Soil Organic Nitrogen Enrichment Following Soybean in an Iowa Corn–Soybean Rotation

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 5 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 (r 2 5 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 betweenMay AA concentrations and corn yield. Microbial N compounds measured (May 1997) as glucosamine, galactosamine, and ornithine were also positively correlated with corn yield (r 2 5 0.84, r , 0.01; r 2 5 0.94, P , 0.001; r 2 5 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 but increased following soybean growth in 1998 by 320 kg N ha. 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. PUBLIC CONCERN that excessive N fertilization may contribute to NO3 enrichment of ground and surface water has stimulated interest in developing analytical methods that can improve our understanding of N cycling and the accuracy of N fertilizer recommendations for corn (Mulvaney et al., 2001). Estimating plant-available soil N not derived from current fertilizer applications is very difficult because of the complicated nature of the soil N cycle. For example, mineralization of indigenous soil N has been shown to increase with application of N fertilizers (Broadbent, 1965; Kissel et al., 1977; Hills et al., 1978; Martens and Dick, 2003). Power et al. (1986) using labeled fertilizer and plant residues found that over 80% of the N assimilated by corn during a growing season originated from indigenous soil N. Unfortunately, identification and measurement of a soil N fraction that could provide sufficient N to support plant growth has been difficult because this fraction clearly responds to seasonal temperature and moisture variations. Predicting the formation of plant-available N is complicated by the opposing processes of mineralization and immobilization and their interactions with seasonal temperature and precipitation (Jansson and Persson, 1982). Numerous attempts have been made to identify a labile pool of soil ON through chemical fractionation of the soil N hydrolysates (Keeney and Bremner, 1964; Porter et al., 1964; Kahn, 1971; Meints and Peterson, 1977), but with little tangible success. Recently, Khan et al. (2001) reported that determination of soil ASs released by chemical hydrolysis provided a means to separate soils that respond and do not respond to N fertilizer during corn production. Currently, preplant and presidedress N tests are recommended by Minnesota and Iowa, respectively, to provide an estimate of plant-available N and to guide fertilizer recommendations needed to optimize corn grain yield, but inherent limitations with those tests have resulted in frequent failures to predict actual N needs (Mulvaney et al., 2001). Soybean in Iowa can be given a credit of up to 56 kg N ha for a subsequent corn crop (Blackmer et al., 1997). This credit was based on N rate tests that showed optimum N fertilizer rates were less for corn following soybean than for corn following corn and postulated to be due to N fixation by soybean or a change in mineralization patterns (Green and Blackmer, 1995). However, the N credit theory has been questioned because estimates of N fixation and subsequent removal with the grain suggest that soybean actually results in a net removal of N from many soils (Heichel and Barnes, 1984). Recent research by Mayer et al. (2003) has found that legume growth resulted in a large influx of N to the soil in the form of root exudates. Use of N isotopes showed that at legume maturity, 15 to 21% of belowground N was identified in microbial biomass or as identifiable root fragments, while the remaining 79 to 85% of belowground N was not identified. The N forms in the soybean exudates were not evaluated but would be expected to be composed mainly of AAs, the main form of N found in mature soybean tissue (Martens and Loeffelmann, 2003). Accurate estimates of the amount of soybean N returned to the soil for utilization by the following corn crop would greatly improve the prediction of N fertilizer needed for optimum yields. Recently, improved methodology for analysis of soil ON composition has shown that up to 92% of the total N present in well-drained soils can be identified as AAs, ASs, and NH4 originating from fixed soil NH4 and AA extraction (Martens and Loeffelmann, 2003). Our D. Martens (deceased), USDA-ARS, Southwest Watershed Research Ctr., 2000 E. Allen Rd., Tucson, AZ 85719; D. Jaynes, T. Colvin, T. Kaspar, and D. Karlen, USDA-ARS, Natl. Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011. Received 6 Apr. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:382–392 (2006). Soil Fertility & Plant Nutrition doi:10.2136/sssaj2005.0112 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: AA, amino acid; AS, amino sugar; GalX, galactosamine; GluX, glucosamine; MSA, methanesulfonic acid; ON, organic N. R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 382 Published online February 2, 2006