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Nitrogen Fertilization and Cropping System Impacts on Soil Quality in Midwestern Mollisols

^a Dep. of Natural Resource Ecology & Management, Iowa State Univ., Ames, IA 50011
^b USDA-ARS, National Soil Tilth Lab., Ames, IA 50011
^c Dep. of Agronomy, Iowa State Univ., Ames, IA 50011

View larger version (37K): [in a new window]   Fig. 1. Available inorganic N (0–15 cm) at the Nashua, IA long-term experimental site. Means (± 1 SE) in 2001 varied by month in interaction with cropping system and N fertilization. Within each month and N treatment, significant differences among cropping systems are denoted by different letters (Tukey's test, P = 0.05). Within each month and cropping system, significant effects of N fertilization are denoted by an asterisk above the 180 kg N ha–1 bar (P < 0.05). All undesignated cropping system and fertilization effects are not significant.

View larger version (43K): [in a new window]   Fig. 2. Potential net N mineralization at the Nashua, IA long-term experimental site. Means (± 1 SE) in 2001 varied by month in interaction with cropping system and N fertilization. Within each month and N treatment, significant differences among cropping systems are denoted by different letters (Tukey's test, P = 0.05). Within each month and cropping system, differences between N fertilization treatments are denoted by an asterisk above the 180 kg N ha–1 bar (P < 0.05). All undesignated cropping system and fertilization effects are not significant.

View larger version (40K): [in a new window]   Fig. 3. Microbial biomass C at the Nashua, IA long-term experimental site. Means (± 1 SE, across N fertilization treatments) that differ among cropping systems are denoted by different letters (Tukey's test, P = 0.05). Soil moisture (•) and soil temperature () are means over all treatments.

View larger version (47K): [in a new window]   Fig. 4. Cation exchange capacity and exchangeable Ca and K at the Nashua and Kanawha, IA long-term experimental sites. Values are means (± 1 SE) across cropping systems. Differences among N treatments are denoted by different letters (Tukey's test, P = 0.05). ‘NP’ refers to the native prairie reference site, not included in the ANOVA.

View larger version (44K): [in a new window]   Fig. 5. Total soil N stocks (0–15 cm) at the Nashua and Kanawha, IA long-term experimental sites. Means (± 1 SE) that differ among cropping systems are denoted by different lowercase letters and among N treatments by uppercase letters (Tukey's test, P = 0.05).