QUANTIFYING TOTAL, N, AND NON-N RELATED CROP-ROTATION EFFECTS WITHOUTN-15

The N benefits attributed to legumes in crop rotations are often overe stimated because the technique for quantifying this benefit without N- 15 includes rotation effects not directly related to N. This study was conducted to test an approach proposed in the literature for delineat ing such effects without requiring the use of N-15. Potato (Solanum tu berosum L, cv. Norwis) was grown continuously and in two-year rotation s with alfalfa (Medicago saliva L. cv. Nitro), hairy vetch (Vicia vill osa Roth), or white lupin (Lupinus albus L, cv. Ultra). Potato was fer tilized with 0, 90, or 180 kg N ha(-1) as (NH4)(2)SO4. The total, N, a nd non-N effects of crop rotation were calculated based on tuber dry w eight and plant N uptake for the rotation systems and the tuber dry we ight vs. plant N uptake relationship for continuous potato. The total effect of crop rotation generally declined upon increasing N fertilize r from 0 to 90 kg N ha(-1), apparently due to decreasing importance of the N effect with added fertilizer. Non-N related effects mostly rema ined static across fertilization regimes. Those effects attributed to N generally declined upon adding 90 kg N ha(-1) in a relatively dry ye ar; and upon adding 90 and 180 kg N ha(-1) in a year with more favorab le precipitation. Calculated N effects with no N fertilizer reflected the quantity and quality of legume crop residues. This method does all ow assessment of total, N, and non-N related rotation effects. However , the approach appears sensitive to climatic conditions, and data shou ld only be subjected to analysis where a yield response to N exists.