COS 29-4 - An atypical association: How unorthodox plant-fungal relationships may contribute to phosphorus loss in the subtropics

Abstract: We conducted a two-year long fungicide experiment designed to chemically exclude arbuscular mycorrhizal fungi (AMF) from plants grown in-field. AMF are known for improving growth and yield in a variety of terrestrial plants, largely because they facilitate access to nutrients like phosphorus. We sampled from paired field plots treated biweekly with Topsin-M fungicide to quantify the degree to which these plant-fungal relationships are necessary for phosphorus acquisition. Samples were evaluated for percent root length colonization, ambient soil-P availability, and final P concentration in plant tissues. We found that samples grown from fungicide treated plots had lower percent (%) P in aboveground tissue. Surprisingly, we found that the common forage Paspalum notatum was particularly reliant on fungi in high-P soils. Generally, plants are considered to rely most heavily on these fungal mutualists in areas of poor phosphorus availability, where AMF convey a competitive advantage by improving access to limited nutrients. As phosphorus availability increases, many plants reduce their carbon allocation to these fungal symbionts as AMF no longer convey a competitive advantage. Uncharacteristically, P. notatum appears to maintain its association with fungi even in soils with extremely high plant-available P, and we found extensive root colonization in our samples that persisted even in high ambient P conditions. Furthermore, this association appears to be a key part of maintaining high plant phosphorus concentrations in high-P soils. While the presence of fungi increased P uptake by ~18% in low P soils (< 10 mg/kg; Mehlich 1 extraction) and 22% in soils of intermediate availability (10-30 mg/kg), fungal associates increased plant % P concentrations by approximately 43% in soils with high plant-available phosphorus ( >50 mg/kg).

Previous work has observed that the warm-season C4 grass Paspalum notatum increases phosphorus loss from soils in heavily-fertilized subtropical ranchlands. The persistence of plant-AMF relationships in these phosphorus-rich soils may result in an increase of soluble phosphorus beyond what might ultimately be taken up by the plant. In this case, the surplus phosphorus is at an increased risk for loss downstream – a challenging problem that contributes to the eutrophication of vulnerable freshwater. Given our results, we suggest that the persistence of phosphorus-acquiring AMF beneath P. notatum may be one explanation for the increased loss associated with the species.