Virtual nitrogen as a tool for assessment of nitrogen management at the field scale: A crop rotation approach

The efficient management of nitrogen (N) requires accurate quantification of the critical components of the N balance. N budgeting was conducted on a production farm at Gorzno, Poland during the 2004-2007 growing seasons for 17 crop sequences: ten oil-seed rape and winter wheat, six maize grown for grain or silage, and one onion monoculture. The measured N components were: i) soil N mineral content measured at 0.0-0.3, 0.3-0.6, and 0.6-0.9 m depths in the soil profile prior to spring growth and immediately after harvest, ii) applied N from fertilizer and farmyard manure, and iii) N content in the harvested crop products and crop residues. Two budgeting procedures, soil surface balance (SSuB), and soil system balance (SSyB), were used to evaluate the impact of cropping sequences on N management. The total N input ranged from 223 to 313 kg N ha(-1), primarily derived (40-50%) from the soil mineral N (N-min). The N output depended on the N content in the harvestable crop component (R-2 = 0.93). The average unit productivity of external N in the soil-crop system was calculated to be 35.4 kg cereal units (CU) kg(-1) N. The critical level, defined as that level at which N resources were fully utilized by the crop, were 51.7 kg CUs kg(-1) N. Concomitantly, the average productivity of the total pool of plant available N was 19.9 kg CUs kg(-1) N, with a critical level of 29.8 kg CUs kg(-1) N. The difference between the average and critical values, a measure of N inefficiency in the soil-crop system, was used to quantify the un-workable pool of N herein defined as virtual N (N-v). Cropping sequences with oilseed rape and winter wheat were less efficient in utilizing externally applied N compared to maize, which greatly increased the amount of N in the harvested product. The efficiency of N present in the soil-crop system can be improved by implementing two key strategies. The first strategy reduces N fertilizer input based on the total N content in the soil-crop system at the beginning of the growing season. The second strategy targets increasing the content of N in the primary yield component, which depends on N uptake and utilization efficiency from soil pools. The concept of virtual N, based on quantification of the un-worked N-min pool, is a basis for sound management of N in a particular cropping sequence. The accuracy of N-v determination based on SSyB characteristics was much more reliable than the SSuB procedure. The results offer holistic tools to develop N fertilizer management practices that reduce the application of excess N, in turn improving both N use efficiency and economic return to farmers while reducing the impact of the production system.