Effect of soil tillage, crop rotation and nitrogen application rates on plant-N content of spring wheat (Triticum aestivum L.) in the Swartland wheat-producing area of the Republic of South Africa
Studies on nitrogen content in spring wheat were conducted during the 2000 and 2001 growing seasons as part of a long-term tillage and crop rotation trial. Four tillage methods were used, namely conventional tillage (CT), tine tillage (TT), minimum tillage (MT) and no-tillage (NT). Crop rotation systems used were continuous wheat (WW) and wheat/ lupine/wheat/canola (WLWC). Three rates of nitrogen fertiliser (60, 100 and 140 kg N ha-1) were applied as sub-plots. Wheat plants were sampled at tillering stage (S1), stem elongation (S2), flag leaf (S3) (2000 growing season only) and anthesis (S4). In general nitrogen content (% of plant component) decreased as the plant reached maturity. Nitrogen content expressed as g plant-1 and kg ha-1 was affected by tillage method in both growing seasons, but the response depended on the amount of precipitation during the growing season. During the low rainfall year, highest N contents were found in the NT treatment. In contrast to this, CT resulted in the highest N content during high rainfall years. Significant differences in total nitrogen content in the plant (g plant-1) due to crop rotation were observed only at the tillering and flag leaf stages. At both stages, plants from the wheat/lupine/wheat/canola system resulted in significantly higher values than those from the monoculture (wheat/wheat) plots. A non-linear increase in the N content of the wheat crop with an increase in the N-application rate from 60 to 140 kg N ha-1 indicated a decrease in N-use efficiency at higher application rates during both years.