (L.) Merr.] than for corn following corn. Our objectives were to quantify the ON pools within a 16-ha Iowa field and to correlate those results with corn yield. Spring and fall measurements of ON content (0–15 cm soil) as amino acids (AAs), amino sugars (ASs), and NH4+ were made using samples collected between 1997 and 1999 from 10 soil map units. The chemical extraction method determined an average 87% of the total N content (n = 10 soils) as identified ON but gave reduced ON recovery from depression soils that experienced periods of water ponding. The total AA concentrations measured in May were positively correlated (r2 = 0.84, P < 0.01) with corn yield during a dry year (1997) and 7 out of 10 soils provided near maximum yields. A wetter 1999 boosted overall corn yields 6.6% but resulted in a poorer relationship between May AA concentrations and corn yield. Microbial N compounds measured (May 1997) as glucosamine, galactosamine, and ornithine were also positively correlated with corn yield (r2 = 0.84, 
< 0.01; r2 = 0.94, P < 0.001; r2 = 0.93, P < 0.001, respectively). The ON concentration decreased during corn production from May to September 1997 an average of 367 kg N ha–1 but increased following soybean growth in 1998 by 320 kg N ha–1. The chemical extraction methodology identified soils that may not require the full amount of N fertilizer currently being applied, thus decreasing the potential for N loss to surface and ground water resources without decreasing opportunities to achieve optimum yield.
Abbreviations: AA, amino acid • AS, amino sugar • GalX, galactosamine • GluX, glucosamine • MSA, methanesulfonic acid • ON, organic N
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