Nitrification inhibition by common plants in New Zealand’s agricultural landscapes

Abstract

Nitrification, the production of nitrate by the bacterial oxidation of ammonium, is an important economic and environmental issue in New Zealand. Nitrification can lead to high levels of fertiliser nitrogen loss from soil through nitrate leaching. This leaching can result in nitrate contamination in surface and ground water, and may be exacabated by New Zealand’s recent ban of the nitrification inibitor dicyandiamide (DCD). This provides an imperative to investigate alternative methods of reducing nitrate contamination from NZ’s agricultural systems. Some plants, known as Biological Nitrification Inhibitors (BNIs), can decrease nitrification rates through production of phytochemicals. There are no studies investigaing whether NZ-native plants are BNI’s. However, the phytochemical profiles of manuka (Leptospermum scoparium) and kanuka (Kunzea ericoides) indicate that they may have BNI properties. Other species commonly found in the New Zealand agricultural landscape, namely macrocarpa (Cupressus macrocarpa), monterey pine (Pinus radiata), and Eucalyptus spp. may also inhibit nitrification. Strategic plantings of such trees on farms may reduce the risk posed by nitrate leaching. This study aimed to determine the extent that these common species inhibied nitrification using plant extracts in a rapid biological assay. A bioassay was developed using a Nitrosospira enriched cell culture and each assay ran for 48 hours. Plant extracts were produced for the assay by grinding leaves suspended in liquid nitrogen with a mortar and pestle, followed by extraction with water for half an hour. Aliquots of filtered supernatant were then added to the cell culture. Nitrate measurements were taken at the end of the assay, and species were compared to L. perenne and unamended cells to determine the extent of nitrification inhibition. The bioassay was used to test the aforementioned species, as well as an un-amended cell solution negative control and two species that were not be expected to be BNIs, namely kowhai (Sophora tetraptera) and perennial ryegrass (Lolium perenne). DCD was used as a positive control. Results indicated that L. scoparium, P. radiata and C. macrocarpa decreased nitrate production relative to the control, L. perenne and S. tetraptera extracts. P.radiata, L. scoparium and C. macrocarpa decreased nitrate produced relative to L. perenne by 60%, 67% and 84%, respectively. Unpublished results of a recent lysimeter trial supported the findings that these species are BNI’s. These results indicate that these species are potentially effective inhibitors of nitrification and could be used for mitigation of nitrogen loss in agricultural systems, which is of benefit to both the environment and agriculture.