LONG-TERM NITROGEN USE AND NITROGEN-REMOVAL INDEX IN CONTINUOUS CROPSAND ROTATIONS

Cropping systems, nitrogen (N) fertilizer levels, and climate largely dictate patterns of N use and influence problems arising from N fertil ization. Nitrogen use was assessed in cropping systems with a nitrogen removal-index (NRI), defined as the ratio of N removed in the grain t o total N supply including that from N fixation by legumes grown in ro tation. Results are reported from analyses of NRIs of cropping systems that comprised a 12-yr continuous and sequential growing of maize [Ze a mays L.], soybean [Glycine max. (L.) Merr.], sorghum [Sorghum bicolo r (L.) Moench], and oat/clover [Auena sativa (L.)/80% Melilotus offici nalis (L.) Lam., 20% Trifolium pratense] in eastern Nebraska. Rotation s involving maize or sorghum had higher NRIs than continuous cereals a t 0 N application levels. Increasing N rates reduced NRI and resulted in an increase of residual nitrate in all but the continuous soybean s ystem. Also, NRI was highest in continuous soybean, lower in continuou s maize, and lowest in continuous sorghum. Rotations and lower N rates both contributed to higher NRI and lower soil residual nitrate. Biolo gical windows that comprised the cumulative number of days in the enti re year when soil is moist and temperature above a specific threshold correlated positively and significantly with NRI, whereas MU and Augus t temperature were negatively related. Between 43 and 87% of variabili ty of NRI in maize and soybean systems was attributed to August temper ature plus August precipitation index. Biological window (moist soil, temperature above 5 degrees C) plus May temperature explained up to 76 % of variability of NRI of maize and soybean. Nitrogen removal index f or sorghum was unrelated to weather variables. Estimated additions to the soil organic N reserve from the return of crop residues averaged b etween 16 and 80 kg ha(-1) yr(-1) with higher levels from sorghum and from all treatments with high levels of N fertilizer. Crop rotations g enerally increased the N-removal index, reduced the year-to-year varia bility in N-removal-index, and at 0 N-application rate, increased the return of N in residue to the soil N pool, compared to continuous crop ping of single species. (C) 1998 Elsevier Science B.V.