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Nitrogen contribution to grazing ecosystems by cyanobacteria

Project start date: 15 October 2009
Project end date: 08 June 2012
Publication date: 08 June 2012
Project status: Completed
Livestock species: Grassfed cattle, Grainfed cattle


This Project was seeking to estimate of the significance of nitrogen contribution from fixation and decay by cyanobacteria to grazing ecosystems, and make these estimates along grazing gradients out from watering points taken across several different environments. This was not able to be achieved.
Plant-available N, P and S in the soil is sourced mainly from organic material (mainly from plant residues), but an essential step to release the bound nutrients into an available form for plants is microbiological decomposition.   In semi-arid and arid landscapes, vegetative cover is generally sparse and the open spaces are usually covered by a biological soil crust (BSC) or biocrust, a highly specialized community of cyanobacteria, mosses, and lichen. As soil N levels are typically low in arid lands, the cyanobacteria within the BSC can be another significant source of organic N -- estimates of N contribution to the soil are usually in the range up to 1-10 kg/ha/year.
​This small scale project was unable to complete objectives given the difficulty to extrapolate from snapshots of N fixation to any reliable estimates of seasonal or annual fixation on an area basis.  There were no replicates at the 12 sites, however this is appropriate for a pilot study with limited resources but it does limit the inferences which can be drawn from the data. The limited data available did not provide information that was sufficiently sound to provide results which would be  useful to the rangeland grazing context. Concern was expressed by both the researcher and external reviewer that the data as it stands could easily be over-interpreted.  The data on the amount of nitrogen fixed per year could not be taken as definitive until further work is carried out to support it. This was not done and so results  are not reported. An external project reviewer indicated that questions around  design and variation that existed between sites undermined confidence to develop conclusions.