Changes in intrasystem N cycling from N2-fixing shrub encroachment in grassland: multiple positive feedbacks

Abstract

Nitrogen-fixing species can increase both the availability and cycling of nitrogen (N) in ecosystems. Autumn olive (Elaeagnus umbellata Thunb.) is an exotic woody shrub associated with N2-fixing actinomycetes that forms dense patches in disturbed landscapes (i.e., riparian zones adjacent to crop systems, old fields and agricultural grasslands) throughout the midwestern United States. We used paired plots dominated by either E. umbellata or C3 grassland to test whether the shrub encroachment altered pools and fluxes of nitrogen (N) and carbon (C) in the soil. Annual mean of NO3‐N concentrations in soil water collected from porous cup tension lysimeters every 2 weeks for 1 year was 20 times higher in soil beneath E. umbellata compared to grassland vegetation. Temporal variation in NO3‐N leaching occurred in the shrubencroached plots, with more nitrate leaching in the dormant season relative to the growing season. Potential net N mineralization, nitrification rates, and extractable N in the surface 10 cm of soil were also higher below E. umbellata. Following establishment of N2-fixing shrub patches for 7‐13 years, the soil C:N ratio showed a declining trend due to lower total soil C rather than an increase in N. Labile carbon pools (i.e., microbial biomass C (MBC) and soil respiration rates) were lower in surface soil below E. umbellata, which demonstrated an additional positive feedback between encroachment of E. umbellata and N export. Less demand for mineralized N due to associated N2 fixation, coupled with higher rates of nitrification and lower microbial demand for N collectively contributed to higher export of N below the E. umbellata patched relative to the grassland system. Thus, areas invaded by this exotic N2-fixing species may function as N sources rather than the N conserving systems typically expected early successional communities following agricultural abandonment. # 2006 Elsevier B.V. All rights reserved.