Harnessing the nitrogen cycle through novel solutions

Nitrogen is a primary nutrient regulator of plant productivity and the potential for soils to lose nitrogen remains an economic and environmental concern.

ACHIEVEMENTS

  • Ammonia oxidising bacteria dominated and suggests these bacteria are the major regulators of nitrification in these semi-arid soils. Evidence suggests that this nutrient transfer process occurs predominately in the surface 10 cm of soil.
  • To date, most attempts to manipulate the soil nitrogen cycle have occurred during the winter cropping period. Our findings highlight the need to manage nitrogen supply during summer fallow if we are to minimise losses to the environment from semi-arid soils.
  • Nitrapyrin decreased the rate of nitrification and increased retention of ammonium in soil. Nitrification inhibitors were effective at 40°C indicating that new generation inhibitors are now a potential means of slowing nitrification rates in higher temperature soils. Cost benefit requires consideration.
  • Liming was effective at decreasing soil nitrous oxide emissions from acidic soils following summer rainfall in semi-arid regions when emissions result from nitrification.

OVERVIEW

Sustainable land management requires that we optimise nitrogen (N) supply for plant production and also minimise soil nitrogen losses through nitrate (NO3) leaching and nitrous oxide (N2O) gas emissions. To achieve this it is necessary to understand how management practices can influence the relative dominance of the individual microbial pathways of the nitrogen cycle and how soil organic carbon availability is key to developing a less leaky nitrogen cycle.

Nitrogen cycling between soil, plant and microbe involves three major components:

  1. Nitrogen  supply  (mineralisation)  which  is  determined  by  the  rate  at  which  microorganisms convert  solid  soil  organic  matter  into  low  molecular  weight  organic  monomers  (e.g.  amino  acid turnover) and the resultant release of ammonium.
  2. Microbial  demand  (immobilisation  versus  nitrification)  whereby  microbial  utilisation  of ammonium  may  be  retained  (immobilisation)  or  potentially  lost  (nitrification,  with  subsequent leaching or N2O emission) from the soil. This can be expressed as the nitrification to immobilisation ratio with higher ratios indicating more potential for the soil/management system to lose nitrogen.
  3. Plant  nitrogen  capture where  the  available  pool  of  nitrogen  (organic monomers,  ammonium, nitrate) is regulated by the microbial nitrogen pathways mentioned above but also by soil chemical (fixation), hydrological/climatic  (leaching  capacity) and plant  (root depth, extent of root branching) factors which need to be considered.

The project aim was to study carbon and nitrogen processes at the microbial-rhizosphere scale (micro- and nano-scales) within soils from established field treatments. Further the project aimed to link this with an understanding of the location of functionally relevant microbial communities, as these microbes are responsible for nitrogen transformations in soil. In doing so we will improve our understanding and thus ability to manage the fate and cycling of nitrogen between land, water and atmosphere.

FUNDING

Project funded by the GRDC's Soil Biology Initiative II and the Australian Research Council, with support from UWA.

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