Mitigation of yield-scaled nitrous oxide emissions and global warming potential in an oilseed rape crop through N source management

Abstract

Enhanced-efficiency nitrogen (N) fertilizers, such as those containing nitrification or urease inhibitors, can mitigate the carbon (C) footprint linked to the production of bioenergy crops through a reduction in direct nitrous oxide (N₂O) emissions and indirect N₂O losses. These indirect emissions are derived from ammonia (NH₃) volatilization, which also have important environmental and health implications. The evaluation of the global warming potential (GWP) of different N sources using site-specific data of yield and direct and indirect emissions is needed for oilseed rape under rainfed semi-arid conditions, especially when meteorological variability is taken into account. Using urea as a N source, the N₂O mitigation efficacy of the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) alone or combined with the nitrification inhibitor 2-(3,4-dimethyl-1H-pyrazol-1-yl) succinic acid isomeric mixture (DMPSA) was evaluated under field conditions in a rainfed oilseed rape (Brassica napus L.) crop. Two additional N sources from calcium ammonium nitrate (CAN), with and without DMPSA, were included. The GWP of the treatments was estimated considering the emissions from inputs, operations and other direct and indirect emissions of greenhouse gases (GHGs), such as methane (CH₄) and the volatilization of NH₃. We also measured the abundance of key genes involved in nitrification and denitrification to improve the understanding of N₂O emissions on a biochemical basis under the conditions of our study. The results show that due to the intense rainfall after fertilization and a rewetting event, N₂O losses from fertilizers without inhibitors were greater than those previously reported under Mediterranean conditions, while NH₃ losses were low and not affected by the urease inhibitor. The cumulative N₂O emissions (which were greatly influenced by a rewetting peak three months after fertilization) from the urea fertilization were significantly higher than those from CAN. The presence of NBPT significantly reduced N₂O losses by an average of 71%, with respect to urea. The use of DMPSA with CAN resulted in an abatement of N₂O emissions (by 57%) and a significant increase in oil yield in comparison with CAN alone. All inhibitor-based treatments were effective in abating N₂O emissions during the rewetting peak. The abundances of the nitrifier and denitrifier communities, especially ammonia-oxidizing bacteria (AOB), significantly decreased relative to the urea or CAN treatments as inhibitors were applied. Under the conditions of our study, the sustainability of a bioenergy crop such as oilseed rape can be improved by using inhibitors because they mitigated N₂O emissions and/or enhanced the oil yield.

Keywords: Carbon footprint; Denitrifying bacteria; Micrometeorological measurements; Nitrous oxide emission; nirK; nosZI.