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. (C) 2015 Elsevier B.V. All rights reserved.