Forest fragment size and nutrient availability: complex responses of mycorrhizal fungi in native-exotic hosts

In the past few decades, it has been widely accepted that forest loss due to human actions alter the interactions between organisms. We studied the relationship between forest fragment size and arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE) colonization, and the AMF spore communities in the rhizosphere of two congeneric Euphorbia species (native and exotic/invasive). We hypothesized that these fungal variables will differ with fragment size and species status, and predicted that (a) AMF and DSE colonization together with AMF spore abundance and diversity would be positively related to forest fragment size; (b) these relationships will differ between the exotic and the native species; and (c) there will be a negative relationship between forest fragment size and the availability of soil nutrients (NH4+, NO3-, and phosphorus). This study was performed in the eight randomly selected forest fragments (0.86-1000 ha), immersed in an agricultural matrix from the Chaquean region in central Argentina. AMF root colonization in the native and exotic species was similar, and was positively related with forest fragment size. Likewise, AMF spore diversity and spore abundance were higher in the larger fragments. While DSE root colonization in the native host was positively related with forest fragment size, DSE colonization in the exotic host showed no relationship. Soil nutrients contents were negatively related with forest fragment size. In addition, NH4+ and NO3- were negatively correlated with AMF spores abundance and root colonization and with DSE colonization in the native species. The results observed in this study show how habitat fragmentation might affect the interaction between key soil components, such as rhizospheric plant-fungal symbiosis and nutrient availability. These environmental changes may have important consequences on plant community composition and nutrient dynamics in this fragmented landscape.