Potential of manuka and kanuka for the mitigation of nitrous oxide emissions from NZ dairy farms

Abstract

Nitrous oxide (N2O) is a greenhouse gas with a global warming potential 298 times that of carbon dioxide. Since pre-industrial times, atmospheric levels of N2O have increased from 270 to 319 ppb. More than a third of emissions are anthropogenic, most of which are due to agriculture. This loss of nitrogen as N2O represents an important economic loss to the agricultural industry. N2O is primarily produced in soils via the processes of nitrification and denitrification, both of which are biologically driven processes. It is possible that these processes may be perturbed by plants, such as the New Zealand natives: Leptospermum scoparium (manuka) and Kunzea ericoides (kanuka) that are known to contain antimicrobial compounds. Potentially, the strategic planting of manuka or kanuka on NZ dairy farms may reduce N2O emissions. This study aimed to test whether manuka and kanuka affected soil microbes and specifically altered the production of N2O.

A greenhouse pot trial was conducted using Lolium perenne (perennial ryegrass pasture), manuka and kanuka plants to test whether the latter two reduced the N2O fluxes from the soil when ~200 kg N ha-1 dairy shed effluent was added as a nitrogen source. A closed-chamber method was developed using 20 L plastic buckets to enable collection of gas samples from the whole pot system. N2O concentrations were determined using gas chromatography fitted with an electron capture detector. A preliminary trial indicated that manuka plants may reduce N2O fluxes from soil. However, a full-scale trial did not produce measurable N2O fluxes. This was attributed to the high volume to soil surface area (0.17 m3:0.02 m2) of the chambers used, meaning they provided insufficient sensitivity to determine the low N2O fluxes.

Following the unsuccessful greenhouse study, a field trial was carried out using an already established method of field N2O collection. Again a closed-chamber method was used, but, these chambers had a much lower volume to soil surface area ratio (0.26 m3:0.18 m2). N2O emissions from soil beneath 5 year-old kanuka trees were compared with bare ground soil (control) treated with either water or dairy shed effluent. Soil samples were taken from adjacent plots which received the same treatments. For effluent-treated plots, N2O fluxes were higher from the control compared with kanuka plots; cumulative fluxes were 65 and 13 mg N2O-N m-2, respectively. Soil nitrate levels were higher under kanuka than control plots, mean values were 17.1 and 3.3 µg g-1 soil, respectively. These findings may indicate an inhibition of denitrification beneath kanuka.

A further experiment demonstrated that 7 days after inoculation, soil Escherichia coli levels in manuka and kanuka pots (average 8 000 cfu g-1) were reduced compared with pasture control pots (65 000 cfu g-1).

These experiments demonstrate that manuka and kanuka affect the functioning and survival of soil microorganisms and reduce N2O emissions under some conditions. Further research should focus on elucidating the mechanisms responsible for this inhibition, and test the range of environmental conditions where these plants may be used effectively.