NITROGEN AND PHOSPHORUS IN SURFACE RUNOFF AND SEDIMENT FROM A WHEAT-PEA ROTATION IN NORTHEASTERN OREGON

Runoff and erosion from cultivated fields are suggested as major sourc es of nitrogen (N) and phosphorus (P) entering lakes and streams. Loss of these nutrients can be impacted by soil and crop management practi ces. There is relatively little information on nutrients leaving agric ultural fields in the dryland areas of the Pacific Northwest, especial ly as impacted by different crops and crop rotations. Nutrients in sur face runoff and soil leaving a series of plots in northeast Oregon wer e evaluated for 5 yr. Objectives were to evaluate the effects of a whe at-pea (Triticum aestivium L.-Pisum sativum L.) rotation on N and P in surface runoff and eroded soil. Cumulative N and P lost in runoff wer e less than 3 and 0.2 kg ha(-1), respectively. Total N lost from conti nuous fallow (CP) plots, even though they were never fertilized, range d from 20 to 100 kg ha(-1) yr(-1). This was much more N than was lost from winter wheat (WW) (5-70 kg ha(-1) yr(-1)) and spring pea (SP) (25 -40 kg ha(-1) yr(-1)), because there was much more sediment loss from CF than from WW or SP plots. The same scenario occurred with P as with N, with loss from CF of 8 to 48 kg ha(-1) yr(-1), WW of 2 to 28 kg ha (-1) yr(-1), and from SP of 8 to 16 kg ha(-1) yr(-1). All fertilizer w as surface-applied and 25 to 76% of the fertilizer applied was lost in 5 yr, depending on the time of fertilizer application and the type of crop growing. Nutrient loss from these fields can be controlled by co ntrolling soil erosion.