The significance of litter loss and root growth on nitrogen efficiency in normal and semi-dwarf winter oilseed rape genotypes

Winter oilseed rape (Brassica napus L.) has a high capacity for nitrogen (N) uptake, but still leaves substantial amounts of N in the soil after harvest, partly due to the N lost through shed leaves. Reduced leaf litter losses or increased N uptake might therefore be possible ways of increasing the N efficiency of winter oilseed rape. In order to determine genotype differences in litter loss and root growth, we examined biomass production, leaf litter loss, N content, yield, root distribution and root depth of winter oilseed rape, as well as soil N content, in a two-year field experiment conducted on sandy loam soils. Three genotypes were studied: a semi-dwarf (SD) hybrid and two normal hybrids. All genotypes were treated with low and high N fertilization levels of 120 and 280 kg N ha−1, respectively. Root growth was studied by means of minirhizotrons to 2.3 m soil depth. Lost leaves contributed significantly to the N surplus left in the soil after oilseed rape. Estimations based on litter loss measurements revealed that more than 100 kg N ha−1 potentially could be lost during the growing season. Generally, estimated total litter loss and accumulated N loss did not differ between the studied genotypes, whereas more N was lost from the high N treatments. In spring, the nitrate (NO3−) levels throughout the soil profile were low, indicating the efficiency of oilseed rape in taking up available NO3− and reducing the risk of leaching losses during autumn and winter. The SD genotype was not found to have less leaf biomass, but had lower yields leading to lower N use efficiency (NUE) as well as lower N uptake efficiency (NUpE). Thus, the hypothesis that SD would have less biomass and consequently reduced litter loss leading to increased N efficiency was rejected. Generally, SD took up less N and tended to leave more NO3− in the upper soil layers at harvest. At maturity, roots reached at least 2.3 m depth, with no significant genotype differences in depth and intensity. This implied that genotype differences in N efficiency and yield was not related to root growth differences, indicating that root growth is not the most obvious target in order to improve N efficiency of winter oilseed rape. Improving remobilization of N from the large amount of biomass lost during the growing season or decreasing the amounts of biomass produced and lost might be more obvious and effective targets.