Cyanobacterial species richness and Nostoc highly correlated to seasonal N enrichment in the northern Australian savannah

Boodjamulla National Park research station is situated in north-west Queensland in the dry savannah where the climate is dominated by summer monsoons and virtually dry winters. Cyanobacterial crusts almost entirely cover the flood plain soil surfaces in between the tussock grasses. Cyanobacteria fix dinitrogen that is liberated into the soil in both inorganic and organic N forms. Seasonality drives N-fixation and in the savannah, this has a large impact on both plant and soil function. In this research project, we examined the cyanobacterial species richness and bioavailable N spanning the seven months of a typical wet season. We hypothesised that cyanobacterial richness and bioavailable N would peak at the time of the heaviest rains and gradually decline in the latter stages of the wet season. We also anticipated that the abundance of N-fixing cyanobacteria would be correlated to N-fixation and N-enrichment of the surface soils. Over the wet season cyanobacterial richness ranged from 6–19 species. N-fixing Scytonema accounted on average across the season for 74 % of the biocrust in varying proportions throughout the season. Cyanobacterial richness was highly correlated with N-fixation and bioavailable N in 0–1 cm. It was established key N-fixing species such as Nostoc , Symploca and Gloeocapsa significantly enriched soil N although Nostoc was the most influential. Total seasonal N fixation by cyanobacteria demonstrated the variability in productivity according to the number of wet days as well as the follow-on days where the soil retained adequate moisture. Based on total active days per month we estimated that N-soil enrichment via cyanobacteria would be ~ 5.2 kg ha −1 annually which is comparable to global averages. This is a substantial contribution to the nutrient deficient savannah soils that are almost entirely reliant on the wet season for microbial turnover of organic matter. This seasonal pattern in atmospheric N-fixation and transformation to a bioavailable form was also present in C-fixation results from parallel research. Such well-defined seasonal trends and synchronisation in cyanobacterial species richness, N-fixation, bioavailable N and C fixation provide significant contributions to multi-functional microprocesses and soil fertility.