Soil nitrogen dynamics as affected by landscape position and nitrogen fertilizer

The influence of landscape position on the dynamics of N in the soil-plant system has not been adequately studied. Our aim with this study on a predominantly Black Chemozem soil was to evaluate the effect of slope position (upper vs. lower) and N fertilizer application (none vs. 60 kg N ha(-1)) on soil and wheat (Triticum aestivum L.) N through the growing season. Landscape position had a dominant effect on soil NO3- and soluble organic N (SON) concentrations, especially in the surface 15 cm. These pools of soil N and net N mineralization were greater at the lower than at the upper slope position. The landscape effect is attributed to higher organic matter content (as measured by organic C) and water availability in lower compared with upper slope positions. Nitrogen application had no measurable effect on soil NO3- and SON concentrations. Exchangeable and non-exchangeable NH4+ were little affected by slope position or N fertilization. Nitrogen application increased wheat N uptake; however, its influence was less than that of slope position, especially on N accumulation in wheat heads during grain-filling. Although N application increased wheat yields, landscape position exerted the greater influence: grain yield was less on upper than lower slope positions due to earlier onset of crop maturity. During grain filling, net N mineralization was suppressed at the upper slope position and by N application. The increase in crop yield and N uptake due to N application was not significantly different between slope positions. This study demonstrated that landscape position had a greater influence on N dynamics and availability than the application of typical amounts of fertilizer N and that the two effects were mostly independent of each other.