6 October 2009 'World first' soil micro-organism discovery at Lincoln University

 

Research into the microbiological mechanism by which the nitrification inhibitor DCD (dicyandiamide) reduces nitrate and nitrous oxide production from soils with nitrogen levels elevated by animal urine and other sources, has led to a world-first discovery by a Lincoln University-led team of scientists.

The scientists, led by Professor Hong Di and Professor Keith Cameron, have discovered that the nitrate and nitrous oxide reductions are due largely to DCD's inhibiting effect on ammonia-oxidising bacteria.

This is in contrast to a recent hypothesis that archaea (single cell microorganisms akin to bacteria but with a separate evolutionary history) were the main oxidising agents at work in nitrogen-rich grassland soils.

The oxidation of ammonia to nitrate - a process known as nitrification - is a fundamental one in the nitrogen cycle and the role of bacteria and archaea in this process is not well understood.

The aim of the research project, conceived by Professors Di and Cameron, was to investigate the relationship between nitrification rate and populations of Ammonia-Oxidising Bacteria (AOB) and Ammonia Oxidising Archaea (AOA) in grazed grassland soils with high nitrogen loading from animal urine.

The "breakthrough" discovery by the scientists has just been published in the prestigious international science publication Nature Geoscience (2: 621-624, 2009).

The team members are Professor Hong Di and Professor Keith Cameron (leaders) of Lincoln University; Dr Jupei Shen of the Research Centre for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China; Dr Chris Winefield of Lincoln University; Dr Maureen O'Callaghan, AgResearch, Lincoln; Dr Saman Bowatte, AgResearch, Palmerston North; and Professor Jizheng He of the Research Centre for Eco-Environmental Sciences, Beijing, China.

The new finding is part of the on-going research at Lincoln University, headed by Professor Di and Professor Cameron, on improving the environmental sustainability of dairy farming by reducing nitrate leaching into groundwater from cow urine patches and reducing similarly sourced emissions of the greenhouse gas nitrous oxide into the atmosphere.

It is work funded by the Foundation for Research, Science and Technology, and the scientific paper which has come out of the bacteria/archaea investigation is a collaborative one with the Chinese Academy of Sciences and AgResearch.

The researchers used soil samples collected from six New Zealand locations representing high-fertility grassland grazed under intensive dairy farming. The samples came from pastures receiving nitrogen inputs annually through fertiliser and biological fixation by white clover plants. The locations were Northland, Waikato, Rotorua Lakes, Canterbury, West Coast and Southland.

The new study indicates that archaea may be specially adapted to unfavourable environmental conditions, for example, a limited availability of nutrients. Bacteria, on the other hand, display a contrasting growth advantage in high-fertility soils. They flourish where soils receive rates of nitrogen boosted by animal urine.

Because of the abundance of Ammonia Oxidising Archaea in soils, it was assumed that archaea may play an important role in breaking down ammonia. However, the new research at New Zealand's specialist land-based university, Lincoln University, has shown that this is not the case.

While Ammonia Oxidising Archaea were present in large numbers in the soil samples, neither their abundance nor their activity increased with the application of an ammonia substrate (in the form of urine}. In contrast the number and activity of Ammonia Oxidising
Bacteria increased many-fold in response to the addition of ammonia.

"The implication of our discovery for nitrate and nitrous oxide reduction is that because it is bacteria rather than archaea that drive their generation it is the bacteria that we should focus on in the development of mitigation technologies," say Professors Di and Cameron.