Dryland soils are characterized by resource island patterns in which biologically relevant elements such as nitrogen are concentrated under plants. However, the source of new nitrogen in the system is often atmospheric deposition or N-fixing soil organisms, in which case the spatially dominant interspaces—often covered by biological soil crusts in various stages of development—are crucial to controlling N input and distribution. Through experiments, isotopic approaches, and field measurement of nutrients, we have been investigating how N enters the system, moves from biocrust to soil, and is laterally transported to maintain patterns of nutrients. Specifically we address the questions: (1) Are fungi and cyanobacterial filaments key transport vectors (the fungal loop hypothesis)? (2) What is the role of roots in N movement and do they prefer specific N forms? and (3) What is the role of rainfall leaching in moving N from biocrusts to subsurface soil?
Results/Conclusions
Natural abundance and tracer isotopic data, and fungal hyphal exclusion mesh experiments, when taken together, show some evidence for lateral nutrient movement across the soil surface, but not unqualified support for the fungal loop hypothesis. While fungi are present in these soils, they are low in biomass and may be more important for near-root uptake than for longer distance nutrient transport. Roots, in contrast, were present in almost every soil sample we collected, and hence more effective at exploring the three-dimensional structure of the soil than we previously thought. Greenhouse trials showed that three different forms of labelled nitrogen (15N-nitrate, ammonium, and glutamate) were readily taken up by roots and transported to plant leaves within 24 hours, with slightly faster uptake rates for nitrate. Finally, we see substantial evidence that nutrients (such as N, P) are leached from many different types of biocrusts to the subsurface soil during simulated rainfall events. Our results suggest that this vertical transport followed by root uptake are likely the key drivers of nutrient movement and the maintenance of nutrient heterogeneity in dryland soils.