Effect Of Tall Fescue Endophyte Strains And Nitrogen Fertilization On Soil Aggregate Stability

Recently, it has been reported that the infection of tall fescue with Neotyphodium spp. endophytes may induce changes in host grass physiology, especially in root morphology that could affect soil aggregate stability. Therefore, the effects of N management under stockpiled “Jesup†tall fescue (Schedonorus phoenix) infected with and without different endophytes (Neotyphodium coenophialum) on short-term response of aggregate stability in a Marietta loam soil (Fine-loamy, siliceous, active, thermic Fluvaquentic Eutrudepts) was investigated. The experimental design was split-split block replicated four times. Endophyte strains in tall fescue evaluated were: endophyte-infected wild type (E+), novel endophyte infected (E502, E514, E542, and E584) and endophyte free (E-). Nitrogen treatment consisted of a control and two N sources [urea and ammonium nitrate (NH4NO3)] applied at two different rates (56 or 112 kg ha-1 of actual N) applied in mid-September and mid-October. Aggregate stability was measured as water stable macro-aggregate (>250 µm) and micro-aggregate (125-250 µm) using a wet sieving apparatus. Endophytes in tall fescue did not affect macro-aggregates, but did affect micro-aggregates. Stable micro-aggregates decreased in following order E584 > E502 > E+ > E514 > E- > E542. The application of nitrogen fertilizer in September increased macro-aggregates by 4% and decreased micro-aggregates in the soil by 8% when applied as ammonium nitrate in comparison to urea. In contrast, application of N fertilizer in October increased micro-aggregates by 5% applied as ammonium nitrate in comparison to urea, but had no effect on macro-aggregates. These results indicate that the direction and magnitude of the change in stable aggregates in a soil could be influenced by the effect of management, climatic and biological factors on the wetting and drying cycle in the soil. Future studies are needed to determine if the effect of N fertilizer on aggregation could be attributed to the effect of the biomass yield on the soil moisture content at rewetting.