As plant cover declines in arid and semi-arid regions of the world, soils in the interspaces of plant canopies are often colonized by biological soil crusts (‘biocrusts’) that develop in the top few millimeters of soil surfaces. Biocrusts are comprised of communities of cyanobacteria, bryophytes, lichens, and fungi and are estimated to cover approximately 12% of the terrestrial land surface. Biocrust play a significant role in the nitrogen (N) cycle contributing N inputs via biological fixation, promoting internal N transformation processes, and releasing N via dissolved, gaseous, and erosional loss processes. In this presentation we report on recent advances in our understanding and ability to address uncertainty in quantifying N accumulation and loss pathways in biocrusts across spatial and temporal scales and how these advances influence our current thinking of N budgets in dryland environments.
Results/Conclusions
There is consistent evidence that N accumulation occurs in biocrusts from early successional cyanobacterial crusts to late successional cyano-lichen or lichen/moss communities in the top few mm of the soil surface. These patterns, however, are much more pronounced in cool desert ecosystems relative to hot desert ecosystems. What is less clear is how N loss changes over successional sequences of biocrusts and across spatial scales. Gaseous N loss through nitrification and denitrification pathways is recognized as in important N loss pathway from biocrusts. More recently, views of biocrust N budgets have been revised to incorporate observations that persistent gaseous N loss in such forms as N2O, NO, and HONO, even under conditions in which biocrusts are inactive may be a significant N loss pathway. At the landscape scale, more infrequent and episodic water erosion and runoff events may move significant amounts of N in dissolved form in runoff waters in addition to nutrient rich sediment production from biocrusts. We discuss the implications of N inputs via fixation versus the “slow” leakage of N from biocrusts in gaseous N loss pathways and the more episodic N losses in wind and water erosion events for dryland N budgets. Although we have made significant advances in understanding biocrust N cycling in drylands, there are still multiple knowledge gaps that will be discussed.