Assessment of arbuscular mycorrhizal fungal propagules and colonization from abandoned agricultural fields and semi-arid grasslands in riparian floodplains

Abstract

Prior to the onset of extensive grazing and clearing for agriculture, riparian floodplains of southeastern Arizona, USA, historically supported large grassland communities dominated by Sporobolus wrightii Munro ex Scribn., big sacaton grass. Large-scale abandonment of agricultural operations has occurred in this region in the past 50 years, but natural re-establishment of big sacaton into abandoned fields has typically been slow. This study assesses whether arbuscular mycorrhizal fungi (AMF) may be one factor in recovery rates of abandoned fields within three riparian areas in southeastern Arizona (San Pedro river, the Babocomari river and Cienega creek). In each riparian area, soil samples were collected along paired 100 m transects, one in an extant grassland and one in a neighboring abandoned agricultural field. At one site (San Pedro), a third transect was established in a second adjoining abandoned agricultural field to enable comparisons between fields showing differing rates of recovery at a single site. Roots were assessed for mycorrhizal colonization using the modified intercept method and the modified mean infection percentage (MIP) method was used to determine inoculum potential of each soil sample. No significant differences were found between the levels of mycorrhizal colonization in roots collected from the grassland and those collected from the abandoned field for any of the sample sites. Levels of colonization did differ among the three sample sites, with higher levels seen at the San Pedro site than at the Babocomari and Cienega sites. With one exception, MIPs were greater in the soil from the abandoned agricultural field than in the soil from the nearby grassland. In the exception, no difference was detected in inoculum potentials at the San Pedro site between the paired grassland and the abandoned field that had the lower level of sacaton recovery. MIPs were also greater in the soil from the abandoned agricultural field at Babocomari, where Salsola tragus L. growth for cattle feed was promoted by discing every 3–4 years. These results indicate that higher inoculum potentials in abandoned fields were not always linked to the high levels of recovery of native vegetation.