Effects of eight years of crop rotation and tillage on nitrogen availability and budget of a sandy loam soil

The effects of tillage and crop rotations on soil N availability and economy were evaluated over two rotation cycles to address the paucity of such information. From 1993 through 2000, soil was sampled to 120 cm in the autumn from four crop rotations : (i) continuous wheat (Triticum oestivum L.), (ii) field pea (Pisum sativum L.)-wheat-canola (Brassica rapa L.)-wheat, (iii) red clover (Trifolium pratense L.) green manure-wheat-canola-wheat/red clover, (iv) fallow-wheat-canola-wheat, and analysed for KC1-extractable N. The rotations were managed under a conventional tillage (CT) or a no-till (NT) system, and were fertilized based on soil test results. A N budget was constructed for each cropping system comprising N added in seed and fertilizers, and by symbiotic fixation and N exported in the grain. More nitrate accumulated under CT than NT, resulting in lower N fertilizer application rates for CT plots. Soil mineralizable N was higher under NT than CT, and was not influenced by crop rotations. The trend for residual soil nitrate among crop rotations was: fallow rotation > green manure rotation > continuous wheat > field pea rotation, due mostly to residual nitrate following the first phase of the rotations. There was no interaction of tillage with rotation. The continuous wheat and field pea rotation maintained a balanced N budget. The red clover rotation resulted in net N import in each rotation cycle of approximately 25 kg ha^-1 under CT and 37 kg ha^-1 under NT; net N export from the fallow rotation was 30 kg ha^-1 under NT and 46 kg ha^-1 under CT.