NITROGEN TRANSFORMATIONS, UTILIZATION, AND CONSERVATION AS AFFECTED BY FALLOW TILLAGE METHOD

While we know that fallow tillage method affects water conservation an d soil erosion control, effects on soil and fertilizer N dynamics are less well known. In this paper, we summarize results from our investig ations of N transformation and cycling for two experiments established in 1969 and 1970 near Sidney, NE. In this research, the effects of th ree fallow tillage methods (moldboard plow, sub-till, and no-till) on changes in soil properties, N dynamics, and subsequent crop growth wer e studied. One experiment was on land, that had been broken from sod b efore 1920, seeded to crested wheatgrass [Agropyron cristatum (L.) Gae rtn.] in 1957, and cultivated again since 1967 with the above three fa llow tillage methods. Wheat (Triticum aestivum) was grown in a wheat-f allow sequence with and without 45 kg fertilizer N ha(-1). The second experiment was on land that was in native mixed prairie sod until 1969 , and included a comparison of the three above tillage methods in a wh eat-fallow system with plots remaining in native sod, During the first 13 years of study, total soil N loss from the surface 30 cm of soil ( compared to the native sod) was only about 3% for the no-till system, compared to 8 and 19% for the sub-till and plow systems, respectively. For both experiments, considerable mineralization and nitrification o f organic N occurred during the fallow period, with greatest rates for plowed fallow. No-till immobilized more labeled and total N in soil o rganic matter and in visible and partially decomposed crop residues on and near the soil surface. However, by harvest of the second crop gro wn after N-labeled fertilizer N was applied, Little if any of the labe led N was found in visible and partially decomposed crop residues or a s soil inorganic N, suggesting that most of the fertilizer N applied w as immobilized in soil organic N or was lost. Deep sampling (to 15 m) of the Native Sod plots showed that several hundred kg of NO3-N ha(-1) had leached beneath the crop root zone, presumably during wet years a fter fallow. For plowed plots, the quantity of NO3-N beneath the root zone approximated the loss in total soil N from the root zone, suggest ing there was a little net loss of soil N by denitrification or ammoni a volatilization. For no-till, quantities of NO3-N beneath the root zo ne exceeded the loss of total soil N during cultivation, suggesting si gnificant N-fixation occurred by unknown mechanisms. These results sho w that fallow tillage system does affect soil and fertilizer N cycling and transformations and the availability of N to crops. (C) 1998 Publ ished by Elsevier Science B.V. All rights reserved.