Nitrous oxide emission from plants a source or a conduit of nitrous oxide

There are several reports of plant-mediated N2O emission, mainly from rice or temperate arable systems, but there is current controversy as to the mechanism involved in this emission. Whilst emissions from the plants themselves have been shown to account for up to 87 per cent of emissions measured from plant-soil systems (Pihlatie et al, 2005), the mechanisms involved are unknown. Aerenchyma are thought to be implicated in the transfer of N2O from soil (Rusch and Rennenberg, 1998), with significant transfer of N2O reported through rice plants (Yan et al, 2000). Chang et al (1998) and Pihlatie et al (2005) also proposed that, as N2O is a soluble gas, it can be transferred from roots in the transpiration stream of upland plants, to leaves and then emitted to the atmosphere. Pihlatie et al (2005) demonstrated using a 15N-enrichment approach that all of the 15N-N2O emitted from Fagus sylvatica leaves was derived from soil-applied 15NH415NO3. These authors pertinently point out that if N2O emission from plants is a common phenomenon, then inventories based solely on emissions from the soil surface may underestimate ecosystem fluxes, and if transpiration is the main mechanism, then it may be possible to predict canopy fluxes from soil N2O production rates and mineral N availability. However, it is unknown how the spatial arrangement of N2O production in the rhizosphere relates to the potential for plant N2O uptake, the zone of root uptake of N2O, or the ability of plants to take up this N2O under dry conditions.

It is also possible that N2O can be formed in leaves during nitrite photoassimilation (Smart and Bloom, 2001; Hakata et al, 2003) or by plant endophytic bacteria. If endophytic N2O production occurs it is likely that this is denitrifi-cation, as a number of bacteria that form associations with plants are denitrifiers, particularly in root nodules, and may in part account for reported N2O uptake from the rhizosphere. Thus Daniel et al (1980) and Fernandez et al (2008) reported denitrification ability in a large number of rhizobia isolates. However, this source of N2O is unlikely to significantly contribute to net N2O emission, compared with transport of N2O through the plant arising from stimulation of denitrification in the rhizosphere following inorganic N addition.

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