Phosphorus fertilization effect on timothy root growth, and associated arbuscular mycorrhizal development

Root architecture and mycorrhizal associations are plant traits that determine soil P uptake by plants. However, the response of these important plant traits to mineral P fertilization under grasslands production is poorly understood. We assessed the effect of P fertilization at 0, 20, or 40 kg P ha–1 applied in the spring of each year since 2010 on root attributes, arbuscular mycorrhizal (AM) root colonization, plant growth, and plant P concentration in timothy (Phleum pratense L.) swards seeded in 2009 on a Labarre clay loam in Normandin, QC, Canada. Soil cores for root samples (height of 7.5 cm and inner diameter of 8 cm) were collected in June 2013 after the first harvest and in August 2014 after the second harvest. The results showed that P application increased dry matter (DM) yield and shoot P uptake at the first harvest time but not at the second. Shoot P concentration increased with increasing P application, particularly in August 2014. Phosphorus fertilization had no significant influence on root architectural attributes, colonization by hyphae or arbuscules, or total AM root colonization at both sampling times. However, in August 2014, AM vesicle density in roots was significantly greater with 40 kg P ha–1 than without P fertilization. We conclude that timothy shoot and root growth varied with sampling time but that root architectural traits and the AM colonization of timothy roots were influenced little by P fertilization in eastern Canada.