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Nitrogen and Phosphorus Runoff Losses from Variable and Constant Intensity Rainfall Simulations on Loamy Sand under Conventional and Strip Tillage Systems

^a USDA-ARS, J. Phil Campbell, Sr. Natural Resource Conservation Center, Watkinsville, GA 30677
^b USDA-ARS, Southeast Watershed Research Lab., Tifton, GA 31793
^c Univ. of Georgia, Coastal Plain Experiment Station, Tifton, GA 31793

View larger version (17K): [in this window] [in a new window]   Fig. 1. Rainfall intensity patterns for constant and variable intensity treatments.

View larger version (22K): [in this window] [in a new window]   Fig. 2. Soil-extractable Mechlich I P (STP), water-soluble P (WSP), and NO3–N before rainfall simulations, with depth and between tillage treatments: strip tillage (ST) and conventional tillage (CT). Errors bars are standard deviations.

View larger version (43K): [in this window] [in a new window]   Fig. 3. (A) Cumulative flow-weighted concentrations of NH4–N for 70 min of runoff, and (B) cumulative NH4–N losses. The symbol at each 5-min period represents a composite sample for that time period and treatment. Gray shading indicates a significant difference (p < 0.05) between tillage treatments strip tillage (ST) and conventional tillage (CT). Black shading indicates a significant difference (p < 0.05) between rainfall intensity patterns, constant rate (Ic) and variable rate (Iv). The symbol ¤ indicates interaction was significant (p < 0.05) between tillage and intensity pattern for runoff periods (5 min) above which are labeled. In graph A, the inserted graph depicts runoff rates for each treatment studied.

View larger version (19K): [in this window] [in a new window]   Fig. 4. Soil KCl-extractable NH4–N with depth. Errors bars are standard deviations. (A) Soil concentrations determined from soil samples taken immediately before rain; (B) soil concentrations determined from soil samples taken 1 h after end of simulation.

View larger version (59K): [in this window] [in a new window]   Fig. 5. (A) Cumulative flow-weighted concentrations of NO3–N for 70 min of runoff, and (B) cumulative NO3–N losses. Gray shading indicates a significant difference (p < 0.05) between tillage treatments, strip tillage (ST) and conventional tillage (CT). Black shading indicates a significant difference (p < 0.05) between rainfall intensity patterns, constant rate (Ic) and variable rate (Iv). The symbol ¤ indicates interaction was significant (p < 0.05) between tillage and intensity pattern for runoff periods above which are labeled.

View larger version (60K): [in this window] [in a new window]   Fig. 6. (A) Cumulative flow-weighted concentrations of total N (TKN) for 70 min of runoff, and (B) cumulative total N (TKN) losses. Gray shading indicates a significant difference (p < 0.05) between tillage treatments strip tillage (ST) and conventional tillage (CT). Black shading indicates a significant difference (p < 0.05) between rainfall intensity patterns, constant rate (Ic) and variable rate (Iv). The symbol ¤ indicates interaction was significant (p < 0.05) between tillage and intensity pattern for runoff periods (5 min) above which are labeled.

View larger version (74K): [in this window] [in a new window]   Fig. 7. (A) Cumulative flow-weighted concentrations of PO4–P for 70 min of runoff, and (B) cumulative PO4–P losses. Gray shading indicates a significant difference (p < 0.05) between tillage treatments, strip tillage (ST) and conventional tillage (CT). Black shading indicates a significant difference (p < 0.05) between rainfall intensity patterns, constant rate (Ic) and variable rate (Iv). The symbol ¤ indicates interaction was significant (p < 0.05) between tillage and intensity pattern for runoff periods above which are labeled.

View larger version (82K): [in this window] [in a new window]   Fig. 8. (A) Cumulative flow-weighted concentrations of total P (TKP) for 70 min of runoff, and (B) total P losses. Gray shading indicates a significant difference (p < 0.05) between tillage treatments, strip tillage (ST) and conventional tillage (CT). Black shading indicates a significant difference (p < 0.05) between rainfall intensity patterns, constant rate (Ic) and variable rate (Iv). The symbol ¤ indicates interaction was significant (p < 0.05) between tillage and intensity pattern for runoff periods above which are labeled.