Biological crusts: A forgotten component of the global carbon and nitrogen cycle?

Background/Question/Methods

Cryptogamic covers are composed of cyanobacteria, green algae, lichens, bryophytes, fungi and bacteria in varying proportions. As cryptogamic ground covers, including biological soil and rock crusts as well as bryophyte and lichen carpets they occur on many terrestrial ground surfaces. Cryptogamic plant covers, comprising epiphytic and epiphyllic crusts as well as foliose or fruticose lichens and bryophytes are spreading over large portions of terrestrial plant surfaces. Photoautotrophic organisms within these crusts sequester atmospheric CO₂ and many of them inhabit nitrogen-fixing cyanobacteria, utilizing atmospheric N₂ to form ammonium which can be readily used by vascular plants. In a thorough literature search, we compiled all available data on the photosynthetic properties of cryptogamic covers and developed a model to calculate their net primary production.

In a detailed long-term study, the net primary production of biological soil crusts (BSC) is analyzed.

Results/Conclusions

We obtained a total value of 3.9 Pg a^-1 for the global net uptake of carbon by cryptogamic covers, which corresponds to approximately 7% of the estimated global net primary production of terrestrial vegetation (Elbert et al., in press). This value is of the same magnitude as the global annual carbon turnover due to biomass burning, which has been estimated at 3.6 Pg a^-1. The corresponding study on the nitrogen assimilation of cryptogamic covers revealed a global estimate of ~49 Tg a^-1, accounting for as much as half the estimated total terrestrial biological nitrogen fixation.

In the long-term study, the microclimatic conditions (water status, temperature, light intensity) of four different types of BSC have been monitored over one whole year. These data reveal that BSC are active for a total duration of approximately 35 days during the year, experiencing mean temperatures of only 14.6°C in an active state. Microclimate data are combined with ecophysiological characterizations of the four crust types to obtain their annual balance of net primary production. Calculation of the net primary production on a spatial scale is being accomplished in a remote sensing approach. Based on hyperspectral remote sensing data we have developed an algorithm to classify biological soil crusts of the Succulent Karoo. Knowing the percentage of each crust type, we depict and calculate the long-term productivity of each crust type with high accuracy.

Literature:

Elbert W, Weber B, Burrows S, Steinkamp J, Büdel B, Andreae MO, Pöschl U (in press). Contribution of cryptogamic coverst to the global cycles of carbon and nitrogen. Nature Geoscience.