Grassland Restoration Increases Mutualistic Benefits That Soil Biota Provide to Plants

Premise of research. Many plant species engage in mutualistic symbioses with soil biota such as arbuscular mycorrhizal (AM) fungi and rhizobium bacteria. Agricultural practices such as chemical fertilizer application and tilling can decrease the mutualistic functions of soil biota, but whether restoration of agricultural lands causes soil biota to increase mutualistic functions is not frequently studied. Methodology. To evaluate whether ecological restoration of agricultural lands increases mutualistic benefits that soil biota provide to plants, we grew a mycorrhizal- and rhizobium-responsive host plant (Trifolium pratense) in a common background soil that had been inoculated with soil biota from grasslands that had been restored in the past 9–12 yr and adjacent cultivated fields. Because both AM fungi and rhizobium are nutritional symbionts, we grew plants in each soil biota treatment under both low- and high-fertilizer amendments to assess whether mutualistic services would be reduced when nutrient availability was high. Pivotal results. Inoculation with soil biota from restored grasslands increased aboveground plant biomass by ~19% compared with cultivated fields, and this positive effect was observed at both low and high fertilizer levels. In the low-fertilizer treatment, percentage colonization of roots by AM fungi was 1.8 times higher in treatments receiving restored grassland versus cultivated field inoculum, but there was no difference in AM fungal colonization under high-fertilizer treatments. Rhizobium nodulation of roots did not differ between restored grassland and cultivated field inoculum sources in either fertilizer treatment. Conclusions. These results show that the mutualistic benefits of soil biota can increase following the restoration of previously cultivated fields to grasslands. The positive effect of soil biota on plant biomass was most likely caused by AM fungi rather than rhizobium bacteria. Increases in mutualistic benefits provided by soil biota can occur within a decade following grassland restoration of formerly cultivated agricultural fields.