Copper modulates nitrous oxide emissions from soybean root nodules

Abstract Agriculture is an important source of the greenhouse gas nitrous oxide (N2O) due to the over- or non-synchronised application of nitrogen to crops. Symbiotic nitrogen fixation (SNF) by legume-rhizobia symbiosis can be an effective strategy for N2O mitigation, but several environmental factors such as copper (Cu) availability might affect N2O emissions derived from legume crops. The aim of this research was to study how Cu can modulate N2O emissions by soybean root nodules. Soybean plants inoculated with Bradyrhizobium diazoefficiens USDA110 were grown in the presence of 4 mM KNO3 and a battery of Cu2+ concentrations added during growth (0, 5, 10, 20, 40, 60 or 100 μM). N2O emissions were measured by gas chromatography in both, nodulated roots and detached nodules, after root flooding. Also, 15N isotope dilution was assayed for SNF and N2O determinations. Results showed that an excess of added Cu during growth significantly affected plant physiology, nodulation and SNF of the soybean-B.diazoefficiens symbiosis, being 20 μM the threshold that soybean plants can tolerate without suffering Cu stress. Meanwhile, Cu addition reduced statistically N2O emissions by soybean nodules. This reduction was correlated with Cu accumulation in nodules, which affected the denitrifying enzymes activities. Cu excess produced a simultaneous decrease of nitrate and nitrite reductase activities, but an increase of nitrous oxide reductase activity. Finally, the modulation of bacteroidal nitrate reductase activity is proposed as an effective target for the strategies for mitigation of N2O emissions derived from soybean crops, probably more effective than nitrous oxide reductase activity.