In many arid and semi-arid ecosystems, canopy trees and shrubs have a strong positive influence on soil moisture and nutrient availability, creating islands of fertility where organic matter and nutrients are high relative to areas outside the canopy. Previous studies of canopy effects on soil processes have rarely considered how landscape context may modulate these effects. We measured the effects of velvet mesquite trees (Prosopis velutina) on soil moisture and the biogeochemistry of nitrogen at different positions along a topographic gradient from upland desert to riparian zone in the Sonoran Desert of central Arizona. We also examined how landscape position and patterns of precipitation interact to determine the influence of P. velutina on soil moisture, N availability assessed using ion exchange resins, net N mineralization and net nitrification, and microbial biomass C and N. P. velutina clearly created islands of fertility with higher soil organic matter, net N mineralization and net nitrification rates, and microbial biomass under mesquite canopies. These effects were consistent across the landscape and showed little temporal variability. Magnitude and direction of effect of mesquite on soil moisture changed with landscape position, from positive in the upland to negative in the terrace, but only when soil moisture was >4%. Resin N showed responses to mesquite that depended on precipitation and topographic position, with highest values during wet seasons and under mesquite on terraces. We suggest changes in proximity of P. velutina to groundwater lead to shifts in biogeochemical processes and species interactions with change in landscape position along a topographic gradient.
|Original language||English (US)|
|Number of pages||13|
|State||Published - Apr 2005|
Bibliographical noteFunding Information:
We thank Tiffany Miley, Jill Koehler, Dana Swoveland, Marcia Kyle, Ayoola Folarin, Amy Novotny, Cathy Kochert and Tom Collela for invaluable assistance in field and laboratory. Ryan Sponseller made valuable comments on an early version of the manuscript that significantly improved our interpretation of the data. We thank Jim Elser, Bill Fagan, and other members of the genes-to-ecosystems team for assistance with the development of the ideas behind this project. This work was funded by the NSF-IRCEB Biological Stoichiometry Genes-to-Ecosystems project (IBN-9977047) directed by Jim Elser.
- Islands of fertility
- Microbial biomass C and N
- N biogeochemistry
- Net nitrification
- Nitrogen mineralization
- Prosopis velutina