Spatial and temporal processes affecting nitrogen availability at the landscape scale

Nitrogen dynamics in soils are affected by spatial and temporal processes. Drainage class is generally regarded to be the most significant source of v ariability for N in temperate humid climates. A 5-year study was conducted including four rates of N fertilizer and three drainage classes within a 15 ha maize (Zen mays L.) field. Variance component analysis showed that N re sponse was minimally affected by drainage class, but showed strong yearly v ariations, apparently related to early-season precipitation. Annual field-a veraged economic optimum N rates had a range of 65 kg ha(-1) with lower rat es being associated with years with low early-season precipitation. A calib rated LEACHMN model and site-specific weather data were used to evaluate th e effects of early-season weather conditions on N rate and availability. Du ring wet years, soil N availability was reduced by approximately 35-50 kg h a(-1) compared to dry years, largely independent of drainage class. For wel l-drained soils, most losses were attributed to leaching (especially in yea rs with wet early-season), while poorly drained soils mainly experienced de nitrification. It is concluded that limited benefits may be gained from spa tially variable N applications within fields based on drainage class or soi l type, but considerable economic and environmental gains are possible from yearly adjustment of supplemental N rates based on model simulations of N dynamics using information on early-season weather conditions. (C) 2001 Els evier Science B.V. All rights reserved.