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Fate of Nitrogen-15 in a Long-Term Nitrogen Rate Study

I. Interactions with Soil Nitrogen

^a Northern Plains Agric. Res. Lab., 1500 N. Central Ave., Sidney, MT 59270
^b Dep. of Crop Sci., 1102 S. Goodwin Ave., Univ. of Illinois, Urbana, IL 61801
^c Dep. of Nat. Resour. and Environ. Sci., 1102 S. Goodwin Ave., Univ. of Illinois, Urbana, IL 61801

View larger version (24K): [in a new window]   Fig. 1. Location of 15N microplots and harvest area within main plots of the long-term N rate study at Monmouth, IL.

View larger version (23K): [in a new window]   Fig. 2. Distribution of total mineral N (NH4 + NO3 + NO2) by soil depth before spring N application. Each data point represents a mean mineral N concentration for the corresponding soil depth increment (0 to 15, 15 to 30, 30 to 60, 60 to 90, or 90 to 120 cm). Error bars show the LSD (P < 0.05) for mean comparisons within a depth increment. NS indicates differences were not significant.

View larger version (43K): [in a new window]   Fig. 3. (a) Monthly average precipitation (crosshatched bars) and (b) monthly average air temperature (solid bars) recorded at the University of Illinois Northwest Research Center during the year preceding (1993) and 3 yr during which the experiment was conducted (1994–1996). The solid lines show the 100-yr average (a) precipitation and (b) air temperature for Monmouth, IL

View larger version (19K): [in a new window]   Fig. 4. Distribution of fertilizer-derived mineral N (NH4 + NO3 + NO2) by soil depth following crop harvest. Each data point represents a mean mineral N concentration for the corresponding soil depth increment (0 to 15, 15 to 30, 30 to 60, 60 to 90, or 90 to 120 cm). Error bars show the LSD (P < 0.05) for mean comparisons within a depth increment. NS indicates differences were not significant.

View larger version (23K): [in a new window]   Fig. 5. Distribution of total mineral N (NH4 + NO3 + NO2) by soil depth following crop harvest. Each data point represents a mean mineral N concentration for the corresponding soil depth increment (0 to 15, 15 to 30, 30 to 60, 60 to 90, or 90 to 120 cm). Error bars show the LSD (P < 0.05) for mean comparisons within a depth increment. NS indicates differences were not significant.

View larger version (35K): [in a new window]   Fig. 6. Recovery of labeled fertilizer N from mineral and Kjeldahl N minus inorganic (KMI) N fractions of a 1.2-m soil profile following a 1994 application of 15NH415NO3. Lowercase and uppercase letters are for comparing (P < 0.05) KMI-N and mineral N means, respectively, within a sampling event. Least significant differences (P < 0.05) for comparing mineral N and KMI-N means within a N treatment and across sampling dates are 4 and 13 kg ha–1, respectively. Parenthetical values are standard deviations for their corresponding means.

View larger version (35K): [in a new window]   Fig. 7. Recovery of labeled fertilizer N from mineral and Kjeldahl N minus inorganic (KMI) N fractions of a 1.2-m soil profile following a 1995 application of 15NH415NO3. Lowercase and uppercase letters are for comparing (P < 0.05) KMI-N and mineral N means, respectively, within a sampling event. Least significant differences (P < 0.05) for comparing mineral N and KMI-N means within a N treatment and across sampling dates are 6 and 14 kg ha–1, respectively. Parenthetical values are standard deviations for their corresponding means.